The use of diagnostic diagrams can be both helpful and restrictive. They may be very simple and easy to follow, but they don't cover the subject adequately. The other approach is to cover the entire subject, which results in a very involved and difficult diagram to follow and understand. We are all looking for a simple solution to a complex problem. It would be so easy to have a computer program that outlines the appropriate differential diagnosis. You then feed in the results of the examination and the laboratory data, and the computer prints out the proper diagnosis. Medical history-taking, however, is more an art than a science, which skill is honed over years of practice and experience. The diagnostic trees are a less sophisticated approach to the problem than the computer but can be helpful. If they are used, knowing their limitation, then they serve a real purpose. We have tried to strike a balance between the two extremes of design and to develop one that is practical.
History (Fig. 6.1)
Figure 6.1 Diplopia history.
The proper investigation of a patient with the complaint of diplopia is truly a test of the physician's history-taking ability. The primary part of the patient interview is to determine if the patient is using the term "double vision" correctly. True binocular diplopia resulting from the malalignment of the two eyes requires a different workup than monocular diplopia (Fig. 6.2). If the patient still claims to see double with either eye closed or that diplopia is still present when one particular eye is closed and the other is open, then we have monocular diplopia. As simple a differentiation as that may seem unimportant, but most patients have not examined their problem that specifically. If the problem is monocular and due to some opacity in the media, ask the patient if the "double vision" is more like a ghost image on TV with one clear image and another more faded outline around it? The distortion of the transmitted image from the ocular media can be a corneal scar, keratoconus, cataract, vitreous opacities, or macular disease. Distortion of images has been reported on a cerebral basis but is extremely rare and will be equal in both eyes. Distortion due to macular disease is frequently described by patients as double. Ask the patient if the image is like a view seen in a cracked mirror. True binocular diplopia images are usually two images equally clear. This may not be always true if there is a secondary problem in one of the eyes, such as a cataract.
Figure 6.2 Monocular diplopia evaluation, part 1.
Once the history denotes monocular diplopia, then tests such as a pinhole, careful refraction, keratometry, Amsler grid, slit lamp examination, contact lens evaluation, and careful fundus examination are the direction the workup should take.
If the history denotes true binocular diplopia, then the next historic step is to find out if it is constant or intermittent. This may not be as straightforward a question as it seems. If it is intermittent, inquire about the circumstances under which it occurs. Does it develop later in the day and do the images get further apart as the day progresses? If it does, then myasthenia gravis is high on the list of differential diagnoses. Myasthenia gravis is an even stronger consideration if there is a ptosis and the patient is a young female. The patient may notice weakness in her shoulders when combing her hair, which is highly suggestive of the diagnosis. The evaluation of a patient with this list of symptoms takes a different direction. This includes the clinical fatigue test, EMG studies, blood assay for antiacetylcholine receptor antibody, and the edrophonium test (Tensilon).
If the diplopia occurs in separate episodes, then inquire under what circumstances it occurs. Ask particularly if there are any other neurologic symptoms such as weakness, sensory changes in an arm or a leg, perioral paresthesias, or difficulty finding a word or a period of unconsciousness that occur at the same time as the diplopia. This type of diplopia probably represents a transient ischemic attack. Evaluation of cardiac status with an echocardiogram, blood pressure monitoring, and carotid ultrasound evaluation is appropriate. However, before ordering such tests, at least listen to the patient's heart for a gross arrhythmia, check the blood pressure and listen for a carotid bruit. Even though diplopia is a sign of posterior circulation disease, that is no guarantee that the arteriosclerotic disease is limited only to that portion of the vascular system. Listening to carotid bruits may therefore be quite revealing. Another form of intermittent diplopia may be in a situation where the diplopia is in only one direction and therefore appears to occur intermittently. This is commonly seen in partial sixth and fourth nerve palsies. This may be apparent to the examiner in how the patient holds his or her head during the interview. The patient may not even be aware of a compensating head position.
