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Inspection of the Eyes and Head Position Inspection of the Lids and Lid Fissures When examining the eyes, attention should be given to the lid fissures, their width, and their direction, by means of an imaginary line connecting the inner and outer canthus. If the two lid fissures are different in width, the possibility of ptosis or pseudoptosis of the upper lid with the narrow lid fissure must be considered and the two conditions differentiated. A weakness of elevation of one eye will cause the lid fissure to be narrower than that of the unaffected eye. The patient may have true ptosis of the upper lid, especially if the superior rectus muscle is involved. However, the lid may only appear to be ptotic as a result of narrowness of the lid fissure caused by the hypotropic position of the globe (Fig. 12–1A). This is known as pseudoptosis and can be established by having the patient fixate with the affected eye (Fig. 12–1B). If pseudoptosis is present, the lids will open to their normal width. The lids of the unaffected eye will widen abnormally as the elevators of that eye receive excessive innervation according to Hering’s law

(see p. 64), and the left globe moves into a hypertropic position. To correct a pseudoptosis, the only requirement is that the eyes be brought to the same level by operating on the appropriate extraocular muscles. Any operation on the levator muscle of an eye with pseudoptosis is a serious mistake. One should also ascertain whether the width of one lid fissure changes when the patient moves the eyes to the right or left, as in retraction syndrome (see Chapter 21), when the jaw is moved, or when the patient speaks or chews, as occurs in the jawwinking phenomenon of Marcus Gunn. In infants the epicanthus frequently is more or less pronounced with a semilunar fold of skin running downward at the side of the nose and its concavity directed toward the inner canthus.39 The epicanthus varies considerably in width and may approach and obscure the inner canthus, which may create the appearance of esotropia when none is present (Fig. 12–2A). This is a common cause of pseudostrabismus. In time, the bridge of the nose develops, and in whites the epicanthal fold normally disappears. The examiner may demonstrate to anxious parents that pseudostrabismus disappears by lifting the skin

168

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FIGURE 12–1. Pseudoptosis in a patient with right hypotropia caused by a paretic right superior rectus muscle and secondary left hypertropia. A, Patient fixating with left eye; apparent ptosis of right upper lid. B, Patient fixating with right eye; lid fissure wide open. Left hypertropia.

from the nasal bridge (Fig. 12–2B). Mongoloid and antimongoloid positions of the lid fissures are occasionally observed in patients with the A and V patterns of strabismus (see Chapter 19) and should alert one to the presence of such patterns.   FIGURE 12–2. Pseudostrabismus. A, A prominent epicanthus may obscure some or all of the usually visible nasal aspects of the globe, thus giving the false impression that esotropia is present. B, For explanation, see text. (From Noorden GK von: Atlas of Strabismus, ed. 4. St Louis, Mosby–Year Book, 1983, p 29.)

Position of the Globes—Angle Kappa The best means of estimating the relative position of the eyes is to have the patient fixate a penlight at near vision and then at distance while the light is held so that reflections from the cornea can be obtained. If reflected images from the two corneas appear centered under both conditions, one can assume that the eyes are properly aligned in distance and near fixation. Estimation of the angle of strabismus by fixation on a light should be used only when examining uncooperative patients or infants too young to sustain fixation of an accommodative target at near, since the state of accommodation is uncontrolled with this method. Unusually narrow or unusually wide interpupillary distances should be noted. Narrow ones may create the impression that an esotropia is present. Of course, actual heterotropias may coexist with abnormal interpupillary distances. Facial asymmetries also may create the impression that a hypertropia is present. In such instances the lid fissure and the whole eye may appear to be higher on one side than the other. However, further examination will reveal that, contrary to the impression given by the patient’s appearance, there is no hypertropia, or a hyperphoria may actually be present in the eye opposite the one thought to be involved. Gross manifest deviations in primary position are readily detected by inspection. However, small deviations may escape detection, or the presence

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of a deviation may erroneously be assumed to exist because of the presence of a large angle kappa. An angle kappa is caused by failure of the pupillary and visual axes of the eye to coincide (Fig. 12–3). The pupillary axis is the line passing through the center of the apparent pupil perpendicular  to the cornea. The visual axis (or the line of sight) connects the fovea with the fixation point.74 The angle kappa is formed at the intersection of these two axes at the center of the entrance pupil. The angle kappa has also been referred to as the angle lambda in the older literature.48; 53, p.80 The visual axis does not always coincide with the optical axis (defined as the line connecting the optical centers of cornea and lens) with which it forms the angle alpha at the nodal point and the angle gamma at the center or rotation of the eye. All these angles are geometric constructions (Fig. 12–4), and only the angle kappa can actually be measured and is of practical importance. As a rule, the pupillary axis touches the posterior pole of the globe slightly nasal and inferior to the fovea. As a result, when an eye fixates a penlight, the reflection from the cornea will not be centered but will be located in a position slightly nasal to the center. This is termed a positive angle kappa (Fig. 12–5). The student may find it helpful to remember "positive to nose." A sufficiently large positive angle kappa may simulate an exodeviation and produce pseudostrabismus. An existing exodeviation will look worse than it actually is, or it may mask all or part of an esodeviation.

FIGURE 12–4. Definition of angles. C, center of rotation; F, fovea; N, nodal point; O, point of fixation; P, center of pupil; X, point of cornea that lies in the central pupillary line; AB, optical axis; AP, central pupillary line; OC, fixation axis; OF, visual axis; angle ONA, angle alpha; angle OCA, angle gamma; angle OPA, angle kappa; angle OXA, angle kappa, as measured clinically. Angle lambda not defined. (From Lyle TK, Wybar KC: Lyle and Jackson’s Practical Orthoptics in the Treatment of Squint [and Other Anomalies of Binocular Vision], ed 5. Springfield, IL, Charles C Thomas, 1967.) If the fovea’s position is nasal to the point at which the optical axis cuts the globe’s posterior pole, the corneal reflection of a light fixated by that eye will appear to lie on the temporal side of the pupillary center. In this case the term negative angle kappa is used. A negative angle kappa may simulate an esodeviation and again produce a pseudostrabismus, may make an existing esotropia look worse than it actually is, or may mask all or part of an exodeviation. Pseudoesotropia

FIGURE 12–3. Angle kappa. A, When the observer places his or her eye in line with the light located on the subject’s line of sight, the reflection of that light appears displaced nasalward on the cornea. B, When the examiner brings his or her eye and the light into line with the patient’s pupillary axis, the reflection of the light appears centered.

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FIGURE 12–5. The angle kappa. The angle is called positive when the light reflex is displaced nasalward and negative when it is displaced templeward. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 33.) secondary to nasal displacement of the fovea may be caused by high myopia. A negative angle kappa is less common than a positive angle kappa, but it is not correct to say that a negative angle kappa is always pathologic.28; 104, p.290 This statement, which has been repeated in many texts, especially orthoptic texts, gives the wrong impression. Fundus examination does not always reveal visible anomalies when a negative angle kappa is present. However, because of traction of the retina in cases

of retinopathy of prematurity, a true pathologic ectopia of the macula is accompanied by a positive angle kappa. The macula is pulled in the temporal direction, causing pseudoexotropia (Fig. 12–6). Other causes of ectopic macula include scarring from Toxocara canis retinitis or congenital retinal folds. The condition may be bilateral and may occur in siblings.43 A vertical angle kappa, simulating a hyperdeviation, is usually (but not always11) caused by superior or inferior displacement of the macula from a retinal scar. Measurement of Angle Kappa For clinical purposes it suffices to observe the position of the corneal light reflection while the patient fixates monocularly on a penlight. To avoid parallax, the examiner’s eye must be aligned with the fixation light. A more accurate determination of the angle formed between the visual and pupillary axes can be made by observing catoptric (Purkinje) images using, for example, Tscherning’s ophthalmophacometer,122 which consists of a telescope on a graduated arc provided with suitable lights. With the visual axis of the subject coincident with the axis of the telescope, the Purkinje images are displaced sideways or

FIGURE 12–6. Pseudostrabismus caused by ectopic macula. A, The patient appears to have a large right exotropia (XT). The Hirschberg test showed an XT of 20
to 25
. B, No shift of OD occurs when OS is covered. C, Fundus photographs reveal an ectopic macula. The tip of the fixation target (X) indicates the position of the fovea, which is displaced several disk diameters templeward. The retinal blood vessels are pulled over templeward. This patient had been born prematurely and for several weeks was kept in an incubator with high oxygen concentration. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 35.)

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vertically, depending on the orientation of the phacometer. Fixation on a small object moved along a graduated arc brings the Purkinje images into the center of the pupil, and the optical axis is now coincident with the telescope’s axis. The angle kappa is measured by determining the distance in degrees by which the fixation object has to be moved along the arc. This elegant and accurate instrument is no longer available and has been replaced by less precise but clinically useful methods. Shifting of fixation and therefore of the visual line is used to measure the angle with a major amblyoscope. A special slide, bearing a horizontal row of letters and numbers separated by intervals of 1° or 2°, is inserted into one arm of the major amblyoscope. The patient is asked to fixate the zero, and the position of the corneal light reflection is observed. The patient shifts fixation to each of the letters or numbers in turn until the light reflection is centered on the cornea, which gives the angle kappa in degrees. The angle is positive for the right eye if fixation has to be shifted to the numbers (left), negative if it has to be shifted to the letters (right), and vice versa for the left eye (Fig. 12–7). The angle kappa also may be measured clinically using a procedure similar to Tscherning’s laboratory method. A seated patient’s head is adjusted in front of a perimeter arc. One eye is occluded, and with the other eye the patient fixates the central fixation mark on the arc. The examiner closes one of the patient’s eyes and places a small penlight or ophthalmoscope light firmly under the lower lid of the open eye. The patient first places his or her head and the light in line with the visual axis and observes the reflection from the cornea. If this reflection is not centered, the patient’s head is moved with the light until centering is achieved. At this point the light coincides with the optical axis. If the patient has to move the light to the left for the right eye (temporally in relation to the patient) or to the right for the left eye, the angle kappa is positive. In contrast, when using the major

amblyoscope test, which is based on a different principle, the patient must turn the right eye to the left to compensate for a positive angle. Size of Angle Kappa Donders35 found a positive angle kappa that varied from 3.5° to 6.0° with an average of 5.082° in emmetropic eyes and from 6.0° to 9.0° with an average of 7.55° in hypermetropic eyes. In myopic eyes the angle kappa was generally smaller, averaging around 2.0°, and may even be negative.33 Donders’s findings that emmetropes and hypermetropes tend to have a larger angle kappa than myopes was confirmed in a more recent study by Giovianni and coworkers49 (Table 12–1). Although mean values reported by these authors are smaller than those of Donders, they are in line with an average angle kappa of 2.6° as measured by Franceschetti and Burian46 in a random population of 334 subjects. Clinical Significance of Angle Kappa Since it may simulate, conceal,* or exaggerate a deviation, the angle kappa must be considered to obtain the best estimate of the actual deviation in patients in whom this determination is made by the corneal reflection test. When the deviation has been so determined because of low visual acuity in one eye, an operation to improve the patient’s appearance is usually indicated. In such cases it is best to disregard the angle kappa and its measurements. Cosmetic operations are performed to make the eyes appear straight, not for _________________________________________ *Prof. Schweigger of Berlin is reported to have said of the angle kappa (Wiesinger126), ille mihi praeter omnes angulos ridet (this corner [angle] smiles at me beyond all others) (Horace, Odes II, vi, 13) because of its role in the improved appearance of some patients after not fully successful operations.

FIGURE 12–7. Amblyoscope slide for the measurement of angle kappa.

