Binocular Vision &

Eye Muscle Surgery Qtrly°

Major Review: Intermittent Exotropia;

Basic and Divergence Excess Type

Summer of 1993

Volume 8 (No.3): 185-216

inability to maintain alignment. Thus, eye closure before exodeviation might occur to eliminate the need for ARC or the possibility of diplopia/ confusion or discomfort avoidance from the stimulus.

In any case, strong non-visual stimuli, such as talking, trigger exodeviation while strong visual stimuli, i.e., large stereo targets, eliminate exodeviation. Therefore, during cover testing, when attention is high, neither a manifest nor a latent exodeviation may be observed when one in fact exists. Exodeviation is easier to elicit with a muscle light than with an accommodative target. Binocular realignment is often obtained with a blink of the eyes which can occur secondary to appropriate stimulus presentation, i.e., stereo target or upon verbal command. The vergence response is quick and accurate.

Parents, unfortunately, often take this as a sign that the child is lazy or inattentive. Unlike esodeviations, a parental report of XT is probably ,nore valid than the examiner’s findings.

Ergographic measurements of both sustained and repeated measurements of convergence in X(T), according to Berens et al (74), do not show a decrease in the amplitude of convergence nor does sustained and repeated convergence produce symptoms over time. Convergence ranges in X(T), surprisingly, are normal when compared to an orthophoric patient.

If Sheard’s criterion is applied (where the demand should be equal to twice the reserve), then the average fusional amplitude of an X(T) should be at least 58 pd (2X the mean deviation of 29 pd). X(T) patients lacking these large fusional ranges can easily and readily be trained to have 80A fusional convergence. However, these enlarged fusional ranges only have minimal effects on the exodeviation.

Fusional ranges are larger with stereo targets. At near, stereo targets initiate realignment (from exodeviation). However, during reading or CRT (VDT) viewing, both stereopsis and sharp accommodative cues are eliminated, thus making it difficult for either the accommodative or the vergence system to respond appropriately. The result is often asthenopia.

Base-in prism or divergence fusional amplitudes are usually less than the angle of deviation (1,38). This difference might be due to the control system initiating a divergence movement. Accommodation

-vergence and fusional divergence may result in NRC (75). An exotropic deviation, however, is most likely initiated by

a supranuclear eye movement which is related to the version control system with resultant harmonious ARC. (See Morgan’s theory under sensory-motor findings, ARC p 198.) The difference in correspondence during different types of vergence movements provides further evidence that the X(l) may not be a result of either relaxation of convergence or loss of fusional divergence. Furthermore, if X(T)s were the result of a relaxation or failure of convergence, then one would expect the angle of deviation to be less than the fusional divergence amplitude. However, this is not the case.

Breinin & Moldaver (76) have performed electromyograpic studies which clearly demonstrated that the lateral rectus muscle has a maximum firing potential which occurs during exodeviation while the medial rectus muscle decreases its firing rate (at the same time). l’his finding is consistent with divergence being an active process, and not a relaxation of convergence.

Flax (77,78) has made the observation that X(Y)s will maintain binocularity when there is an advantage to being binocular (at near with stereopsis) and will often deviate when there is no real advantage to binocular alignment, as in distance viewing. The deviation often increases when fixation changes from 20 to 200 feet where stereoscopic vision becomes poor (70).

The first author, Cooper, (1) stated that during alignment X(T)s have normal binocular findings, i.e., they have normal stereoacuity, bifoveal fixation, and NRC.

Gross & von Noorden (79) recently reported that 60% of X(T) patients have poorer than 60 seconds of arc stereoacuity (i.e., less than normal limits). This is at odds with other reports and our previously noted observations.

Rosenbaum & Stathacopoulos (80) measured both near stereoacuity in X(Y)s with the Titmus and Randot Stereoacuity Tests; and distance stereoacuity with the “BVAT’ (Mentor’s CRT monitor acuity measuring system). They found an average near stereoacuity on the Titmus Test of 41.1 sec arc and 31.1 sec arc on the Randot Test. Distance stereoacuity findings were reduced compared to normals however.

This reduction in distance stereoacuity was likely due to the significant number of X(T)s deviating at distance when measurements were taken. For purposes of

calculation, they also recorded a failure to perceive stereopsis as “400 sec arc.” This would tend to raise the mean numerical value artificially. (The median value would have been a more valid statistical measure since this measure is not biased by the extremes.)

Common to many such studies, it should also be noted that since the smallest disparity target on the Titmus Test is 40 seconds of arc, and on the Randot Test, 20 seconds of arc, neither test provides a true measure of stereoacuity. The normal near mean stereoacuity for the general population is 20 sec arc SD ±10 sec arc. Therefore, these tests cannot accurately measure stereoacuity, since 50% of the population have a stereoacuity better than 40 sec arc, which is the limit of the Titmus Test.

Appreciation of random dot stereo- grams demonstrates that during times of normal binocular alignment there is bifoveal fixation (81).

Suppression, if present during normal binocular alignment, occurs only during dichoptic viewing (each eye sees a different image, e.g., physiological diplopia, bird and cage targets) of first degree targets. This suppression may be noted during cheiroscopic tracing and measurements of physiological diplopia. Pritchard & Flynn (82) have reported that 74% of X(T)s suppress physiological diplopia and 6% have intermittent suppression. Since normal binocularity is found with stereoscopic targets, and suppression is observed with first degree targets, sensory responding in this condition is stimulus mediated.

During exodeviation sensorial functioning changes dramatically. Fine stereopsis is lost (1). The patient may exhibit: NRC (83-85); an altered (adapted) projection value such as in harmonious or unharmonious ARC (86-91); or a lack of retinal correspondence (91,92). These binocular findings may be altered by the presence of regional or total suppressions. These suppressions may be shallow or deep.

X(T) patients, according to Cass (27) and Costenbader (12), have extension of the peripheral field of view which is known as panoramic viewing. Cooper & Feldman (86) used a translucent hemisphere to present peripheral visual stimuli, and an EOG to monitor eye position during normal alignment and exodeviation. They demonstrated that during deviation the exotrope has an extension of the binocular

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