Intermittent diplopia can result from the breakdown of a phoria. The clinical clue is that the muscle measurements are comitant, which suggests an old problem or a phoria converted to a tropia. This breakdown of a phoria can occur after head trauma, severe illness, or fever. How often have we heard about the measles settling in the patient's eyes and causing strabismus.
Patients often use the term double vision in such situations as convergent spasm and momentarily having blurred or double vision When refixing at distance. Students can have a similar complaint at near with prolonged reading, which is covered under the title of convergence insufficiency. At best this is a very obvious diagnosis with good history taking.
If the diplopia is constant, then ask Whether the images are vertically or horizontally separated. If the patient response is horizontal, then the diagnosis of the offending muscle is much easier. It represents either a lateral rectus or, less commonly, a medial rectus muscle paresis. If the medial rectus is an isolated problem, it rarely represents a partial third-nerve paresis. The latter is more commonly due to a myopathy such tea myasthenia gravis. It is less commonly due to thyroid disease, which primarily affects vertical muscles initially. The medial rectus palsy may also be due to a central mechanism such as internuclear ophthalmoplegia. In a similar circumstance, skew deviation represents the vertical counterpart of a central mechanism causing diplopia rather than a final common pathway.
Evaluation of vertical diplopia is much more complicated than that of horizontal diplopia. Invariably when patients are asked if the diplopia is horizontal or vertical, they answer that it is both or oblique in nature. This is because there are multiple muscles involved or an underlying horizontal phoria that is now revealed. The three-step test is the usual method of identifying which is the appropriate muscle. This test is described below.
Monocular Diplopia Examination (Fig. 6.3)
Figure 6.3 Monocular diplopia evaluation, part 2.
Once the question leads to a monocular form of blurred vision, it is not uncommon for the patient to have trouble realizing it any more specifically. The pinhole test is one of the easier tests to start the evaluation. If the pinhole test significantly improves the patient's vision, then a careful refraction is the next test to perform. Since refractive errors are very common and usually not pathologic causes of blurred vision, it frequently solves the problem without further workup. If the test is performed at a phoropter, the solution may not be the refraction but rather the relative pinhole effect and constriction of the field resulting from looking through a small aperture. This can be avoided by transferring the prescription from the phoropter to a trial lens arrangement where the constrictive field is not a compensatory factor.
A true pinhole test can be clone with a single or multiple pinhole technique. Multiple-hole technique is usually easier for the new patient. Even with the multiple pinhole test, some patients cannot perform very well and give an answer as though it was a negative response. Decreased vision from refractive disease is improved with a pinhole, while macular disease is the same or usually worse. Nerve disease usually remains about the same with or without the pinhole.
The Amsler grid is a valuable test for showing macular distortion to a patient, as is illustrated in Chapter 17 (Figs. 17.3, 17.4, 17.5). Show these two illustrations to patients who have difficulty in interpreting what they see. It is also helpful to let patients with a monocular complaint compare the one normal eye with the abnormal eye. A response of distortion usually indicates macular disease, but cataracts such as irregular cortical spokes or posterior subcapsular opacities can cause distortion. Small changes in the patient's lens may not be impressive on ophthalmoscopy or even on biomicroscopy. An irregular reflex on retinoscopy may be more suggestive of the problem. Small changes can be especially bothersome in irregular corneal astigmatism such as is seen with keratoconus.
The patient's blurred vision complaint may be a poorly expressed description of a field defect. These defects are usually paracentral and may be vascular, edematous, inflammatory, or hemorrhagic and found on a careful fundus examination. If the defects are the same in both eyes, an occipital lobe location may be the source of the problem. Glaucoma defects are usually negative scotomas and not noticed by patients. However, when an arcuate scotoma comes very close or directly into fixation, the patient will report an acute complaint. The central involvement is usually a late sign of glaucoma. When this event occurs, you expect to see advance cupping, which would be the clue to the diagnosis. In low-pressure glaucoma, central defects occur early, and as such, cupping is not as obvious.
Subtle macular disease, such as central tivrous retinopathy or retinal epithelial detachment, is best seen with a fundus contact lens or a 90 diopter lens.