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TABLE 12–1. Distribution of the Angle Kappa in 483 Subjects with Emmetropia, Hypermetropia, and Myopia

aligning the visual axes properly to facilitate binocular vision. If one were to aim at aligning the visual axes in a patient with a large angle kappa to facilitate binocular vision, the relative postoperative position of the eyes might be cosmetic overcorrection or undercorrection. No mother would appreciate an exotropic appearance of her previously esotropic daughter, even if the ophthalmologist could assure her quite correctly that the visual axes are now parallel. Observation of Head Position Patients with comitant heterotropias, especially those with comitant horizontal heterotropias, usually carry their head in a normal position, but there are exceptions. In patients with nystagmus, the frequency and amplitude of the nystagmus may be reduced or there may be no nystagmus when the eyes are directed to one or the other side (see Chapter 23). In this position visual acuity is optimal. The patient keeps the head turned to one side (e.g., to the left) to bring the eyes into this optimal position (say, dextroversion) when looking straight-ahead. Patients who have a high amblyopia of one eye occasionally tend to turn their head in a direction away from the amblyopic eye, especially when reading or looking intently at an object. Patients with infantile esotropia, manifest-latent nystagmus, and strong fixation preference for one eye often have their face turn toward the side of the fixating eye (see Chapter 16). Abnormal head positions in connection with incomitant and paretic deviations are usually assumed in the interest of obtaining binocular cooperation or avoiding diplopia. Abnormal head positions take either the form of tipping the chin up or down, a head turn (i.e., a turn around a vertical axis), or a head tilt to one shoulder. For example, a patient with an A or V pattern of deviation (see Chapter 19) may tend to carry the head with the chin depressed or elevated.

On the other hand, a patient with a right lateral rectus paresis may turn the head to the right, causing levoversion to bring the eyes into a position in which the right lateral rectus muscle receives no impulses to contract. With these positions of head and eyes, patients avoid diplopia and gain binocularity. Bielschowsky wrote that "the patient chooses the least inconvenient position of the head by which the paretic muscle is sufficiently relieved so that binocular single vision can be obtained."13, p.99 In many instances a patient will turn or tilt the head in the direction of the field of action of the paretic muscle. However, if fusion cannot be attained with a compensatory head posture, the head may be positioned to produce maximal separation of the double images. These and other aspects of compensatory head posture are discussed further in Chapter 20. While an anomalous head posture should alert the examiner to search for nystagmus; a paralytic horizontal, vertical, or cyclovertical strabismus; cyclotropia; or an A or V pattern, normalcy of the head position does not rule out any of these conditions. Moreover, an ophthalmologist must be aware that there are ocular causes unrelated to strabismus for an anomalous head posture, such as an uncorrected refractive error or anomalous retinal correspondence with a vertical angle of anomaly.24 Nonocular causes include fibrosis of the sternocleidomastoid muscle, unilateral hearing loss, or psychogenic torticollis. Several instruments have been developed to quantify the inclination of the head in degrees.104, 114, 115, 128 Among these a cervical range of motion (CROM) device used in physical medicine27 and in assessing craniomandibular disorders15 also measures the degree of head turn and chin elevation and depression. Such instruments are useful in documenting and quantitating the effect of various surgical procedures on an abnormal head position. Kushner70 has modified the CROM

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to assess limitations of ductions and the range of single binocular vision at near and distance fixation.* Determination of Presence of a Deviation—Cover and Cover-Uncover Tests Inspection alone clearly is not always sufficient to determine a manifest misalignment of the visual axes. An epicanthus, facial asymmetry, or a wide angle kappa may simulate or conceal a deviation. The simple cover and cover-uncover tests establish whether orthotropia or an ocular deviation is present, whether a deviation is latent or manifest, the direction of a deviation, the fixation behavior, and even whether visual acuity is significantly decreased in one eye. A cover is placed briefly before the eye that appears to fixate while the patient looks at a small object, a figure pasted on a tongue depressor, or a 6/9 visual acuity symbol. The test should always be done for distance and near fixation to establish any differences between the two conditions. As a cover, one may use the palm of the hand or some form of occluder or paddle. Covering one eye of a patient with normal binocular vision interrupts fusion. If the patient has a heterotropia and the fixating eye is covered, the opposite eye, provided it is able to do so, will make a movement from the heterotropic position to take up fixation, and the covered eye will make a corresponding movement in accordance with Hering’s law. An exotropia is present when the eye taking up fixation moves toward the nose, an esotropia when it moves toward the temple, and so forth. If there is no movement of the fellow eye, that eye is then covered and the other eye is observed (Fig. 12–8). When it has been established that no manifest strabismus is present (no movement of the fellow eye when either eye is covered), a cover-uncover test will determine whether the patient has a latent deviation (Fig. 12–9). Again, one and then the _________________________________________ *Performance Attainment Associates, 3550 LaBore Rd., Ste 8, St. Paul, MN 55110-5126; or Binoculus 740 Piney Acres Circle, PO Box 3727, Dillon, CO 80435-8727. Phone or fax USA 970-262-0753, email: perxbvq@colorado.net, website: BinocularVision.com

FIGURE 12–8. The cover test. A, Position of patient’s eyes before the test. B, Cover placed over OS from the left does not elicit a fixation movement of OD: no deviation of OD is present. C, Cover placed over OD from the right does not elicit a fixation movement of OS: no deviation of OS is present. D, OD moves outward to fixate when OS is covered: esotropia. E, OD moves inward to fixate when OS is covered: exotropia. F, OD moves downward when OS is covered: right hypertropia. G, OD moves upward when OS is covered: right hypotropia. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 39.) other eye is covered while the patient maintains fixation. However, the examiner now directs attention to the covered eye just as the cover is removed. If the patient has a heterophoria, the covered eye will deviate in the direction of the heterophoric position. When the eye is uncovered, it will move in the opposite direction to reestablish binocular fixation, that is, toward the nose in exophoria, downward in hypertropia, and so forth. Once the diagnosis of manifest strabismus has been made, it is possible to establish the degree

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FIGURE 12–9. The cover-uncover test. A, Cover has been removed from OD, and no movement of OD can be detected: no latent deviation of OD. B, Cover has been removed from OS, and no movement of OS can be detected: no latent deviation of OS. If conditions A and B are present, the patient has no phorias that can be detected with this test. C, When uncovered, OS moves outward to fixate: esophoria. D, When uncovered, OS moves inward to fixate: exophoria. E, When uncovered, OS moves down to fixate: left hyperphoria. F, When uncovered, OS moves up to fixate: left hypophoria. The cover-uncover test must be performed on both eyes. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 43.)

of alternation with the cover-uncover test. The fixation behavior may vary from extreme monocularity, as in patients with deep amblyopia or strong ocular dominance, to free random alternation. In the case of strong dominance the just-uncovered eye will immediately resume fixation as the fellow eye returns to its deviated position. In the case of free alternation the formerly deviated eye will continue to fixate after removal of the cover. If the usually deviated eye continues fixation for some time, for instance, until the lids close during a blink, weak but definitive alternation is present. The possible results of the cover and cover uncover tests may be summarized as follows: 1. On covering the seemingly fixating eye:    a. No movement of the other eye: there was binocular fixation before applying the cover.    b. Movement of redress of the other eye: a manifest deviation was present before applying the cover.

2. On uncovering the eye:    a. Movement of redress of the uncovered eye (fusional movement); no movement of the other eye: heterophoria is present.    b. No movement of either eye; uncovered eye deviated; opposite eye continues to fixate: an alternating heterotropia is present.    c. Uncovered eye makes movement of redress and assumes fixation; opposite eye deviates; preference for fixation with one eye: a unilateral heterotropia is present. The cover test also allows one to establish by observation whether a gross eccentric fixation (see Chapter 14) is present in a patient with heterotropia. When the fixating eye is covered in such usually esotropic patients, the deviated eye will make no movement of redress or only a small, incomplete one. Infants often object to having their heads or faces touched. In such instances the examiner may place a paddle at some distance in the path of one

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eye while holding a light or some other fixation object with the other hand. This test, which has been termed the indirect cover test, does not permit full interruption of fusion but is useful in infants and small children with heterotropia (Fig. 12–10). When the cover test is applied to the fixating eye in a strabismic infant and the patient consistently pushes the cover aside or performs searching, nystagmoid movements with the fellow eye, chances are high that visual acuity of that eye is low and amblyopia must be suspected. This application of the cover test is of inestimable value in the diagnosis of amblyopia in infants (see Chapter 14). A pseudoptosis (see Fig. 12–1), if present, will disappear when the fellow eye is covered. The cover test may not reveal ultrasmall deviations as seen in microtropia (see Chapter 16), but in most patients this limitation does not reduce the value of this simple test. A clinically useful modification of the cover test was introduced by Spielmann112 after having been mentioned briefly by Javal.66 Instead of an opaque cover a translucent occluder is used through which the examiner can observe or even photograph the covered eye, but through which the patient sees only diffuse light without contours. By using the Spielmann occluder the diagnosis of heterophoria is simplified because the deviation of the covered eye can be directly observed by the examiner without having to remove the cover (see Fig. 8–1). Covering both eyes with translucent occluders permits a quick preliminary determination of

whether an esotropia is of refractive-accommodative or nonaccommodative origin (Fig. 12–11). In the first case the eyes will straighten after covering both eyes; in the second the esotropia will persist. Further applications of the cover test with translucent occluders are discussed in Chapters 18 and 23. Measurement of Deviation Tests used to diagnose strabismus usually are classified as objective and subjective. Objective tests as performed in clinical practice, and even certain laboratory tests, reduce cooperation of the patient to the ability to hold steady fixation. The ophthalmologist performs certain manipulations, makes observations, and draws conclusions from these observations. When using subjective tests the ophthalmologist also performs certain manipulations, but the patient’s response determines the results; that is, the patient must make observations and report them. The opinion is widespread that objective tests are more reliable and therefore preferable to subjective tests.17 Objective is equated with "good," subjective with "bad." This is erroneous. In common usage objectivity in making a judgment has come to mean that the judgment is not tainted by one’s prejudices and feelings. However, in subjective tests for measuring the state of the sensory and motor visual system, a patient’s feelings, prejudices, and sense of value are no more suspect than the feelings, prejudices, or value judgments of the examiner. Also, the

FIGURE 12–10. The indirect cover test. The occluder is placed between the patient’s eye and the fixation object. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 41.)

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FIGURE 12–11. A patient with refractive-accommodative esotropia may have A, manifest esotropia without glasses; B, orthotropia when the stimulus to accommodate excessively is suspended with translucent occluders of Spielmann112; and C, orthotropia with glasses.

premise that the patient is a less keen or more biased observer than the ophthalmologist is by no means necessarily correct. The truth is that most subjective tests are inherently more precise than objective tests. The enormous precision of the great body of psychophysical investigations of the visual system, which are all based on subjective testing, is the best evidence for the value of these tests. Many diagnostic and therapeutic decisions in ophthalmology are based on an assessment of visual acuity, which depends entirely on a patient’s response. Is this response less reliable or trustworthy than, say, the patient’s localization of double images on any of the diplopia tests? The sensory system can be evaluated clinically only by subjective responses of the patient, although there are laboratory methods that permit the sensory system to be assessed objectively. When using subjective tests, one expects a patient to be able and willing to cooperate. Verbal responses to a change in a stimulus situation cannot be expected of an infant or someone who is severely mentally defective nor can objective tests requiring attentive fixation be performed on such patients. All tests have their limitations; not all tests are suitable for every age level and every person. This does not make one test intrinsically better or worse than another. The art of the ophthalmologist consists of judiciously applying tests in each case that provide the maximum amount of correct information needed for appropriate treatment. Prism and Cover Test The prism and cover test, or the alternate cover test, is deservedly popular. It is also known as the

screen cover test, but this term is misleading and should be avoided.72 To perform this test, a cover is placed alternately in front of each eye while the patient maintains fixation. The eye that is uncovered makes a movement of redress in the direction opposite that of the deviation. The amount of the deviation is grossly estimated, and a prism of a strength less than the estimated deviation is placed in the appropriate direction in front of one eye. To measure esotropia, the prism must be placed base-out, for an exotropia base-in, for a right hypertropia basedown in front of the right eye or base-up in front of the left eye, and for a left hypertropia basedown in front of the left eye or base-up in front of the right eye. This manipulation reduces the movement of redress, and the prism strength is increased until the movement is offset (Fig. 12–12). Combinations of horizontal and vertical deviations are frequent. In such patients it is best to first neutralize the horizontal deviation with prisms and then to add prisms to stop the vertical deviation. At that point, it may be necessary to further correct the horizontal deviation. The amount of prism strength required to offset all movements of redress is a measure of the deviation. Cyclodeviations cannot be measured in this fashion, and must be determined either subjectively or with the major amblyoscope. Physiologic Basis Redress in the prism and cover test is a psychooptical reflex movement that occurs when the eye fixates. The sensory origin of this reflex movement stems from stimulation of a peripheral retinal area in the deviated eye by the

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FIGURE 12–12. The prism and cover test. A, Right esotropia. B, When OS is covered, OD moves outward to take over fixation. C, When the cover is transferred to OD, OS moves out to take over fixation. D, A prism, base-out, is held before OD; the cover is transferred to OS. There is still outward movement of OD when taking over fixation, although the amplitude of this movement is decreased by the effect of the prism (compare with B). E, The cover is again transferred, and a prism of greater power is held before OD. F, Transfer of cover to OS does not elicit fixation movement of OD. The deviation is offset by the prism, and the power of this prism equals the deviation. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 49.) fixation object. Fixation causes the eye to turn in such a way that the fixated object is imaged on the fovea. The movement is quantitative and is directly proportional to the distance of the fovea from the stimulated peripheral area. Placing prisms of increasing power in front of the eyes brings the image of the fixated object closer and closer to the fovea, causing a corresponding decrease in the movement of redress. When the prism strength equals the amount of deviation, the image falls on the fovea. There is no longer an incentive to move the eye, and the movement of redress ceases (Fig. 12–13).

