Case 6.1 Paralysis of the Superior Oblique Muscle
In step 1, a patient complaining of diplopia is found to have the left eye higher on the alternate-cover test (Fig. 6.12). Possibly involved are the weak depressor muscles (the superior oblique and the inferior rectus muscles) in the left eye, or the weak elevators (the superior rectus and the Inferior oblique muscles) in the right eye.
In step 2, the diplopia is determined to be worse in right gaze. Of the four muscles selected in step 1, the only ones having maximum vertical action in right gaze are the left superior oblique and the right superior rectus muscles. Both are intorters.
In step 3, the diplopia is found to be worse on left head tilt, which brings into play the left eye intorters and the right eye extorters. Since only intorters were selected in step 2, malfunction of the left superior oblique muscle is indicated, because the other intorter in that eye (the left superior rectus muscle) was eliminated in step 1.
Figure 6.12. Three-step test for paralysis of the left superior oblique muscle.
Case 6.2 Paralysis of the Superior Rectus Muscle
In step 1, a patient with a vertical diplopia is found on alternate-cover testing to have the left eye higher (Fig. 6.13), The muscles involved must be either the weak elevators of the right eye (right superior rectus and right inferior oblique muscles) or the weak depressors of the left eye (the left superior oblique and left inferior rectus muscles).
Figure 6.13. Three-step test for paralysis of the right superior rectus muscle.
In step 2, the diplopia is determined to be worse in right gaze. Of the muscles selected in step 1, the only purely vertical muscles in right gaze are the left superior oblique and the right superior rectus, both of which are intorters.
In step 3, the diplopia is found to be worse on right head tilt, which involves the right eye intorters and the left eye extorters. Since only intorters were selected in step 2, the intorter of the right eye, the right superior rectus, must be the muscle at fault.
The foregoing cases illustrate the most common forms of isolated paralysis to which the three-step evaluation method is applicable. A similar program can be worked out for the inferior rectus and inferior oblique muscles. However, these muscles are more commonly involved in cases of local disorders of the orbit, such as inflammation, tumors, and blowout fractures of the floor; only rarely are they causative factors in isolated paralysis owing to a neuropathic process (Fig. 6.14).
Figure 6.14. Position of eyes in head-tilt position with paresis. (Courtesy of Dr. Caleb Gonzalez, Strabismus and Ocular Motility, & Wilkins, 1984.)
The diagnosis of vertical diplopia is not always easy. Not infrequently, more than one muscle may be involved, as in a partial third cranial nerve paralysis. Where diplopia is minimal, failure to make sure that the patient's head is in the correct erect straight-ahead position is a common error. A mild degree of vertical and oblique diplopia can be compensated for by unconscious head tilting on the part of the patient, which may obscure results. On the other hand, head tilting is much more common in superior oblique, than in superior rectus, muscle paralysis, and observation of abnormal head posture prior to formal testing may facilitate diagnosis.
The oblique character of diplopia tends to confuse the diagnostic program, and it will be totally misleading if it is simultaneously considered in the course of the evaluation. When both vertical and oblique separation of images occurs, only the vertical component should be considered. If so much horizontal separation exists that it is difficult or impossible to determine whether the vertical component is greater in left or right gaze or to evaluate the significance of the torsional component in step 3, the horizontal displacement should be removed by prism correction. The extent of this correction should be such that on testing, the vertical movement of the eyes is more apparent to both patient and observer. Such a correction is more easily done with loose prisms than with the prism bar, which is cumbersome to move correctly when testing in the various cardinal fields of gaze.
If the vertical diplopia is not of recent onset, steps 1 and 2 of the testing procedure will fall in line, but step 3 will usually be inconclusive, because the head-tilt test may not separate out the last two malfunctioning muscles. Smoothing out or development of comitancy is common and may come on soon after the onset of the initial paralysis. In such an event, an indication of which muscle is involved may be derived from the patient's history, which may reveal local trauma to one eye, associated ptosis with a superior rectus muscle weakness, or a previous third cranial nerve paralysis on one side.
With vertical diplopia testing, it is difficult to follow the three-step, diagnostic procedure while keeping the correct muscles in mind. Thus it is recommended that the three primary observations be made and noted but that the muscle selection for each step be deferred until the testing is completed. At that time, results based on primary observations can be checked and rechecked more simply and efficiently.
During the alternate-cover test, some cases of diplopia are not grossly obvious because the diplopia may be caused by minimal paralysis of the muscle or fusion may overcome or obscure the problem. In these cases, the red lens test can sometimes be of great help. When a red lens is put over one eye as the patient looks at the hand light in the extremes of gaze, he or she sees two images, because the red lens helps to break up fusion. The image as the patient sees it will be in a position opposite to that of the position of the eye. Thus, if the red lens is over the right eye and the patient sees the red image as the lower of the two images, the right eye is the higher eye in terms of the straight-ahead position.
