Binocular Vision &

Eye Muscle Surgery Qtry©

Some X(T)s have a third type of sensory adaptation mechanism in which there is an absence of retinal correspondence during exodeviation. Each eye acts independently without any spatial communication, as if there were two separate worlds. For example, the manifest X(T), when viewing dissimilar right and left targets through a stereoscope, will see both the right and left pictures at the same time without being able to spatially relate them. Depending on stimulus conditions, the X(T) changes from NRC, to ARC, to a lack of correspondence. Triggering mechanisms are stimulus dependent.

The above findings are important in understanding why the X(T) patient deviates whenever stereoscopic information or visual attention is lacking. At near, where stereopsis is more important and associated with attention, proprioception, and touch, exodeviation is expected to occur less frequently. Most deviating X(T)s will align their eyes when presented stereoscopic stimuli such as the Titmus Stereo Test, Random Dot Stereogram, or Straw and Pointer Test (stereoacuity is normal).

The habitual use of disparity-driven vergence to maintain fusion causes the feedback loop to slow vergence to reduce the deviation, keeping the deviation latent in the simulated DEX(T), while proximal convergence reduces the deviation in the true DEX(T). If there is a loss of visual attention, or if fixation changes to distance where there are fewer stereoscopic cues to elicit binocular alignment, exodeviation will ensue. During exodeviation motor changes occur which result in changes in egocentric localization, i.e., ARC.

During exodeviation there is an expansion of the binocular field of view known as panoramic viewing. Panoramic viewing extends the peripheral field of view and the motion detection system of the eyes. With appropriate visual stimuli, touch or attention, orthotropic realignment ensues with resultant NRC.

An anatomical theory of etiology was first proposed by Bielchowsky (8). He maintained that the deviation was due to anomalous positions of the orbits. Parks (108) has suggested that the divergent position is congenital. In support of the anatomical theory, Knapp (109) noted that XT is often associated with craniofacial anomalies such as hypertelorism and

Major Review: Intermittent Exotropia;

Basic and Divergence Excess Type

oxycephaly where the (Inter)pupillary distance (PD) is very large. However, wide PDs should result in a relative convergence insufficiency type XT. Additionally, there is no data to suggest that X(T) patients have more divergent orbital axes (than normals do).

Mechanical theories based upon improper insertions or adhesions of the extraocular muscles have been advocated. However, there have been no systematic studies supporting this theory. Additionally, transposition studies performed on monkeys do not support either a mechanical or anatomical theory of etiology (110,111). In monkeys an XT cannot be created unless the medial rectus muscle is disinserted without spontaneous reinsertion (112). Numerous transposition studies have demonstrated that superior and lateral recti mu.scles can be transposed with normal concomitant horizontal movement restored in a few days (110,111). These studies place both mechanical and anatomical theories on tenuous ground.

Duane’s original concept of active divergence has electrophysiological support. Blodi & Van Allan (113) have demonstrated that when an XT starts to manifest itself, the lateral rectus muscle of the deviating eye begins to fire. According to Brenin & Moldaver (114), the lateral rectus continues to fire while the eye is divergent, having its maximum potential during divergence. Active divergence is also supported by the fact that the angle of deviation is outside the fusional divergence range (38). This finding is unexplainable if divergence were due to a relaxation of convergence.

Tambler, Jampolsky & Marg (115) believe that X(T) is a result of relaxation of convergence. They state that: “Our binocular electromyograms of the intermittent exotropia are similar in every detail to the known diminution of convergence in the normal individual”. The fact that an X(T) patient assumes a position similar to Bell’s phenomenon found when the eyes close during sleep or death further support a theory of relaxation.

Jampolsky (34) and Parks (35,108) have suggested that DEX(T) is a result of a high AC/A ratio. By clinical definition, the use of distance-near findings to calculate an AC/A guarantees a minimum AC/A of 10/1. However, as previou.sly discussed in

Summer of 1993

Volume 8 (No.3): 185-216

the section on AC/A, gradient AC/As in DEX(l) are within normal to high normal limits. Objective AC/A findings are essentially normaL If AC/A ratios are high, then lateral rectus muscle recessions should result in postsurgical ET at near. Also, high AC/A accommodative induced convergence, as reported by Seaber (116), should result in sustained blur whenever DEX(Y) patients move their eyes from a deviated position to alignment. This is a rare occurrence. Lastly, AC/A models do not explain the initial distance deviation.

Posner (117) observed that phylogeny and ontogeny demonstrate a migration of the visual axes from the lateral to the frontal position while oculomotor control has moved from the midbrain to the cortex. According to Posner, manifest XT represents a loss of cortical decebralization which is an atavistic trait. Though this theory is conjecture, it is consistent with the binocular functioning of the chameleon, which is a naturally occurring X(T). (See further below.)

Jampolsky (22,23,34) and Knapp (83,94) assumed a simplistic approach to differentiate X(l) from exophoria using suppression. Jampolsky (22) states: “Intermittent exotropia evolves from an exophoria as a result of the development of hemiretinal suppression during visual infancy”. This statement assumes that X(T) has NRC when deviated. Though there is evidence of suppression with NRC, the bulk of studies suggest ARC. There is no scientific evidence to support hemiretinal suppression during fusion. If hemiretinal suppression occurs with ARC, the X(T) patient would be living in a world of confusion. Objects temporal to the fovea of the deviating eye would be projected almost straight ahead.

Many authors feel that X(l) is a genetic anomaly. In Costenbader’s study of 412 patients with X(T) 204 were present at birth, 88 appeared before one year of age, 196 appeared between age one and five years, and only 24 appeared after five years of age (12). Similar findings have been reported by Kzystkowa & Pajakowa (28). However, Hall (118) found only 37% of X(T) occurs before the age of 2 years.

More recent studies suggest that X(T)

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