Performance As is true of most tests to diagnose strabismus, the prism and cover test is technically very simple, yet findings can be misleading unless the test is understood and performed correctly. To properly perform this test, use adequate fixation objects and a technique that will ensure maximum dissociation of the eyes. A penlight should never be used as a fixation object. For distance fixation a 6/9 visual acuity symbol is recommended or, in the case of a preliterate patient, electrically operated, moving mechanical toys or projected moving cartoons. For near fixation, a similar visual acuity symbol or some small picture or object can be used. Small cutout figures pasted on the end of a tongue depressor are convenient for testing children, who are asked to identify the object (Fig. 12–14). To maintain the child’s interest, paste pictures on the ends of both sides of the tongue depressor and change the object if the child’s attention wanes. The examiner should place him- or herself at the desired distance, 33 cm, and then ask the child to hold the tongue depressor against the end of the examiner’s nose. This serves two purposes: to keep the examiner’s hands free and to help maintain the child’s interest in fixating. Instead of using a tongue depressor, the examiner may clip a small card to the bridge of his or her glasses (Fig. 12–15). The reason for using fixation objects rather than a simple penlight is to control accommodation during measurement of the deviation at near and distance fixation. One must understand that a patient’s response depends on the stimulus presented, not only during subjective tests, where it is more obvious, but also during objective tests. Another important consideration is the manner in which the test is performed. Maximal dissociation of the eyes must be achieved to make the correct diagnosis, especially in patients with heterophoria. Such patients have a strong compensatory innervation that keeps their eyes aligned and it is not immediately suspended when one eye is covered. It is necessary to dissociate the eyes for some time to bring out the full amount of the deviation. The test must not be performed hurriedly, and the cover should be placed alternately over each eye a few times.

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FIGURE 12–13. Optical principles of prism and cover test. A, Image of object fixated by OD is projected on the nasal half of the retina of OS. B, When OD is covered, OS moves outward to take over fixation. Under the cover, OD performs an inward movement of equal amplitude, following Hering’s law of equal innervation. C, When a prism of sufficient power offsets the nasal displacement of the image, OS will no longer change its position when OD is covered (compare with Fig. 12–12, F). (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 51.)

FIGURE 12–15. Photographically reduced Snellen letters mounted on the frame of the examiner’s glasses.

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Most important, the patient should never be permitted to regain fusion while the cover is being transferred. In patients with an exodeviation, one eye may have to be occluded for 30 to 45 minutes. The difference in measurements before and after occlusion may be significant (see Chapter 17). In contrast to this brief occlusion is the prolonged occlusion test developed by Marlow80 in an attempt to uncover the full amount of heterophoria, particularly a hyperphoria. Marlow occluded the nondominant eye for 14 days and no less than 7 days to accomplish thorough dissociation of the eyes and was convinced of the effectiveness of this procedure in uncovering clinically significant amounts of heterophoria. Prominent ophthalmologists99 of Marlow’s time supported his observations which have been reconfirmed in a recent study, using more critical methods of investigation.87 However, the clinical significance of these findings remains questionable. Duane and Berens37 stated that unilateral occlusion produces an artificial hyperphoria, an opinion shared by Lancaster.73 It has yet to be shown that small degrees of horizontal and vertical heterophoria unveiled only by prolonged unilateral occlusion have any impact on the patient’s ability to fuse without symptoms of asthenopia. Today, the test of Marlow has probably only historical interest and should not be confused, as it is frequently in the German strabismus literature, with the occlusion test introduced by Scobee and Burian for the diagnosis of a pseudodivergence excess type of exodeviation (see Chapter 17). Another way to ensure full dissociation and obtain the full amount of the deviation is to add prism power not only until redress is stopped but also until a reversal of the direction of movement is noted. In so doing, one frequently finds that the endpoint is higher than originally thought. This technique is recommended for routine use. To gain insight into a patient’s deviation, perform the prism and cover test for distance and near fixation with the patient first wearing refractive correction and then with the correction removed. Comparison of these four figures allows one to draw conclusions about the part played by accommodation in the patient’s deviation. The deviation measured in distance fixation while the patient is wearing full correction excludes accommodation. The fusion factor must be excluded as far as possible by a properly performed prism and cover test. With the influence

of accommodation and fusion controlled, one obtains the static or basic deviation or static (basic) angle of squint. If correction of the refractive error is inadequate, accommodation is uncontrolled and one then obtains the dynamic deviation or dynamic angle of squint. Likewise, in the case of insufficient dissociation of the eyes, persistent strong compensatory fusional innervation during the prism and cover test will cause dynamic factors to override and obscure the static deviation. Precise definition of these terms is important to avoid misunderstanding. This has not always been the case in the European strabismologic literature in which different meanings have been given to classic terms in discussions of the nystagmus blockage syndrome.1, 31, 57, 86 For example, dynamic angle has been used synonymously with variable angle and it has been said that "the smallest observed angle is always the static angle." We, on the other hand, define a variable angle of strabismus as a deviation that increases or decreases significantly while the patient is being examined or, when measured on different occasions, while testing conditions remain equal and accommodation and fusion (dynamic factors) are fully controlled. A good example of a variable angle is that which occurs in a patient with an acute nystagmus blockage syndrome who has a variable angle of esotropia of a size that is inversely related to the nystagmus intensity (Chapter 23). It is also not true that the static angle is always smaller than the dynamic angle. For instance, fusional convergence may cause a larger static deviation to decrease at near fixation in patients with a simulated divergence excess type of exotropia (Chapter 22), or a patient with intermittent exotropia may use accommodative convergence to control the deviation at distance fixation (smaller dynamic angle). Controlling the accommodative state by asking this patient to read the 6/6 line on the visual acuity chart at 6 m distance will unmask the larger static deviation. Also, if the angle of a comitant horizontal strabismus is greater in primary position than in extreme lateral gaze, dynamic factors should not be blamed for this difference. We believe this phenomenon can be explained on simple mechanical grounds: the excursion of each eye in lateral positions of gaze is checked by orbital structures.32 Thus a fully adducted eye can no longer exhibit excessive adduction in esotropes, and excessive abduction in exotropic patients can no longer be effective once an eye is fully abducted. Consequently, depending on individual variations of the effectiveness of

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these "brakes," the angle of strabismus in lateral gaze may be smaller than in primary position. This rather lengthy explanation is necessary to define clearly the terms dynamic, static, and variable angle as used throughout this text. Another useful modification of the prism cover test is to repeat it while the patient is fixating at 33 cm distance and looking through 3.00D lenses, clipped over the distance correction or, in the case of emmetropia, placed in the trial frame. With the accommodative demand for near fixation thus eliminated, the deviation at near should now approximately equal that at distance. Single clip-on lens holders are commercially available (Fig. 12–16). It is recommended that the prism and cover test also be performed with either eye fixating. To do this, one eye is made to fixate while the other is alternately covered and uncovered and a prism is placed before the covered eye to stop movement of that eye. The process is then repeated with the other eye fixating. Differences in the amount of prism power required for each eye indicated the presence of a primary and secondary deviation (see Chapter 20) or an incomitance, for example, induced by an operation. The prisms to be used in the test may be loose, in sets, or so-called prism bars or ladders, consisting of a row of prisms of increasing power. When using prism bars, two are necessary: one to measure the horizontal deviation and one to measure sure the vertical deviation. One also may use the FIGURE 12–16. Halberg clip-on lens holder.

horizontal bars with loose prisms held vertically. To establish the basic deviation, the eyes must be tested in the clinical primary position for both distance and near fixation with and without additional plus lenses. Measuring the near deviation with the eyes in the reading position, as Scobee106, p. 300 suggested, is not desirable, because the possible presence of a V pattern of fixation may simulate an accommodative factor when none exists (see Chapter 19). Limitations The prism and cover test presupposes accurate fixation and cannot be performed if the deviating eye is blind or has grossly eccentric fixation. In eccentric fixation, the test provides wrong measurements, as the movement of redress of the deviated eye stops when the stimulus falls on the eccentric retinal area used for fixation and not when it reaches the fovea. Test accuracy is also limited by the optical qualities of the prisms.1, 89, 97 The stronger the prisms, the greater the errors; but from a practical standpoint, it is of little importance whether a deviation measures 75, 80, or 85. When large deviations are present, an error as high as 10 is of no consequence for decisions on the treatment of the patient. When using loose prisms one should remember that significant errors are produced when a lowpower prism is added to a high-power prism. According to Thompson and Guyton118 the effect produced by adding a 5 glass prism to a 40 glass prism is not 45 but 59. This error can be minimized by holding one prism before each eye. These authors also point out that the amount of deviation neutralized by an ophthalmic prism is variable depending on how the prism is held. For instance, a 40 glass prism with a posterior face held in the frontal plane gives only 32 of effect. Glass prisms are calibrated for use in the Prentice position; that is, the posterior face of the prism is perpendicular to the line of sight of the deviating eye. Plastic prisms, on the other hand, are calibrated for use in the frontal plane position, that is, parallel to the infraorbital rim. When measuring large angle horizontal deviation with a prism bar one must be aware of the fact that even slight oblique shifts of the bar can induce a vertical displacement of the image, mimic a vertical deviation, and cause vertical diplopia.