Pupil examination is important to help differentiate optic nerve disease from other causes. This is particularly helpful when the cause for the reported decrease in vision is not obvious during the examination. The possibility of functional disease comes to mind as part of the differential diagnosis when no obvious cause is found. However, relative afferent pupillary defect removes that diagnosis from consideration and identifies the optic nerve as the source of the decrease in vision.
Binocular Diplopia Examination (Figs. 6.4, 6.5)
Figure 6.4 Binocular diplopia evaluation, part 1.
Figure 6.5. Binocular diplopia evaluation, part 2.
In a case of true binocular diplopia, instruct the patient to move the eyes in the cardinal directions of gaze. Restriction of an eye In one of these cardinal directions will usually identify the affected muscle. The investigation from this point takes two directions. The first test is to measure the horizontal and vertical deviations in the cardinal fields of gaze. In the face of a large horizontal deviation, a small hyperdeviation may be missed unless measurements are made. This is even more important to define when the ductions tire full and the affected muscle is not obvious. If the eye restriction is obvious, then you need to differentiate a neuropathic from a inyopathic or restrictive cause. A restrictive cause is best diagnosed by a forced duction test. Adequate topical anesthesia is applied to the eye and a tooth forceps is used to move the affected eye in the direction it does not move voluntarily. The best area to pick up with the forceps is next to the limbus. Farther away from the limbus, the conjunctiva is loose and can be moved without moving the eye. The test conclusion in that situation would be inconclusive. Did the eye not move because of restriction or because the loose conjunctiva just moved and didn't create enough force to move the globe? I find that the usual topical anesthetics are not enough for total anesthesia in the perilimbal area to have the patient relax during the test. Since I am a neuroophthalmologist, I have cocaine in the office, which at one time I used for doing pupil testing. I don't use it for that purpose anymore, but I find it useful in forced duction tests. Put the cocaine on a cotton-tip applicator and apply it to the area of the conjunctiva that you plan to pick up with the forceps. This technique reduces the amount of cocaine absorbed by the patient and lessens any serious allergic reaction. It is always best to put a drop of any other topical anesthetic in the eye before applying the cocaine, since cocaine is more uncomfortable before total anesthesia occurs. An adequate forced duction test is one in which you can move the eye freely or there is obvious resistance. The question to be asked is whether the resistance is from a primary orbital or muscular process or if the patient is resisting. The way to differentiate is to look at the normal moving eye for reference. When the patient has a problem moving the left eye out totally, observe the right eye on left gaze. If there is restriction of left eye movement but the right eye is in full adduction, this restriction of the left eye is real. If the patient does not move the right eye, then the restriction may be from lack of cooperation and not from a restriction. The technique of using an anesthetic coated cotton-tip applicator for moving the eye is adequate if the eye moves freely. It is difficult to evaluate the results if there is limited movement, since the applicator has no firm grip on the ocular tissues. As a first attempt, it is worth doing if the eyes move freely, and it is certainly less traumatic than using forceps.
If the ductions are normal and there is still double vision, the cover test is the first step toward the solution. The alternate-cover test determines whether there is any muscle imbalance. This test can be misleading if the examiner is not wary. If there is a phoria or tropia, you would expect to see some corrective movement of the eye as it comes out from under the cover. However, a patient who is not alert or paying attention may not move the recently uncovered eye and bring fusion into play. This gives the false impression of no muscle imbalance. There are two ways to avoid this mistake. If the test is being done with gaze at distance, have the patient look at a series of letters and read them as each eye is uncovered. When the cover is moved from one eye to the other, the patient must move the newly uncovered eye toward the next letter on the screen to see and name it. If the patient reports the correct letter and did not move the newly uncovered eye to see it, there is no phoria or tropia. The patient has to move that eye to pick it up if he or she has a phoria or tropia. When the test is done at near, use a light or small object for the patient to fix on or maybe even his or her own finger. Before moving the cover from one eye to the other, move the object of regard back and forth slowly. Observe whether the patient is following it as instructed. You then stop the movement of the fixation object. This is followed immediately by moving the cover over the other eye and observing any corrected eye movement. This alternate-cover test merely shows a muscle imbalance and does not reveal whether it is a phoria or a tropia.