If the patient maintains fusion even with the red lens in position, the Maddox rod, which changes a beam of light into a line by multiple prisms, should be used. With the Maddox rod, the patient sees a bar of light in one eye and a pinpoint of light in the other and obviously cannot fuse these two dissimilar images. If the rod is red, this feature further helps the patient to discriminate between the two objects. The Maddox rod can be rotated so that the line of light is vertical in one situation, thus allowing the patient to experience horizontal separation of images. The Maddox rod can then be rotated 90° to measure the vertical separation.
Torsion represents a special aspect of diplopia. Recognition of the presence of torsion can lead to more accurate diagnosis of fourth cranial nerve palsy, the most common cranial nerve palsy affecting ocular motility seen by the strabismologist. When taking the history of a patient with diplopia, complaint of spontaneous torsional diplopia should raise the suspicion that bilateral fourth nerve palsy may be present. Finding a "V" pattern and noting a left hypertropia on head tilt to the left and a right hypertropia on head tilt to the right confirms the presence of bilateral fourth-nerve palsy. Because this condition may be masked or partially masked, one should maintain suspicion when significant vertical diplopia symptoms including report of torsional diplopia are present, even if the head-tilt response is equivocal.
Spontaneous torsional diplopia also occurs with superior oblique myokymia. In this condition, the patient will observe a rhythmic or pulsating incyclodiplopia caused by repetitive, purposeless contraction of one of the superior oblique muscles. This condition is usually self-limiting. When symptoms persist, treatment consists of Tegretol therapy and surgery; simultaneous superior oblique tenectomy and inferior oblique myectomy has been advocated as a surgical treatment.
Torsion with or without spontaneous diplopia can be measured after dissociation is produced by placing a Maddox rod, one red and one white, in front of each eye. The rods are placed with the long axes vertical to produce a horizontal image of the fixation light on the retina. Either the patient or the examiner adjusts the axis of the rod until the two lines are seen to be parallel or superimposed; when this occurs, the line(s) from each light produced by the Maddox rod falls along the principal horizontal meridian of the retina. It may be necessary to rotate both lenses to produce parallel lines. The following is a rough rule of thumb for differential diagnosis of fourth-nerve palsy: (a) the clinical picture of fourth cranial nerve palsy including head tilt usually with a fuller face on the side opposite the direction of the tilt and noncomitant hypertropia but no cyclotropia on testing with the double Maddox rods, clear-cut history of trauma, and a vague time of onset indicates probable congenital superior oblique palsy; (b) the clinical picture of fourth-nerve palsy with a history of trauma or a sudden unexplained onset of torsion measuring less than 15° with the double Maddox rod but no subjective torsion probably is acquired superior oblique palsy; and (c) a history of vertical diplopia and head trauma in a patient with torsion measured at more than 15° with the Maddox rod and/or subjective torsional diplopia, a "V" pattern, and chin-down position indicates probable bilateral superior oblique palsy.
A quick method for differentiating superior oblique palsy from other vertical muscle palsies employs an extension of the Blelschowsky head-tilt test, which is carried out as follows:
- When a patient presents with a vertical tropia in the absence of restriction (free ductions in both eyes), the lateral versions are evaluated first. The adducted eye during the lateral version that produces greater vertical tropia points to the ipsilateral oblique muscle and the contralateral rectus muscle as the two potentially paretic muscles.
- The patient's head is then passively tilted 45° to the right and to the left. If the vertical deviation increases with the head tilted toward the higher eye, the oblique muscle is paretic; If the vertical deviation is greater with the head tilted toward the lower eye, the rectus muscle is paretic.
There are four vertically acting muscles in each eye. Two are elevators, the inferior oblique and the superior reclus, and two are depressors, the superior oblique and inferior rectus. By carefully following the above simple scheme, it is possible to isolate the single most likely vertically acting paretic muscle in just a few seconds. The clinical reality is that in almost every case when this test is applied, the superior oblique is paretic. When results are equivocal, a bilateral superior oblique palsy is often the most likely diagnosis. It is less common to find isolated paresis of one of the vertical recRis muscles or of the inferior oblique muscles, although these do occur and can be found on occasion. However, in most instances when the diagnosis is something other than a fourth-nerve palsy, some other contributing cause usually is present, such as prior surgery or trauma.