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Thompson117 (see also Scattergood and coworkers103) also drew attention to the artifacts introduced by spectacles lenses in measurement of strabismic deviations. Plus lenses decrease and minus lenses increase the measured deviation. This effect becomes clinically significant with corrective lenses with powers of more than 5D (see also Adelstein and Cüppers1). Test accuracy is limited further by the minimum movement of redress that the examiner can detect with the naked eye. Some experienced ophthalmologists have claimed they can detect shifts as small as 1.0 or even 0.5. Ludvigh77 showed that with cooperative patients, experienced observers, and optimal conditions of illumination, 2 would seem to be the limiting amount of observers, and optimal conditions of illumination, 2 ers the much less ideal conditions under which the test is generally performed, it is probably safer to set the limit at  3  to 4. Much the same results were obtained by Romano and von Noorden.98 In some instances, on assuming fixation, either eye overshoots the mark and returns to fixate the object of attention with a secondary corrective eye movement. For example, in esotropia the eye makes a larger outward movement than one corresponding to the angle of squint and then must turn inward to assume fixation. In these cases the endpoint of the test is not exact, but an approximate measurement can usually be obtained. According to Mehdorn and Kommerell,81 this "rebound saccade" may be caused by failure of suppression to be released instantly on covering the fixating eye. Certainly one can reasonably assume that such a mechanism would decrease precision of a preprogrammed corrective eye movement. It is evident also that if nystagmus is present, an accurate determination of the deviation by the prism and cover test may be difficult if not impossible. The nystagmus need not be manifest but may occur only when one eye is covered. This is the so-called latent nystagmus, a rather rapid, jerky form with its quick phase toward the uncovered eye. More about this interesting form of nystagmus is found in Chapter 23. Prism and Cover Test in Diagnostic Positions of Gaze The prism and cover test is useful in determining incomitance in otherwise comitant deviations, confirming by measurement the degree of a paresis, and establishing what muscle or muscles

are involved in a paralytic condition. To this end, the deviation should be measured in the nine diagnostic positions of gaze. The first is the primary position; next come the secondary positions of right, left, up, and down; and last the tertiary positions of up and right, up and left, down and right, and down and left. In the past, the eight secondary and tertiary positions have been called cardinal positions and are so designated, even in some recent textbooks. In discussing the physiology of ocular motility, only the secondary positions, reached by the cardinal movements, are so termed. To prevent confusion it is better to use the term diagnostic positions, since tertiary positions are included. The term has the added advantage of reminding the ophthalmologist that these positions are primarily for establishing certain points in the diagnosis. The practical field of fixation in the unrestricted casual use of the eyes is rather narrow (p. 79). Results of tests made 25° or 30° from the primary position in any direction may not be meaningful for the patient’s use of the eyes, but they are meaningful in making diagnostic points. Deviations present only in extreme positions may not require surgical treatment. To determine whether a deviation is paretic or paralytic, the relative position of the eyes in diagnostic positions should be measured with either eye fixating, as has been described for the primary position. Various techniques have been proposed to diagnose paretic involvement of a vertical muscle,58, 60, 96, 124 all of which include the head tilt test as a final step. The head tilt test, the obliquity of horizontal or vertical double images (see Chapter 15), and similar tests actually are confirmatory rather than primary tests. They are not essential to the diagnosis, although they can be helpful in complex cases such as congenital paralyses with greater or lesser spread of comitance, marked overaction of an antagonistic muscle, and preexisting horizontal or vertical heterophoria or heterotropia. For relatively recent and simple pareses or paralyses of vertically acting muscles, however, answers to the following three questions will definitely establish the diagnosis: (1) Is there a right or left hyperdeviation in primary position? (2) Does the hyperdeviation increase in elevation or depression? (3) Does the hyperdeviation increase in dextroversion or levoversion? These questions are answered by examining the versions and by comparing the position of the eyes in the various directions of

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gaze or, better yet, by measuring them with the prism and cover test. The answer to the first question is obtained from the cover test; the answer to the second informs one whether a right or left elevator or depressor muscle is at fault; and the answer to the third determines whether the vertical rectus or the oblique muscle is involved. Thus the diagnosis is established. Further details about examination of patients with cyclovertical deviations may be found later in this chapter and in Chapters 18 and 20. In practical performance of the prism and cover test in diagnostic positions of gaze, varying procedures are followed. Accommodative targets should be used as fixation objects to minimize variability of the deviation resulting from variations in accommodation. Such a target may be handed to the patient to hold and the hand placed successively in the desired positions, leaving the examiner’s hands free to perform the test (Fig. 12–17). For the experienced ophthalmologist this method may be fully satisfactory, but it does not permit accurate repetition of the test. So-called deviometers therefore have been designed that permit all patients being tested to bring the eyes as closely as possible into the same positions. A perimeter arc on which an accommodative target could be placed for measurement of the deviation in fixation above and below the horizontal plane was described by von Noorden and Olson.88 Such a perimeter arc can be placed in the horizontal and tertiary positions and makes a good deviometer. In the Motility Clinic of the Department of Ophthalmology at Baylor College of Medicine, Houston, Texas, a deviometer built from scrap

metal is used in which retroilluminated slides stimulate accommodation. Each slide is positioned 35° from the primary position119 (Fig. 12–18). One great advantage of using a deviometer is that the prism and cover test can be performed in diagnostic positions under exactly the same conditions on different occasions and thus permit meaningful comparison of test results (e.g., preoperative and postoperative). Measurement with the Major Amblyoscope The angle of deviation can be measured also by using a major amblyoscope. These devices, patterned after the Hering haploscope (see p. 72), are basic orthoptic instruments. They are especially useful for studying the sensory state of the patient and in nonsurgical treatment. The essential parts of major amblyoscopes (Fig. 12–19) are a chinrest, a foreheadrest, and two tubes carrying targets seen through an angled eyepiece, one for each eye. The tubes are placed horizontally and supported by a column around which they are movable in the horizontal plane. A mirror, one in each tube, reflects the image of the target through the eyepiece into the corresponding eye. The distance between the tubes can be adjusted so that the centers of the eyepieces correspond accurately to the patient’s interpupillary distance. When this is done and if the head and chin are properly adjusted, the axes around which the tubes turn should be in line with the center of rotation of the eyes. In addition to adjustments for horizontal positions of the arms, there are controls

FIGURE 12–17. Example of prism and cover measurement outside primary position. Patient is looking up and to the left.

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FIGURE 12–18. The Boyse-Smith deviometer. (From Toosi SH, Noorden GK von: Effect of isolated inferior oblique muscle myotomy in the management of superior oblique muscle palsy. Am J Ophthalmol 88:602, 1979.)

that allow a vertical separation of the targets, as well as cyclorotational adjustment. The amount of all these displacements can be read from scales, which are usually graduated both in arc degrees and prism diopters. The tubes may be locked and moved together horizontally and in some modern models also vertically. The illumination system for each target can be controlled individually to increase or decrease the stimulus luminance to one eye. Keys are provided to manually flash the light, illuminating either target. The flashing also FIGURE 12–19. A major amblyoscope. (Courtesy of Clement & Clark.)

can be controlled automatically in certain models, with a wide range of light-dark intervals. This basic instrument may be equipped with a greater or lesser number of refinements. Some models are designed to produce afterimages or Haidinger’s brushes. The main difference between laboratory haploscopes and major amblyoscopes is the way in which accommodation is controlled. The target carrier in the haploscope can be moved along the arm, which is graduated in diopters. The position of the targets in the major amblyoscope is fixed in the focal plane of a 6.0D or 6.5D lens so that they are at optical infinity, which should prevent accommodation from affecting the deviation. However, the fact that targets are actually a short distance from the eyes causes proximal convergence to enter into play. Consequently, the deviations measured with the major amblyoscope in distance setting are frequently larger than those obtained with the prism and cover test in distance fixation.8, 46, 117 One major British amblyoscope (the Curpax Major Synoptiscope No. 10) uses semitransparent mirrors in lieu of opaque mirrors in front of the eyes, a feature already used in the haploscope of Ames and Gliddon,2 which allows the patient to view a distant object on which the target images in the slide holders on each arm are

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superimposed while deviation is being measured. In this way the influence of proximal convergence is avoided.78, p. 105 To induce accommodation, auxiliary minus lenses are placed in front of the eyepieces. For measurements at near fixation (e.g., of the vergences), minus lenses are used in the amount required to offset the plus lenses in the eyepieces and the arms are set at 18D (for the convergence requirement of a 6.0 cm interpupillary distance), which thus becomes the zero setting for a 33 cm viewing distance. The deviation is measured by moving the arms of the major amblyoscope into such a position that images of the target fall on the respective foveal areas. This is done by moving the arms until there is no further refixation movement of the eyes in an alternate cover test (either by actual covering or by alternately extinguishing the light on one side of the instrument). Vertical displacements of the target carriers measure vertical components of the deviation. Finally, it is also possible to rotate the targets around an anteroposterior axis and thus evaluate and measure cyclodeviations. Conventional amblyoscopes do not permit determination of the angle of strabismus in peripheral positions of gaze whereby the eyes are dissociated by the patient’s nose or orbital margins. Yet such measurements are important, especially in those patients with paretic or paralytic strabismus. These difficulties have been overcome by the Synoptometer (Oculus) of Cüppers, which is a modified amblyoscope that permits measurement of deviations by means of mirrors in peripheral positions of gaze of up to 50° in dextroversion and levoversion, 50° in elevation, and 60° in depression.31, 85 To avoid the distraction of infants and children that is caused by instrumentation or by prisms and occluders switched directly before the eyes, Guyton developed an ingeniously designed remote haploscope to be used in combination with an infrared television-based eye tracker.54, 56 The efficiency of measurement of an ocular deviation with this apparatus in terms of speed and repeatability is superior to conventional methods. Campos and colleagues25 developed a similar system in association with a computerized deviometer which allows one to follow automatically step-bystep the various diagnostic procedures in comitant and paralytic strabismus.

There are many potential research applications for such systems, but it is questionable whether this technological extravagance will eventually replace the older tests in a clinical environment. Corneal Reflection Tests If the deviated eye is blind or has low visual acuity or, in young children, is unable to maintain fixation for longer than a moment, the amount of the deviation cannot be determined by the prism and cover test or by any subjective tests. One must then resort to estimation of the deviation by observing the first Purkinje image using the so called corneal reflection test. The corneal reflection is on the nasal side of the deviated eye in exotropia, on the temporal side in esotropia, below the corneal center in hypertropia, and above it in hypotropia. Hirschberg63 first suggested the use of corneal reflection for measuring ocular deviations, and his test is still widely used. Based on a simple calculation, Hirschberg found that each 1 mm of decentration of the corneal reflection corresponded to 7° of deviation of the visual axis (Fig. 12–20). His assistant, du Bois-Reymond,38 determined with a modified arc perimeter that if the corneal reflection in the deviated eye is found to be in the pupil, the deviation ranges from 0° to 20°, given a pupillary diameter of 3.5 mm. If it is on the iris between the pupillary margin and the limbus, an angle of 20° to 45° may be present. If the reflection appears on the conjunctiva, a deviation of 45° or more exists.38 Brodie16 reexamined the conversion factor FIGURE 12–20. The Hirschberg test. For explanation, see text. ET, esotropia. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 45.)

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given by Hirschberg over 100 years ago and photographed the corneal reflection of normal subjects who were fixating visual targets separated by 10 over a range of 200. A value of 12° per millimeter displacement of the reflection was determined in Brodie’s study, which is similar to numbers reported by subsequent investigators.34, 44, 93, 102 The discrepancy between the traditional (7°/mm) and this new factor may be caused by the fact that a photograph records the true reflex displacement in the frontal plane, which differs from a measurement of the reflex displacement from the corneal apex along the corneal surface.16 The rather wide range of measurements reported by different authors (10.7° to 15.6°) could be explained by the fact that the reference landmarks (corneal center, pupillary center, limbus) and the calibration method were not the same in all studies. Paliaga and coworkers92–94 have revived the method of objective strabismometry, which was quite popular about a century ago. With a millimeter ruler, they measured displacement of the corneal light reflection that occurs in the deviated eye as this eye assumes fixation. The linear displacement of the reflection is then converted into angular values (see also p. 200). Another method is based on the well-established principle of Hering’s law of equal innervation to the two eyes. Corneal reflection is produced in the two eyes by an appropriately placed penlight, which is fixated by the patient’s better eye. The examiner places him- or herself on the side of the deviated eye to avoid parallax errors in observation (Fig. 12–21). Prisms are then placed in front of the fixating eye to center the corneal reflection in the deviated eye. The amount of prism power necessary to achieve this is a measure of the deviation. This test, first described by Krimsky,69 who suggested the name "prism reflex test," is a practical method of estimating the size of the angle of squint in patients with a blind or deeply amblyopic eye with or without eccentric fixation. It is important for the examiner to be seated directly in front of the deviating eye to avoid false readings caused by parallax. In another version of this test, prisms are placed in front of the deviating eye until the corneal reflection is centered. However, observation of the corneal reflection through prisms is difficult, which is why we prefer the procedure outlined.