The cover-uncover test is used to differentiate a phoria from a tropia. A phoria is a deviation of the eyes that is kept under control by the fusional mechanism. It becomes manifest when fusion is interfered with, as during the alternate-cover test. The tropia is a manifest deviation that fusion can't overcome. The cover-uncover test reveals whether the deviation found on the alternate-cover test is a phoria or a tropia. When the cover is taken off one eye rather than switched over to the other eye, there are several responses possible. If there is no movement from either eye, there may be no phoria or tropia. However, we noticed some movement in the alternate-cover test. The hick of eye movement now means that the deviation is a tropia and that the eye originally not behind the cover is still the fixing eye. Confirmation is seen when the cover is moved onto the other fixing eye without interfering with the potential fusional mechanism. If you move the cover onto the second eye by moving directly to the other eye, you always have one eye covered and never allow fusion to work if it can. To prove it is a tropia, take the cover off the first eye and then cover the other eye, leaving an interval for fusion to work when both eyes are uncovered. Now the second eye picks up fixation while the first eye, which was originally fixing, moves in or out or up or down the exact amount the fixing eye moves to pick up fixation.
An example will help to illustrate this test, The alternate-cover test reveals an esophoria or tropia defect. Cover the left eye and the patient must fix with the right eye. Remove the cover and the right eye continues to fix. The left eye, now uncovered, moves in or moves out to align itself with the steady fixing right eye. Uncovering the second eye allows it to move in an appropriate direction and to align itself with the other, which does not move. It proves that fusion plays an important role in aligning the eyes and that the deviation is an esophorla. If there is no movement in the left eye when it is uncovered, then it is a tropia, and the right eye remains the fixing eye. The alternative response is that as it is uncovered, the left eye moves out to pick up fixation, and the right eye that was fixing moves in the exact amount that the left eye moves out to pick up fixation. That deviation is called a tropia because fusion did not overcome it. No matter what the response, it should be repeated several times to be sure of it. It should be done in the cardinal directions, since the deviation may be in only one field of gaze and not the other. Any abnormal or compensating head position should be corrected during these measurements.
Once the deviation is identified as a tropia, then it is easier to switch back to an alternate-cover test when doing it with prisms to measure the extent of the deviation. As the alternate-cover test is repeated, put progressively stronger prisms in front of either eye to compensate for the deviation. The prism is placed so that the apex of the prism points in the direction of the deviation. When an adequate prism is used to compensate for the deviation, no further movement is seen on alternate-cover testing. The test is then performed in the cardinal directions to identify where the defect is the greatest. It is not uncommon in cyclovertical muscles to use compensating prisms in both a horizontal and a vertical direction. We can overcome a fair amount of muscle imbalance in the horizontal direction but very little in the vertical direction. If a patient has a large horizontal phoria, that may be the presenting picture to the examiner. When the examiner finds the horizontal deviation comitant, he or she is led to believe that the deviation is old. For instance, in the presence of a horizontal comitant deviation of 20 diopters, a small vertical tropia of 3 prism diopters can very easily be missed. It is this last vertical diplopia that caused the separation in the eyes and made the latent hori-zontal deviation so obvious. Since fusion cannot overcome the vertical deviation, it no longer can overcome the horizontal deviation either. An examiner who makes the determination on the basis of the comitant deviation will miss the real culprit, which is the new subtle vertical deviation.