Paresis of the sixth cranial nerve may be more common in a neuro-ophthalmology practice than fourth cranial nerve palsy, which is seen primarily in a strabismus practice. Unilateral sixth-nerve palsy produces diplopia only when looking in or toward the field of action of the paretic muscle. Of course, the strabismus is an esodeviation, because the weakness is in an abducting muscle. In the very young, the differential diagnosis of this esodeviation includes essential infantile esotropia, which is usually comitant and does not produce diplopia. A more difficult differentiation includes type I Duane syndrome, which is actually a form of sixth-nerve palsy (sixth-nerve nuclear hypoplasia) with aberrancy of the third nerve in the orbit. Type I Duane syndrome usually has malalignment in the field of action of the underacting lateral rectus, restricted passive adduction, and narrowing of the fissure in adduction both in the involved eye. Most patients with type I Duane syndrome have good fusion with stereopsis when looking slightly away from the involved side. Patients with bilateral type I Duane syndrome may have straight eyes in the primary position with fusion or may have esotropia in the primary position. When doll's head or oculovestibular testing produces full abduction in any of the early-onset esodeviations, sixth-nerve palsy can be ruled out.
The diplopia of unilateral sixth-nerve palsy can be treated with a patch, preferably over the fixating eye, at least part of the time, or with a prism, when the angle is small. Nowadays, many ophthalmologists prefer to inject the antagonist medial rectus with 1.5 to 5 units of botulinum A toxin, usually under EMG control, in the acute phase (within 3 months of onset) to avoid contracture of the antagonist. This treatment is also used in conjunction with extraocular muscle transfer to avoid the need to recess the medial rectus muscle in chronic sixth-nerve palsy.
Third cranial nerve palsy falls into two main categories, congenital and acquired. The congenital variety causes no trouble with diplopia because of early compensatory sensorial adaptations. Acquired third-nerve palsy produces diplopia that is extremely difficult for the patient to overcome. Even in the absence of aberrant regeneration, the combination of muscles involved in the third-nerve palsy makes it difficult to find an area of single binocular vision. With aberrant regeneration this is virtually impossible. Suppression of the involved eye is without doubt the best solution to the diplopia of persistent acquired third-nerve palsy. If suppression is not achieved, occlusion with a patch or frosted lens or use of an opaque contact lens may be the treatment of choice.
Diplopia after Cataract Surgery
In recent years, an increasing number of patients have complained of diplopia after cataract surgery. Cataract surgery often is performed on only one eye, producing good acuity in an eye that had been unused for a long time. If fusional facility has broken clown, the potential for double vision definitely exists after monocular cataract surgery. Good vision with the second eye after cataract surgery also may uncover a preexisting mild, but bothersome, Grave's ophthalmopathy, which produces diplopia. However, in most cases, it is believed that a toxic or mechanical effect or both associated with injectable anesthetics used in the orbit produces myopathy leading to paresis and/or restriction. Whatever the cause or causes of diplopia after cataract surgery, this condition has become a greater problem recently. Some of these diplopia patients can be treated with prisms. If the deviation is large and noncomitant or if the patient does not wish to wear glasses, surgery can be done. These patients can be ideal candidates for adjustable suture surgery done with local anesthesia on an outpatient basis.
Diplopia with Mechanical Restriction
Various causes of mechanical restriction (e.g., thyroid myopathy, blowout fracture, Brown's syndrome, postretinal detachment repair, and orbital mass) can produce diplopia. In almost every case, this restriction produces a noncomitant strabismus. This type of noncomitant strabismus with mechanical restriction can be evaluated by means of passive duction testing and generated muscle force testing.
Generated muscle force testing can be done only with the awake patient. The eye is anesthetized in a manner similar to that for passive duction testing. The patient is first asked to look away from the direction of the field to be tested. The examiner grasps the eye at the limbus on the opposite side of the direction that the eye is to be moved. The patient is then asked to look gradually in the direction of the field to be tested while the examiner stabilizes the eye. Movement of the fellow untested eye into the field of action to be tested is evidence that the patient is putting forth neural input In the eye being tested. This' is a manifestation of Hering's law. A pull or tug on the forceps experienced by the examiner during this maneuver is evidence that generated muscle force is present in the tested muscle. Generated muscle force can also be measured in the face of restricted passive ductions by noting an increase in intraocular pressure when ocular movement is attempted in the restricted field. If there is no increase in intraocular pressure in the presence of restricted movement, decreased generated force can be inferred. Comparing saccadic velocity either by observation of eye movement or with use of an EOG with recording can identify decreased muscle contraction as evidenced by decreased saccadic velocity in the tested eye compared with saccades in the fellow eye.
Accurate appraisal of the state of active and passive eye movements can direct the ophthalmologist toward more appropriate treatment. For example, mechanical restriction must be relieved before other surgical straightening is undertaken. Also, resection of a "dead" muscle will not help movement In the field of action of that muscle. Instead, a muscle transfer may be indicated.