FIGURE 12–21. The Krimsky test. A–C, Prisms, baseout, of increasing power are placed before the fixating eye until the light reflex is centered on the cornea of the deviating eye. D, Optical principles of the prism reflex test. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 47.) Strabismometric methods based on the corneal light reflection remain rather crude and are not as precise as the prism and cover test because the deviation at distance fixation is difficult to measure with this method, accommodation cannot be controlled while maintaining fixation on a penlight, the angle kappa is included in the measurement, and, as shown by Choi and Kushner,29 interpretation of the position of the light reflection on the cornea differs widely even among experienced observers. Be this as it may, the Hirschberg and Krimsky tests are valuable methods to obtain an approximate measurement of the angle of strabismus in patients too young to cooperate with the prism and cover test or when poor visual acuity in one or both eyes precludes adequate fixation. Photographic Methods Barry and coworkers9 developed a method to measure the angle of strabismus in infants and

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children photographically (see also Lang75). The apparatus consists of a camera with three horizontally aligned flashes and a small fixation light. The angle of strabismus is derived from the reflection pattern in the pupil of the first and fourth Purkinje images from each light source. The accuracy is said to be between 2.0 and 4.5, which would make it superior to the Hirschberg test. Friedman and Preston47 use a two-flash Polaroid camera to screen for amblyopiogenic factors, such as strabismus, media opacities, and refractive errors. Photographic methods for visual screening of children lack accuracy because accommodation is not controlled by an appropriate fixation target. Evaluations of the efficacy of this method to detect amblyopiogenic factors have thus far yielded controversial results. One study concluded that the photoscreener holds promise as a useful mass screening tool,91 but others have shown that the photographs may be noninterpretable or that amblyopiogenic factors were missed in 20% of children evaluated with this method.105, 110 Photoscreening is probably more accurate than screening for these factors by the pediatrician,120, 125 but cannot and should not take the place of a complete ophthalmologic examination. Brückner Test Brückner18 introduced a test to diagnose strabismus in infants that is based on judgment of the position of the corneal light reflex and the color of the light reflected from the fundus. A bright coaxial light source emitted by a direct ophthalmoscope illuminates both eyes of the patient simultaneously from a distance of 1 m in a semidarkened room. The position of the corneal reflection and differences in the brightness of the fundus reflex between the two eyes are noted by the observer through the ophthalmoscope. In the presence of strabismus the reflex of the fixating eye is darker than in the deviated eye, a phenomenon that had been previously noted by Worth.127 It has been estimated that this technique can be automated to detect the presence of 2° to 3° of ocular misalignment based on the difference in brightness of the bright pupil images between the two eyes.84 In a second step one eye is illuminated at a time, and the pupil size, its reaction to light, and fixation movements are noted to detect amblyopia. Whether this test is a reliable screening method for strabismus is another matter since it has been reported that asymmetrical fundus reflexes occurring in infants up to 10 months of age may represent a normal stage of

development4 and that the tests yield false positives in nonstrabismic subjects.52 Subjective Tests Subjective tests for estimating the deviation of the visual axes have a long and honorable history. All the great names, and many of the near-great ones, in Germany, England, and the United States have made their contribution to this chapter of the investigation of neuromuscular anomalies of the eyes. The story has been fascinatingly retold by Sloane,111 but in this book the discussion must be restricted to tests in current use. If the two visual axes are not properly aligned, the patient should have diplopia. The diplopia is either spontaneous, as in recent extraocular muscle paralyses or acute comitant strabismus, or it must be elicited artificially if suppression or anomalous correspondence (see Chapter 13) is operative in casual seeing, as is the rule in comitant squint. If correspondence is normal, the distance of the double images may be used as a measure of deviation. All subjective tests for measurement of the deviation are based either on the diplopia principle or the haploscopic principle. Diplopia Tests (Red-Glass Test and Others) In the diplopia type of test (Fig. 12–22), one determines the subjective localization of a single object point imaged on the fovea of the fixating eye and an extrafoveal retinal area in the other eye. In esotropia, where the image of the fixation point in the deviated eye falls on a retinal area nasal to the fovea, there should be uncrossed diplopia. In exotropia, where the image of the fixation point in the deviated eye falls on a retinal area temporal to the fovea, there should be crossed diplopia. If retinal correspondence is normal, double images not only should be properly oriented but also should have a distance equal to the angle of squint. The distance of the double images is then a measure of the deviation; but even with spontaneous diplopia it is difficult if not impossible for the patient to state whether the images are crossed or uncrossed. The two visual fields must be differentiated and for this purpose a red glass is placed in front of one eye (hence, red-glass test; see Fig. 12–22). The patient fixates a small light source and states whether the red light is to

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the right or to the left and above or below the white light. If the white fixation light is in the center of a Maddox cross (Fig. 12–23), the patient must state the numbers near which the red light is seen. If the patient is seated at the correct distance, these numbers indicate the amount of deviation. The red glass not only must differentiate the two fields but also must dissociate them, which is especially important in patients with heterophoria, intermittent heterotropia, and particularly with intermittent exotropia. In these patients, one wants to ascertain the full amount of the deviation. For proper dissociation of the fields, the red glass must be dark enough to make it impossible for the patient to see anything but the red fixation light to prevent fusional impulses from the surroundings of the fixation light. The test is facilitated if it is begun by alternately covering the eyes of the patient to show that a white light is seen with one eye and only a somewhat dim red light with the other eye. When both eyes are uncovered, the patient is more likely to become aware of the double image of the light. Nevertheless, eliciting diplopia in patients with comitant heterotropia is often difficult, but with patience and skill it can invariably be achieved. One must occasionally have recourse to the simple trick of placing in front of the eye a 10 or 15 prism base-up or base-down together with the red filter. As a rule, this throws the image outside the suppression scotoma and the patient immediately recognizes diplopia. Vertical displacement of the retinal image introduced by the prism must be taken into account when this maneuver is used.

In general, in doing the red-glass test, the filter should be placed before the fixating eye, which is less likely to suppress the darkened image of the fixation light, but one should always attempt to repeat the test by placing the red filter in front of the other eye. For various reasons, responses are not always identical. In patients with a paralytic condition, a primary and secondary deviation may be present (see Chapter 20). Retinal correspondence may change with a change in fixation (see Chapter 13). Frequently a dissociated vertical deviation may occur. If the deviation is large, it is sometimes necessary to reduce it with prisms; but it is not advisable to fully correct the angle, since this may lead to the phenomenon of horror fusionis (see p. 136). However, in some instances the ophthalmologist may want to investigate the response of the two foveas to simultaneous stimulation (e.g., before suggesting an operation). For this purpose the deviation must be fully neutralized with prisms. The red-glass test used in conjunction with a Maddox cross can be performed successfully in cooperative children as young as 4 years of age. Such children should not be asked to tell where they saw the light, but they should be asked to go to the scale and put their finger on the place where they saw it. It is then wise to place a vertical prism first base-up and then repeat the test basedown. This makes it easier for the child to locate the position of the double image and serves as a check on the reliability of the report. Alternate use of vertical prisms is recommended in all cases in which the patient’s answer is doubtful.

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FIGURE 12–23. The Maddox cross. A, The test is performed at 5 m but can also be performed at 1 m, in which case the small numbers on the Maddox cross (not shown in this figure) indicate the angle of separation of the two images. This patient has a right esotropia of 4. B, If both foveas have a common visual direction (normal retinal correspondence) the red light will appear in the same visual direction as the number whose image is formed on the fovea of the deviating eye. In this case the light appears on the number 4. C, Homonymous diplopia in an esotropic (or esophoric) patient with a deviation of 4. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 61.)

To determine the presence and amount of  incomitant deviations with the red-glass test, one may chart the so-called diplopia fields. This test is best done by turning the patient’s head so as to bring the eyes into the various secondary and tertiary diagnostic positions, ascertaining in each position the distances of the double images and marking them on an appropriate chart. For gross orientation in near fixation a quick and helpful procedure is to keep the patient’s head straight and to move a penlight up, down, right, left, and so on and to ask the patient whether the distance

of the double images is greater in gaze up, down, right, or left. The test can be made quantitative by using a device suggested by Sloane,111 which consists of a small, hand-held, transparent screen provided with a tangent scale designed for a viewing distance of 0.5 m and having a small fixation light in its center. The patient uses a pointer to indicate the position of the double image. To test in secondary and tertiary positions, it is necessary that the patient’s head be turned. Figure 12–24 shows a diplopia field in a patient with a recent paresis of the left superior rectus muscle.

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FIGURE 12–24. Diplopia field in a patient with a recent onset of a left superior rectus paresis. Vertical diplopia is present only in left upper gaze because of underaction of the paretic muscle and secondary overaction of the right inferior oblique.

Vertical diplopia is present only when the patient looks up and to the left where underaction of the left superior and secondary overaction of the right inferior oblique muscle are present. A special tangent scale devised by Harms,59 further developed by Mackensen,79 and widely used mainly in Germany has a fixation light in its center, covered by a metal box. When this box is removed, in lieu of the round fixation light, a horizontal streak of light may be used to determine the obliquity of double images by placing a red glass in front of the observer’s eye. The amount of obliquity may be read from a scale. When the tangent scale is used with a fixation light, the patient also wears a red glass in front of one eye and indicates the position of the red light by means of a green ring projected by a small projection device handled by the patient. In addition to the usual markings, an oblique cross at 45° on the tangent scale makes it possible to test the vertical deviations with the patient’s head tilted to the right and left shoulders. Proper position of the patient’s head is monitored with a special projector attached to the forehead. A handy, routinely used method of measuring the amount of heterophoria is to replace the red glass

with a white or preferably red Maddox rod. This device, consisting of small glass rods, causes an astigmatic elongation of the fixation light and may be placed to produce a vertical or horizontal streak to measure the horizontal and vertical deviation. If the streak does not go through the fixation light, prisms of increasing strength are placed in front of the eye until it does. The amount of prism power required to achieve this goal is a measure of the heterophoria (Figs. 12–25 and 12–26). The amount of the heterophoria in near fixation also may be measured with the Maddox wing test,78, p. 200 the heterophorometer,13, p. 41 or similar devices, all based on the diplopia principle. As has been pointed out, all tests require that the retinal correspondence be normal. Their application to the study of retinal correspondence is discussed in Chapter 13. Haploscopic Tests Haploscopic tests differ from diplopia tests in their mode of stimulation. Two test objects rather than one are presented to the patient, who is required to place them in such a fashion that they appear superimposed (Fig. 12–27). Again assuming that correspondence is normal, the two objects are placed to stimulate the foveae of the two eyes.

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FIGURE 12–25. A, Maddox rod in testing position for horizontal heterophoria. B, Patient sees the line going through the light: no horizontal phoria is present. C, The line is seen to the left of the light (crossed diplopia): exophoria. Add prisms, base-in, to OD until the line is centered on the light. The power of the prism is read and equals the amount of phoria. D, The line is seen to the right of the light (uncrossed diplopia): esophoria. Add prisms, base-out, to OD until the line is centered. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 53.) The visual fields of the two eyes are differentiated and dissociated in various ways. Each eye may be presented with a different target, as is done when using a major amblyoscope. Also, complementary colors may be placed in the visual field of the patient, either directly or by projection, and each eye may be provided with a corresponding colored filter. Instead of color differentiation, a Polaroid projection system or some other system, such as the phase difference projection haploscope of Aulhorn5 (see p. 74), may be used. Color differentiation is convenient in clinical practice. It is applied, for instance, in the Lancaster71, p. 78 red-green test (Fig. 12–28), which uses a window shade type of screen that can be rolled up when not in use. The screen is ruled into squares of 7 cm so that at a distance of 2 m each square subtends approximately 2°. The squares are all of the same size and the tangential error is not taken

into account. Lancaster claimed that at 2 m distance this error did not produce a significant inaccuracy. The patient is equipped with red-green reversible goggles. Two projectors are used: a green projector, handled by the patient, and a red projector, handled by the examiner. The image formed by the projector is linear. The red filter may be placed in front of either eye to investigate differences caused by changes in fixation. Instead of inverting the glasses, the examiner can exchange projectors with the patient. The examiner projects the line from his or her projector onto the screen at any desired place, the patient’s head is held steady, and the patient is asked to place the streak from his or her projector so that it appears to the patient to coincide exactly with the other streak. If one assumes that correspondence is normal, the two streaks will be separated objectively on the screen by an amount corresponding to the deviation of the visual axes. The positions of the streak shown by the patient are entered into a small chart on which the screen is reproduced. FIGURE 12–26. A, Maddox rod in testing position for vertical phoria. B, No vertical phoria is present. C, Right hypophoria (usually left hyperphoria also). Add prisms, base-down, to OS until the line is centered. D, Right hyperphoria. Add prisms, base-up, to OS until the line is centered. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 53.)