The three-step test to identify the vertical muscle involved is outlined below. Sometimes the patient is confused with identical images when asked which one disappears when one eye is covered. If the images are far apart, sometimes the patient will ignore the other image and not be aware of diplopia. A red lens can be used over one eye to try to bring the patient's attention to the problem. The patient sees two objects in different colors and can identify two separate objects. Sometimes the red object is not seen. This may occur if the patient suppresses the image or if the red lens is in front of the poorer-seeing eye. In either case, switch the red lens to the other eye, and the patient may then appreciate a red and white image. If the diplopia is intermittent, it is appropriate to do away with the fusional mechanism while doing the mechanism measurements. The red lens can do this, but frequently the patient can force fusion even with a red lens in front of one eye. This intermittent complaint can be seen particularly in myasthenia gravis and may not be uncovered by the red lens test. The Maddox rod changes a single point of light into a bar. The two dissimilar images of a bar seen by one eye and a point of light presented to the other cannot be overcome by the fusional mechanism. Once the preliminary test identifies a tropia, the next logical step is identifying which muscle. This is done using the three-step test.
Identification of Muscles Involved
When it is determined that true diplopia is present, the offending muscle must be identified. To do this, examine each eye separately, instructing the patient to follow a hand light into all the cardinal fields of gaze (duction) while one eye is covered. If a muscle is severely affected, some limitation of movement will occur in one or more fields, which will facilitate identification of the offending muscle. If limitation is not obvious, the muscle is weak only in regard to maintaining fusion with the other eye (version). Since ductions are stronger than versions, muscular weakness affects fusion first.
Limitation of movement can be caused by structural changes in the muscles that prevent their stretching, such as a tumor mass or hemorrhage. Tumor or hemorrhage may cause diplopia for several reasons, including neuropathic or myopathic changes and displacement of the globe. Blowout fractures of the orbital floor with entrapment of the muscle tendons also produce diplopia.
The forced duction test as described above is a simple, easily performed, and useful diagnostic technique for evaluating limitation of ocular rotation and determining whether lack of innervation or some other condition is responsible.
Horizontal movement of the eye is accomplished by means of the medial rectus muscle, which is innervated by the third cranial nerve, and by means of the lateral rectus muscle, which is innervated by the sixth cranial nerve. The medial rectus muscle moves the eye toward the nose; the lateral rectus muscle turns the eye out. Diplopia involving these muscles is purely horizontal. When the medial rectus muscle is involved, the images are crossed; that is, the left image disappears when the right eye is covered.
Diplopia involving the lateral rectus muscle produces homonymous, or uncrossed, images and may be evident only when an attempt is made to look into the field of action of that muscle. Therefore, examination should include right and left as well as straight ahead gaze.
With right lateral rectus muscle weakness, diplopia may not occur in left gaze. However, an uncrossed diplopia will occur In right gaze, and the right image will disappear when the right eye is covered.
Four muscles produce vertical diplopia (Fig. 6.6). Three of these, the superior rectos and the inferior oblique muscles, which move the eye up, and the inferior rectus muscle, which moves it down, are controlled by the third cranial nerve. The fourth muscle, the superior oblique muscle, which moves the eye down, is innervated by the fourth cranial nerve.
Figure 6.6. Actions of cyclovertical muscles.
Unlike the medial and lateral rectus muscles, which have a single function, these muscles cause the eye to intort or extort and, in addition, assist the medial and lateral rectus muscles to move the eye medially or laterally. Because of these secondary functions, vertical diplopia is rarely just vertical; the images are usually obliquely located, at least to some degree (Figs. 6.7, 6.8).
Figure 6.7. Binocular vertical movements. (Courtesy of Dr. Caleb Gonzalez, Strabismus and Ocular Motility, Williams & Wilkins, 1984.)
Figure 6.8. Binocular torsional movements. (Courtesy of Dr. Caleb Gonzalez, Strabismus and Ocular Motility, Williams & Wilkins, 1984.)
The terms intorsion and extorsion refer to the rotation of the eye around an imaginary axis running from the posterior orbital apex through the center of the globe and out through the cornea when the head is tilted toward either shoulder. This movement keeps the visual axis parallel to the ground (Fig. 6.9).
Figure 6.9. Anatomic paths for torsional gaze. (Courtesy of Dr. Caleb Gonzalez, Strabismus and Ocular Motility, Williams & Wilkins, 1984.)