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FIGURE 12–27. Principle of haploscopic tests. (From Burian HM: Normal and anomalous correspondence. In Allen JH, ed: Strabismus Ophthalmic Symposium, II. St Louis, Mosby–Year Book, 1958, p 184.)

Since the projected image is a line, the patient’s response may indicate the presence of cyclotropia when the streak is tilted. The Lancaster red-green test is most useful in patients with ocular paralysis. It is least useful in patients with heterophorias or intermittent heterotropias since suppression is not perfect. Reducing the ambient illumination lessens the unwanted effect of fusional stimuli. The same holds true when the eyes are dissociated with Polaroid material.19 When such tests are used, the position of the patient’s head must be fixed and maintained in a headrest. Even slight tipping of the head will reduce the angle between the analyzer in front of the patient’s eyes and the polarizer in front of the projectors and reduce the extinction. The Hess screen test 62 (Fig. 12–29) is based on the haploscopic principle. It was popularized by Lyle, in particular for diagnosing possible paretic or paralytic conditions in patients with normal correspondence. To perform this test, a black cloth 3 ft wide by 31⁄2 ft long, marked out by a series of red lines subtending between them an angle of 5°, is used. At the zero point of this coordinate system and at each of the points of intersection of the 15° and 30° lines with one another and with corresponding vertical and horizontal lines, there is a red dot. These dots form an inner square of 8 dots and an outer square of 16 dots. An indicator is provided consisting of three short green cords knotted to form the letter Y. The end of the vertical green cord is fastened to a movable black rod 50 cm long. The ends of the other two cords are kept taut by black threads that pass through loops

to small weights at corresponding upper corners of the screen. This arrangement enables the patient to move the indicator freely and smoothly over the whole surface of the screen in all directions. The patient wears red-green goggles and is seated 50 cm from the screen, preferably with his or her head fixed in a headrest. The patient now sees the red dots with one eye and the green cords with the other and is instructed to place the knot joining the three green cords over each of the red dots in turn. The examiner marks the positions indicated by the patient on the small card with a reduced copy of the screen. The points found by the patient are connected by straight lines and permit the examiner to determine which, if any, muscles react abnormally. To change fixation, the red-green goggles are reversed with the red filter now in front of the left eye. Measurements of the angle of strabismus that are based on image separation with red and green glasses or other haploscopic methods have become justifiably popular in many countries, especially in Europe. Provided the patient is cooperative, these tests are precise and repeatable on different occasions. However, they are less popular in the United States, partially because they are somewhat time-consuming and require a prolonged attention span and reliable patient responses, which excludes a large segment of pediatric patients with strabismus problems. Subjective determinations of the angle of deviations with the major amblyoscope, also

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FIGURE 12–28. Lancaster red-green test. A, Equipment for the test. B, Charts to record results. (Courtesy of Luneau and Couffignon, Chartres, France.)

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FIGURE 12–29. The Hess screen test. A, The screen. B, Chart for recording the results (From Lyle TK, Wybar KC: Lyle and Jackson’s Practical Orthoptics in the Treatment of Squint [and Other Anomalies of Binocular Vision], ed. 5. Springfield, IL, Charles C Thomas, 1967.)

based on the haploscopic principle, are aimed primarily at establishing the sensory state of the patient and are discussed in Chapter 13. Measurement of Cyclodeviations Qualitative Diagnosis Based on Position of Double Images The measurement of cyclodeviations in clinical practice relies largely on subjective tests. In patients who have spontaneous diplopia or who can be made to appreciate diplopia by interrupting fusion with alternate covering of the eyes, cyclodeviations can be grossly estimated by holding a ruler horizontally with the straight edge in front of their eyes and slightly below the midline. If one of the cyclovertical rotators is involved, there will be vertical diplopia. By alternately covering the eyes, the examiner then ascertains to which eye the higher or lower image belongs and asks whether the two rulers seen by the patient appear closer on the right or on the left. To avoid any misunderstanding, one should draw a horizontal line and then let the patient add the obliquely seen line. For quantitative determination of cyclotropia the Lancaster red-green test may be used.

The tilt of the retinal image is opposite the tilt of the horizontal line, as seen by the observer. Therefore, when the line is seen slanted toward the nose (to the left for the right eye or to the right for the left eye), an excyclodeviation is present. Tilting of the line down toward the temple will indicate the presence of an incyclodeviation. For correct interpretation refer to the diagram shown in Figure 12–30. A simple mnemonic rule is that the line is always tilted in the direction in which the offending muscle would rotate the eye if it were acting alone. Since, for example, the superior oblique is an intortor in addition to being a depressor, paralysis of that muscle will cause the image seen by the involved eye to appear lower and slanted toward the nose. Maddox Double Rod Test For quantitative determination of a cyclodeviation, red and white Maddox rods are placed in a trial frame, the red before the right eye and the white before the left eye (Fig. 12–31). The direction of the glass rods is aligned with the 90° marks of the trial frame. A small scratch on the metal frame of the Maddox rods facilitates this alignment. Special care must be taken to avoid tilting the trial frames

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FIGURE 12–30. Subjective appearance and retinal image of a horizontal line seen by the left eye. A, In the absence of a cyclodeviation. B, With incyclotropia. C, With excyclotropia. V and H, The vertical and horizontal meridians of the retina; UT, UN, LT, and LN, the upper and lower temporal and nasal quadrants of the retina; VI, the resulting visual impression.

during the test. The patient looks through the Maddox rods and is shown a penlight, the images of which appear as horizontal streaks. A vertical prism may be added to separate the images for easier identification. If one of the lines (say, the red one) appears slanted toward the nose (Fig. 12–31A), excyclotropia of the right eye is present. The red Maddox rod is then turned by the ophthalmologist (or by the patient) until the red line is seen parallel with the white line (Fig. 12–31B). If at the end of this adjustment the scratched mark points, for example, toward the 100° mark of the right trial frame, the patient has a right excyclotropia of 10°. In the presence of bilateral cyclotropia, for instance, excyclotropia of the right and left eye in bilateral traumatic superior oblique paralysis, both the red and white lines will be seen slanted toward the nose. The settings are repeated two or three times. With good observers the measurements are extremely accurate. To test the cyclodeviation outside the primary position, shift the penlight to the right, left, above, and below, and repeat the test. The Maddox double rod test is valuable as a qualitative test to substantiate a patient’s complaint about image tilting and to quantitatively determine the degree of tilt. However, the dissociating characteristics of the test preclude cyclofusion, which is a most effective compensating mechanism in cyclodeviations.100, 101 Thus the Maddox double rod test may indicate a cyclotropia that,

in some patients, may be clinically insignificant under casual viewing conditions that permit cyclofusion. Moreover, since ocular dominance determines the patient’s response to the Maddox double rod test, contradictory results with respect to the laterality of the paralyzed muscle are common.90 Simons and coworkers109 have shown that the two-color format of the Maddox double rod test, with the red rods placed before the right eye and the clear rods before the left eye, may produce artifactual localization of the image perceived through the red rods in patients with superior oblique paralysis. Thirty-three of 40 patients (83%) localized the excyclodeviation to the eye viewing through the red Maddox rods, regardless of the laterality of the paralysis or the fixation preference. To avoid this artifact and the influence of peripheral visual clues the authors suggested that red Maddox rods be placed before both eyes and that the test be performed in a dark room. To distinguish which eye is cyclodeviated one Maddox rod is then slightly rotated back and forth in the trial frame and the patient is asked whether it is the horizontal or tilted luminous line that is "rocking." Bagolini Striated Glasses To test for cyclotropia under casual viewing conditions, we replace the Maddox rods with the striated glasses of Bagolini.7 Like Maddox rods,

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FIGURE 12–31. The Maddox double-rod test. For explanation, see text. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 57.)

these glasses produce an image of a streak of light, perpendicular to the axis of the striations when viewing a punctate light source, without, however, obstructing surrounding fusible visual details. The glasses are placed in the trial frame with the axes of striation pointing toward the 90° mark. If the patient is unable to fuse the two vertical lines, the glasses are turned until fusion occurs and the amount and direction of the cyclotropia is read off the trial frames as during the Maddox double rod test.100 The use of the Bagolini glasses to test for retinal correspondence is discussed in Chapter 14. Major Amblyoscope To test for cyclotropia, the targets positioned in the arms of this instrument are rotated around an

axis until there is no more movement of redress of the eye that takes up fixation. The amount of cyclodeviation, expressed in degrees, can be read off the instrument. Ophthalmoscopy and Fundus Photography Indirect ophthalmoscopy and fundus photography are useful auxiliary methods to diagnose cyclotropia. As early as 1855 and not long after the invention of the ophthalmoscope by von Helmholtz (1851), von Graefe51 pointed out an apparent vertical displacement of the optic disk in cyclodeviations and discussed using ophthalmoscopy to study the action of muscles involved in cyclorotations of the globe. Normally, the average location of the fovea in relation to the optic nerve head is 0.3 disk diameter

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below a horizontal line extending through the geometric center of the optic disk. From this position, an imaginary horizontal line will cross the optic nerve head just below the halfway point between its geometric center and lower pole (Fig. 12–32A). The range of variation of this relationship in nonstrabismic persons is indicated by the solid lines in Figure 12–32A. Incyclotropia is present when the fovea appears above a line extending horizontally from the center of the optic nerve head (Fig. 12–32B), and excyclotropia is present when the fovea is below a line extending horizontally from just below the lower pole of the optic disk (Fig. 12–32C). The fovea’s position may vary slightly between the two eyes, but a difference of 0.25 or more of a disk diameter in the vertical position should be considered abnormal.14 To measure the amount of torsion accurately while viewing the fundus through a 60D lens on the slit

lamp, Spierer113 suggested projecting a horizontal slit beam on the retina so that it crosses the fovea while the patient fixates on a target straight-ahead with the other eye. In the presence of cyclotropia the examiner tilts the slit beam until it crosses the fovea on one side and the border between the center and lower third of the optic disk on the other side. The amount of tilting needed for this position to occur is then read off the scale mark on the slit lamp. De Ancos and Klainguti3 described a special lens to measure the angular displacement of the lower border of the optic disk with respect to the fovea during indirect ophthalmoscopy. Discrepancies between sensory and motor aspects of cyclodeviations, as expressed in differences between subjective (Maddox double rod test, Bagolini lenses) and objective (ophthalmoscopy, fundus photography) findings, are discussed in Chapter 18.

FIGURE 12–32. A, Average foveal position (dashed line) and range of normal (solid line). B, Fundus in patient with incyclotropia. C, Fundus in excyclotropia. (From Bixenmann WW, Noorden GK von: Apparent foveal displacement in normal subjects and in cyclotropia. Ophthalmologica 89:58, 1982.)