The principal function of these muscles is elevation or depression of the eye, primarily in one position. The superior oblique muscle, for instance, is a depressor only when the eye is adducted. When it is abducted, the muscle acts as an intorter, rotating the eye toward the nose. The primary vertical function of the superior and inferior rectus muscles takes place when the eye is abducted. The primary vertical action of the inferior oblique muscle occurs when the eye is adducted.
VERTICAL DIPLOPIA TESTING
Evaluation of vertical diplopia involves three steps.
STEP 1. Since four muscles in each eye may be implicated, it must first be determined in which eye the visual axis is deviated upward. The answer is easily ascertained by the cover-uncover test while the patient fixes on a light. If it is determined that the condition is a true tropia and not a phoria, one can use the alternate-cover test, which is easier and quicker to perform. As each eye is covered alternately, one will come up and the other come down to fix. The one that comes down is the higher or hypertropic, eye. Even if the other eye is suspected of being the affected eye, the eye that comes down to fix should be selected for this evaluation. From the eight muscles that could cause vertical diplopia, all the combinations of muscles that could cause the test results should be considered. In the higher eye, weak depressor muscles, such as the inferior rectus and superior oblique muscles, may be responsible. On the other hand, the elevator muscles in that eye can be eliminated, since only overactions secondary to weak antagonists—not primary muscle overactions—should be considered. Another possibility is that there are weak elevators in the hypotropic eye, which suggests the superior rectus and inferior oblique muscles. Possible muscle involvement has been reduced from eight to four muscles, two in each eye, and all different.
STEP 2. In step 2, the degree of diplopia in right and left gaze is examined. The vertical muscles function most strongly in one field of gaze. When they are weakened, the greatest vertical separation will be in that field. The vertical action of the superior and inferior oblique muscles occurs primarily in adduction. In abduction, the superior and inferior rectus muscles come into play. If the diplopia is worse in right gaze than in left gaze, select from muscles isolated in step 1. The muscle in each eye that would have the greatest vertical effect in right gaze should be selected.
At this point, it should be determined whether the two muscles chosen are intorters or extorters. If one is an extorter and one an intorter, the error is in muscle selection. Then the diagnostic steps must be checked.
STEP 3. The Bielschowsky head-tilt test (Fig. 6.10) is now used to determine torsional malfunction of the vertical muscles. Because of the partial or complete paresis of one muscle, extra innervation is given to it to help the muscle perform its torsional function.
Figure 6.10. Bielschowsky head-tilt test.
When the head is tilted to the left, the left eye intorts and the right eye extorts; when the head is tilted to the right, the right eye intorts and the left eye extorts. If the diplopia worsens in left head tilt, either a weak intorter in the left eye or a weak extorter in the right eye is responsible. The fact that both muscles chosen in step 2 were either intorters or extorters facilitates selection of the muscles to be considered in step 3. If both intorters were selected in step 2 and the diplopia is worse on left head tilt, the only intorters involved are those in the left eye. Since the two intorters selected in step 2 were in different eyes, the intorter in the left eye is the offending muscle.
To compensate for the double vision, we adjust the position of the head, face, and chin and place all three in a position in which fu-sion may be restored (Fig. 6.11). Depressing or elevating the chin compensates for vertical movement. Turning the face to the right or left compensates for a lack of abduction or adduction. Head tilting compensates for the tortional component. In the case of the medial and lateral rectus muscles, only the face turn is necessary, since there is no vertical or tuitional component. In theory, the head positioning should be universally adapted. It is not. In fact, it is most commonly seen with paresis of the lateral rectus and superior oblique muscles. Even then, patients may not adopt a compensatory head position or may oven assume a reverse head position. This latter type head positioning lets the eyes drift farther apart, since it may be easier to suppress widely separated images than to fuse Images in an uncomfortable position. Knowing the three functions of the vertically acting Muscles of elevation or depression, adduction or abduction, and incycloversion and excyloversion is enough to decide the correct head position.
Figure 6.11. Compensatory head positioning.
To bring this diagnostic method into focus, let us consider two hypothetical cases in which this type of testing would be standard procedure.