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The "New Cyclo Test" The New Cyclo Test,* introduced by Awaya and coworkers,6 is similar in principle to the Awaya test for aniseikonia (see p. 121) and based on haploscopic image separation with red-green spectacles. A red half-moon is viewed through the green glass and a green half-moon through the red glass. In a series of printed figures the green halfmoon is tilted in a stepwise fashion. The patient selects the figure in which the two half-moons appear to be aligned and the amount of cyclodeviation in degrees is then read off this figure. Scotometry Locke76 showed that the vertical displacement of the blind spot in cyclotropic eyes may be used to determine its degree. This method, while precise, is rarely used in clinical practice. Determination of the Subjective Horizontal or Vertical Determination of the subjective horizontal or vertical with and without spatial clues distinguishes between the contribution of such clues to the adaptation to cyclotropia. This method, almost forgotten now but once used widely in clinical practice as part of the workup of a patient with paralysis of the cyclovertical muscles,61, 121 is still a useful diagnostic tool99 (see Chapter 18). The patient whose head is stabilized views with either eye a luminous line that is projected in random oblique positions onto an optically empty screen. The patient then rotates the slide containing the _________________________________________ *Distributor for the Western Hemisphere: Binoculus (see p. 174 for address).

line until it appears exactly horizontal to him or her. The deviation from the objective horizontal as determined with a carpenter’s level is measured by the examiner with a protractor and indicates the degree of cyclotropia. Measurement of Dissociated Vertical Deviations Bielschowsky12 classified vertical deviation into four groups: (1) true comitant hyperphorias and hypertropias, (2) dissociated vertical divergence (alternating sursumduction), (3) paretic vertical deviations, and (4) vertical deviations in the right and left half of the field of fixation caused by primary overaction of an inferior oblique muscle. The diagnostic differentiation of these forms is discussed in Chapter 18, but a few words must be said about the diagnosis of dissociated vertical deviation, commonly abbreviated as DVD. In patients with DVD, the alternate cover test reveals that each eye turns upward under cover in contrast to the situation in vertical heterophoria. After removal of the cover, the eye makes a slow downward movement to reach the midline, at times even going below it, accompanied by incycloduction. The translucent occluder of Spielmann112 is especially useful in the diagnosis of this condition and in demonstrating it to the patient’s parents (Fig. 12–33). As pointed out in Chapter 18 a precise measurement of the vertical excursions of each eye during DVD is nearly impossible because of the variable nature of this condition. The Head Tilt Test The maneuver of comparing the angle of strabismus with the head tilted successively toward

FIGURE 12–33. The effect of occlusion in a patient with dissociated vertical deviation. A, Orthotropia in primary position. B, Elevation of the right eye and C, of the left eye when fusion is suspended with the translucent occluder of Spielmann. (From Spielmann A: A translucent occluder for study of eye position under unilateral or bilateral cover test. Am Orthoptics J 36:65, 1986.)

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one and then the other shoulder, introduced by Hofmann and Bielschowsky64 in 1900 and widely developed by Bielschowsky, is known as the head tilt test. This useful test gives positive findings regardless of whether the patient has any binocularity. The physiologic principles of the head tilt test and its application during the diagnosis of cyclovertical deviations are discussed in Chapter 20. The deviation may differ in various head positions in patients with congenital and acquired paralyses of the cyclovertical muscles when other diagnostic signs have become blurred with the passage of time. Two valuable assets of the test are that the difference in the deviation can be seen and measured by the examiner and that it can be readily used in examining young children who cannot yet respond to subjective tests. In some instances it is difficult to see the difference in the vertical deviation in the head tilt test. In such instances a prism and cover test with the head tilted first to one shoulder and then to the other is advisable. The prism should always be tilted so that it has the same relation to the eye as in primary position (Fig. 12–34). Examination of the Motor Cooperation of the Eyes Ductions and Versions Determination of the deviation amount in the nine diagnostic positions of gaze by means of the prism and cover test or by one of the subjective tests establishes certain important points in the diagnosis. These points must be amplified by a study of the extent to which the eyes, singly or FIGURE 12–34. Head tilt test with prism and cover measurement. Note that the prism is held so that its base and axis are parallel with the palpebral fissure and not with the floor.

together, perform movements in various directions of gaze. When examining ductions, cover one eye and have the patient follow a penlight or other fixation target, bringing the eye to the farthest possible position in the directions right, left, up, down, up and right, up and left, down and right, and down and left. The examiner observes whether movement lags or is excessive in any direction. Bielschowsky13 pointed out that a study of the versions is of more value than a study of the ductions. He stated that it is easier for the patient to overcome a weakness in the action of a muscle by a very strong innervational impulse during ductions than while performing version movements. To study the versions, one places a penlight in the midline before the patient’s eyes and moves it in the various directions, keeping the penlight at such a distance that one can always observe the corneal reflections in both eyes. While doing so, carefully watch for excessive or defective movements in any direction. Remove the patient’s glasses to observe the movement of the eyes in peripheral positions of gaze. In judging the normalcy of adduction and abduction, a gross but useful guideline is followed. In maximal adduction an imaginary vertical line through the lower lacrimal punctum should coincide with a boundary line between the inner third and the outer two thirds of the cornea (Fig. 12–35A). If more of the cornea is hidden, the adduction is excessive (Fig. 12–35B). If more of the cornea is visible on maximal adduction and if some of the sclera remains visible, adduction is defective (Fig. 12–35C). If abduction is normal, the corneal limbus should touch the outer canthus (Fig. 12–35D). If the limbus passes that point and some of the cornea is hidden, the abduction is excessive (Fig. 12–35E ). If some of the sclera remains visible, abduction is defective (Fig. 12–35F ). Guibor53, p. 28 rated overaction in adduction and underaction in abduction on a scale of 1 to 4; however, he assigned no specific figures to the grades of overaction or underaction. Urist123 developed what he called the lateral version light reflex test, which is performed by holding a penlight exactly in the midline of the patient’s head at a distance of about 25 cm. The patient makes extreme dextroversions and levoversions.

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FIGURE 12–35. The ductions of the eyes. A, Normal adduction. B, Excessive adduction. C, Defective adduction. D, Normal abduction. E, Excessive abduction. F, Defective abduction. P, Lacrimal punctum. While the versions are being tested, the fellow eye is kept open and, as a rule, fixates. The left eye is not shown in this figure. Normally, the light reflex in the adducted eye should be on the cornea at 35° temporally (Hirschberg scale) and at 10 mm nasally from the limbus on the sclera of the abducted eye. A much more precise procedure is the limbus test of motility of Kestenbaum,68 intended especially for evaluating the action of paretic muscles. This test avoids some pitfalls inherent to older tests in attempting to determine the shift in relative position of certain fixed points in different positions of gaze. The test is performed by holding a transparent millimeter ruler horizontally in front of the cornea. In measuring abduction, the location of the nasal limbus point is noted on the ruler in primary position and in maximum abduction. The difference immediately gives the degree of abduction in millimeters. Adduction is measured similarly by determining the positions of the temporal limbus. To measure elevation and depression, hold the ruler vertically. The examiner should test each eye with his or her own homonymous eye. Normal values established by Kestenbaum are 10 mm for adduction, abduction, and depression, and 5 to 7 mm for elevation (Fig. 12–36). It is interesting to note that there is no shift of the midpoint of the excursions toward the nose in normal subjects.68 Patients with esotropia (infantile or accommodative; see Chapter 16) and alternating fixation or with manifest-latent nystagmus (see Chapter 23) may employ the left eye for viewing

objects in the right field of vision and the right eye for objects in the left field of vision. No effort is made to abduct the nonfixating eye, which shows an apparent limitation of abduction (Fig. 12–37A and B). This behavior is called crossed fixation and in the older literature is also referred to as tripartite fixation. To distinguish between pseudoparalysis and true paralysis of the lateral rectus muscle, the ductions of each eye are examined while the fellow eye is patched (Fig. 12–37C). In general, the agreement between the measurements in the diagnostic positions and the behavior of the versions is good, but there are exceptions. For example, simultaneous weakness of adduction in one eye and an excess of abduction in the other eye may offset each other and the abnormalities of the versions may not be apparent from the differences in measurements of the deviation.21 In watching pursuit movements of the eyes, one may find that the fixating eye will follow the light, but the deviated eye will remain stationary for some time and then make a sudden movement in the direction taken by the fixating eye. Infrequently, version movements (e.g., a dextroversion movement) are replaced by vergence movements.22, 23 As a rule, children with comitant strabismus will follow an appropriate fixation object without difficulty. However, they frequently have much more difficulty in making version movements in a direction opposite that of the deviation (e.g., in levoversion in a left esotropia). This behavior is mentioned briefly in the discussion of the etiology of heterotropia. Tests that distinguish between innervational and mechanical-restrictive limitations of ocular FIGURE 12–36. The limbus test of Kestenbaum. (From Kestenbaum. A: Clinical Methods of Neuro-Ophthalmologic Examination, ed 2. New York, Grune & Stratton, 1961, p 237.)

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FIGURE 12–37. Crossed fixation. A, Children with esotropia may employ the left eye for viewing objects in the right field of vision and B, the right eye to view an object in the left field of vision. Thus no effort is made to abduct the nonfixating eye, and the examiner must differentiate between a true and a simulated abducens paralysis. C, Momentary occlusion of the fixating eye may not suffice to force the fellow eye to take up fixation. D, Occlusion for several minutes may be required to restore good abduction. (From Noorden GK von: Atlas of Strabismus, ed 4. St Louis, Mosby–Year Book, 1983, p 121.)

rotations (forced duction test, estimation of generated muscle force, saccadic velocities) are discussed in Chapter 20. Elevation or Depression of the Adducted Eye (Upshoot or Downshoot in Adduction) When studying the versions, one often finds the adducted eye to be elevated. The phenomenon may be unilateral or bilateral. This elevation in adduction is called strabismus sursoadductorius and in the more recent American literature is also referred to as upshoot in adduction. It is often overlooked but important to note that overaction of the inferior oblique muscle is but one of several causes for this phenomenon. Other causes are

paralysis of the contralateral superior rectus muscle, dissociated vertical deviation, certain forms of Duane retraction syndrome, excyclotropia of the involved eye, and atopic muscle pulleys. The differential diagnosis of these conditions from primary and secondary overaction of the inferior oblique muscle is discussed in Chapter 18. Depression of the adducted eye, also called strabismus deorsoadductorius or downshoot in adduction, is seen with overaction of the superior oblique muscles, paralysis of the contralateral inferior rectus muscle (see Chapter 18), atopic muscle pulleys, and may also occur with Duane retraction syndrome (see Chapter 21).

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Measurement of Vergences Determination of the vergences is of major importance in examination of the motor state of a patient’s eyes. It provides information about the patient’s ability to cope with a deviation and is also helpful in establishing the type of deviation according to Duane’s classification.36 The characteristics of the disjunctive ocular movements, the fusional movements or vergences, are discussed in some detail in Chapter 4. What follows deals with their practical determination rather than the physiologic basis of the tests. To produce a vergence movement, retinal images must be shifted so that they fall outside the Panum area of the retinal region under investigation. Since the extent of that area in the foveal region is roughly of the magnitude of 6 minutes of arc and since vergences are usually measured with bifoveal fixation, a shift of 1 is ample to produce an image displacement capable of eliciting a fusional movement. Measurement With Prisms The patient is seated comfortably and asked to fixate the appropriate object. A rotary prism is then placed in front of one eye and moved to produce the desired fusional movement. The prism may be hand-held, or it may be part of a phorometer or phoropter. The prism strength is increased slowly and stepwise, and the patient is asked to report when the fixation object appears double. When the patient reports diplopia, at which point the two images are rather suddenly quite far apart, the required amount of prism power is noted. It represents the limit of the patient’s fusional amplitude in the direction tested, the so-called breakpoint. The prism power is then reduced, again slowly and stepwise, and the point at which the patient regains single vision is noted. This is the so-called recovery point. Many ophthalmologists disregard the recovery point, depriving themselves of an important bit of information. The recovery point indicates a patient’s readiness to fuse the images. It should be 2 to 4 below the breakpoint. If the breakpoint for convergence at near is, say, at 24, the recovery point should be 20 to 22. Some patients may not recover until the prism power is much reduced, to 10, 8, or even 0. This is especially common in patients with intermittent deviations and indicates that once fusion is broken they have great difficulty regaining it.

When the convergence amplitudes are measured, the patient will see singly and clearly up to a point. Beyond this point the fixation object will appear blurred but single until the breakpoint is reached, where the fixation point doubles up and again is seen clearly. The point at which the blurring occurs is known as the blur point. It measures the limits within which accommodation can clear the image of the fixation point in spite of increased convergence. The amount of fusional convergence that can be elicited between the blur point and breakpoint represents the absolute convergence. Orthoptists use accommodative targets to determine the blur point and nonaccommodative targets to determine the breakpoint. Vergence movements are slow and tonic. They must be elicited by increasing the prisms slowly to allow the patient to regain fusion after each change. If the prism power is changed too quickly, lower fusional amplitudes are obtained than if the test is properly performed. For a smooth and continuous increase of prismatic power we prefer a rotary prism (Fig. 12–38) rather than a prism bar with a stepwise increase of prismatic power. One must also remember that when a strong impulse to perform a convergence movement has been given, the tonic innervation does not suddenly stop with removal of the stimulus but continues for quite some time. If one starts by measuring the convergence amplitude and this is followed immediately by measurement of the divergence amplitude, divergence is opposed by the lingering tonic innervation to converge. The resulting divergence amplitude is then likely to have a lower value than if it had been measured first because any fusion-induced vergence has an aftereffect41 that is stronger after sustained convergence than after divergence or vertical vergences. The longer the duration of the vergence effort, the longer the rate of recovery from the aftereffect.107 The best means of reducing this effect is to induce a vertical vergence. The following order in testing fusional amplitudes is recommended: prisms baseout (convergence), prisms base-up (deorsumvergence), prisms base-in (divergence), and prisms base-down (sursumvergence). Vergences should always be tested both in distance and near fixation. Frequently they are tested at one fixation distance only (e.g., in distance fixation), especially when the divergence amplitudes are determined with a major amblyoscope. In patients who complain of difficulties in close work, especially when a convergence insufficiency is suspected,

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FIGURE 12–38. Objective measurement of endpoint of vergence. A, Rotary prism at zero. The eyes are parallel. B, Rotary prism has reached 26 base-out. The left eye is in divergent position.

a comparison of the vergences in distance and near fixation is often very enlightening. Since vergences are fusional movements, amplitudes depend on the amount of fusible material in the field of view of the person examined; the greater the amount of fusible material, the larger the amplitudes. They are smallest when a fixating light is seen in a completely dark room,50 and they cannot be elicited with dissimilar targets in a major amblyoscope. In 1948 Fink45 made a careful study of this subject and recommended a 6/9 letter as a suitable fixation object. Actually, a small fixation light serves the purpose well since its doubling up is most easily recognized by the patient, provided the light is surrounded by ample fusible material (such as vertical and horizontal lines) usually available in a well-lighted office. The images of this fusible material occupy the whole of the two retinas and provide an adequate fusion stimulus. Most patients are able to recognize diplopia when the breakpoint is reached, but some do not and instead suppress the image in the deviated eye. Even in such instances the breakpoint can be determined by observation, at least in near fixation. Both eyes will appear properly aligned as long as they follow the vergence stimulus induced, for example, by base-out prisms. As soon as the

breakpoint is reached, one eye will turn out (see Fig. 12–38). The question most frequently asked and most difficult to answer concerns normal limits of the different vergences. It is not possible to state in specific numbers what the amounts of the vergences are or should be. Convergence normally is larger than divergence, and vertical vergences are smaller than either of the two. In some texts normal limits for distance fixation are given as 20 for convergence, 6 to 8 for divergence, and 3 to 4 for sursumvergence and deorsumvergence. Cyclovergence amplitudes may range between 8° and 22°, depending on the size and orientation of the targets being used.30, 55, 67, 95 Horizontal vergences measured in distance fixation are smaller than those obtained in near fixation, and the most useful data are probably still those of Berens and coworkers10 (Table 12–2). These data were taken with prism bars in 104 subjects with normal vision. Sharma and Abdul-Rahim108 reported larger vertical amplitudes (mean 4.63) than those found by Berens and coworkers. Mellick82 used a variable prism stereoscope and the synoptophore (a form of major amblyoscope) and two targets (a fusion target and a stereoscopic target) to study horizontal fusional amplitudes in relation to age. He could find no significant

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TABLE 12–2. Average Vergence Amplitudes of 218 Men

influence of age but did observe that the amplitude of convergence for distance and near vision was twice as large when measured with the synoptophore as when measured with a prism stereoscope. The average figures for his 561 subjects of all ages are presented in Table 12–3. The data of Tait116 obtained from 500 ocularly normal subjects (Fig. 12–39) reflect the distribution of both breakpoint and recovery point. From these figures it is evident that amplitudes of the vergences vary considerably from one person to another, even if function of the binocular system is normal. They are a measure in the motor sphere of a person’s responsiveness to disparate stimulation, as stereoscopic acuity is a measure of a person’s responsiveness in the sensory sphere. It is important to relate the vergence amplitudes to another individual characteristic, the heterophoric position for the distance at which the amplitudes are measured. In this regard, measurements with a rotary prism may be misleading unless they are properly understood. In performing this test, the patient’s eyes start from the primary position, as defined clinically; that is, the eyes intersect in the fixation point, having already overcome any heterophoric position that might be present. The test with the rotary prism tells only how much additional vergence the patient can perform. Case 12–1 may make this point a little clearer.

CASE 12–1          A 27-year-old woman had an intermittent exotropia for distance of 22, which she could control quite well. Measured with rotary prisms for distance, she had a convergence breakpoint at 8 and a divergence breakpoint at 26. This appeared to be an obvious case of insufficient prism convergence and excessive prism divergence, a conclusion that would only be true if the zero position of the rotary prism had a biological significance, which it did not. Actually, this patient was able to overcome by convergence a divergent heterophoric position of 22 to keep her eyes straight. With a rotary prism she overcame an additional 8 of convergence. This patient, then, in fact possessed a convergence amplitude of 30, although her divergence beyond the heterophoric position was only 4. The patient was operated on, and the deviation for distance was reduced approximately to zero. Following the operation, the amplitudes were found to be 30 of convergence and 6 of divergence. Case 12–1 illustrates that measurements of vergence amplitudes with a rotary prism are meaningful only insofar as they are related to the patient’s heterophoric position, yet the absolute values are not completely without significance. They indicate the reserve that the patient has

TABLE 12–3. Mean Values and Standard Errors of Horizontal Vergences in 561 Subjects with Normal Neuromuscular Systems

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FIGURE 12–39. Data of Tait for breakpoint and recovery point in 500 subjects. A, Prism convergence. B, Prism divergence. (From Tait EF: Fusional vergence. Am J Ophthalmol 32:1223, 1949.)

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beyond the parallel position of the visual lines. A  reserve of 8 of convergence, as the above patient had, is clearly insufficient. Such a patient may readily experience diplopia under conditions of stress and annoying asthenopic symptoms. In contrast, one may find remarkably large fusional amplitudes in patients with well-developed binocular vision. This is particularly impressive in vertical heterophorias in which fusional amplitudes may amount to 20 and more, especially in cases of long-standing superior oblique muscle paralysis. In 1900, Hofmann and Bielschowsky65 made a thorough study of vertical fusional amplitudes and concluded that they gradually increase by training up to a maximum value of 5.5° (approximately 11). Ellerbrock,40 on further investigation of vertical amplitudes, discovered that they were greater in magnitude when larger fusional targets were presented and when the rate of separation of the targets was slower. He reported amplitudes as large as 7.5° to 8.4° (approximately 16).42 An example of extraordinarily large vertical amplitudes, which developed in a patient who had a hyperphoria throughout his life, is given in Case 12–2.83 CASE 12–2       A 42-year-old man who complained of blurring and distortion at close work had been given glasses incorporating a correction for a mild hypermetropic astigmatism and 6 base-down for a correction of a right hypertropia (RHT). Our examination revealed a vision of 6/4.5 in each eye with a correction of +1.00D sph -1.00D cyl ax 80°OD and +5.00D sph -1.00D cyl ax 85° OS. With this correction he had 10 of comitant RHT for distance and 12 of comitant RHT for near. He had excellent horizontal fusional amplitudes, and his vertical fusional amplitudes were extraordinarily large (24/16 of sursumvergence and 24/10 of deorsumvergence). He had satisfactory binocular cooperation with a stereoscopic threshold of 40 seconds of arc. The patient was given his refractive correction with a reading add but no prisms since he was so well adapted to his motor anomaly. Over a follow-up period of 6 years, his motor condition has not changed and he has remained free of symptoms.

Measurement With a Major Amblyoscope When vergences are measured with a major amblyoscope, the point of departure is the heterophoric position, that is, the arms are set at the patient’s  angle of deviation. From this position, various vergence movements are induced by moving the arms (for the horizontal vergences) or the targets (for vertical and cyclovergences). This procedure determines total vergence amplitudes, but no information is gained about the fusional reserve beyond the straight position of the eyes. This test can be done also in heterophoric patients by starting with the arms of the major amblyoscope at zero. These measurements correspond to those obtained with the rotary prism. Ordinarily, the major amblyoscope is adjusted for optical infinity. Placing minus lenses of appropriate strength in front of each eye and setting the arms at 18 adapt the instrument for near fixation, and vergences can also be measured for near visual distance. Targets that incorporate fusible material must be used to measure vergence amplitudes when a major amblyoscope is used. Using targets that contain larger or smaller amounts of fusible material allows the effect of the amount of fusible material on the amplitudes to be determined. Fusional Movements Elicited by Peripheral Retinal Stimuli in Strabismus Burian20 applied the peripheral fusion technique (see p. 74) in 75 patients with comitant strabismus and showed that patients with manifest strabismus can display fusional movements. The main results obtained in eliciting fusional movements by peripheral retinal stimuli can be summarized as follows: patients with strabismus in whom peripheral fusional stimuli were effective, as a rule, experienced sensory disturbances (suppression, changes in mode of localization, changes in deviation) when the two retinal centers were stimulated simultaneously; patients who did not follow peripheral fusional stimuli did not have retinocentral disturbances. Although the technique used in Burian’s studies is not directly applicable in ordinary clinical practice to examination of patients, it can be used to some extent in the major amblyoscopes by designing or selecting appropriate targets both for diagnostic and therapeutic purposes.

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Near Point of Convergence The near point of convergence (NPC) is determined by placing a fixation object at 30 to 40 cm in the midplane of the patient’s head; the patient is then asked to maintain fixation on the object. The object is then moved toward the eyes until one of the eyes loses fixation and turns out. The distance at which this occurs is the NPC. It is measured by a Prince ruler or similar device (Fig. 12–40). This NPC lies approximately in the plane of the centers of rotation of the eyes. The eye that maintains fixation at the NPC is generally considered to be the dominant eye, and the deviated one is the nondominant eye. Determination of the NPC is one of many tests suggested for establishing ocular dominance. The NPC should be at 8 to 10 cm. A distance closer than 5 cm is excessive. An NPC farther away than 10 cm is defective or remote. In patients with convergence insufficiency, it may be as remote as 25 or 30 cm or more. As the test is repeated, the NPC often comes closer to the eyes. The NPC is readily trained, except in extreme cases of convergence insufficiency. On the other hand, in a single testing session patients may make a special effort to converge and have a better NPC than they actually use in casual seeing. These statements are justified to some extent when the test is performed objectively. They also apply to the subjective form of the test in which the endpoint is established by the patient’s report of   FIGURE 12–40. Objective measurement of near point of convergence.

diplopia. They very definitely do not apply to modification of the subjective test as described by Capobianco26 and used routinely in our clinic. In this test a moderately dense red filter is placed in front of one of the patient’s eyes, preferably the dominant eye. A penlight is held at a distance at which the patient can fuse the two images. One pinkish light is then seen. The penlight is now advanced in the midline of the patient’s head, and the point is noted at which binocular single vision is lost and diplopia is reported. This test offers a slight obstacle to fusion, minimizes the effect of voluntary convergence, and yields results of diagnostic and prognostic value. In comparing findings in the objective test and the red-glass test, the following observations can be made: (1) In patients with good convergence function, results obtained with two tests are comparable and both are within normal limits; (2) in patients with convergence insufficiency, the subjective NPC is generally more remote than the objective NPC and the difference may be quite large; (3) in patients with convergence insufficiency who have a slightly remote NPC but in whom the NPC established by the red-glass subjective test coincides with the objectively measured NPC, the prognosis for speedy, successful recovery by treatment is good; (4) throughout the treatment the NPC, as measured by the subjective red-glass test, normalizes more rapidly than the objectively determined NPC; and (5) at the successful completion of treatment, the NPC should not only be normal but values obtained in the objective and red-glass subjective tests should be in agreement. Maintenance of Convergence A patient not only must be able to converge the eyes to a near vision distance but also must be able to maintain convergence. This ability may be tested by what is called, inaccurately, the drop convergence test. When NPC is measured, an object is brought closer to the eyes. Accommodative and fusional convergence are stimulated by the object and assist in performance of the convergence movement. After bringing the fixation object into reading distance, ask the patient to maintain convergence after the fixation object is removed ("dropped"). Some patients are better able than others to keep their eyes converged in the absence of a fixation object.

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