Binocular Vision &

Eye Musck Surgery Qtrly°

Major Review: Intermittent Erotropia;

Basic and Divergence Excess Type

Summer of 1993 Volume 8 (No.3): 185-216

may be considered a normal finding up to the age of 6 mos (119). After that age any X(T) is considered indicative of a future strabismus and should be watched carefully. X(T) has a predilection for blacks and women. Unlike El’, X(T) is rarely found after systemic illness.

Besides the early appearance, strong support for X(T) being a result of a genetic anomaly comes from hereditary patterns reported by Jampolsky (22), Parks (108), Posner (117), and Binion (120). Knapp (121) reported a family history in 28% of the X(T) cases and Burian & Spivey (122) in 21.5% of the cases. Knapp (83) further states that the percentage of family related X(T) would be higher if X(T) was more accurately studied: many X(T) cases go unnoticed or unreported due to the infrequency of the XT

Previous theories have not explained all or even many of the sensory and motor findings of DEX(T). The first author has presented and published a theory which attempts to include most of the sensory-motor findings in X(T)(1):

Chavasse (123) noted that during the course of evolution from vertebrates to primates, the eyes moved from a lateral position to a frontal position. He suggested that the physical position of the eyes was related to either protection or aggression. Protective mechanisms were dependent on the largest possible total field of vision while the aggressive reflexes were initiated by binocular fixation of the prey. Chavasse went on to say: ‘The disadvantages of having the total field of vision limited [to achieve maximum binocularity] must have been outweighed by other factors affording enhanced protection, such as the acquisition of neck movement, especially in the upright posture, the reduction of the chance of attack from behind by the assumption of arbored life, the use of the hands both for protection and aggression (p.25)’

Walls (124) and Prince (125) also noted that the position of the eyes, i.e., frontal or lateral, was not based on phylogeny but on predation. Those animals that hunt have frontal placement of the eyes with overlapping binocular visual fields to improve stereopsis, while hunted animals have lat eral placement of the eyes to increase peripheral vision and motion detection. This larger teleologic sensing device is important in detecting danger: for example, the rabbit has almost a full 360° field of vision while man has 180° field of

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vision. Prince (125)) statea tnat animals involved with manual manipulation need to have frontal placement of the eyes for improved stereopsis.

Non-predaceous mammals high on the evolutionary scale survive on the basis of manipulation. Both manipulation and predation require precise spatial localization as is obtained with stereoscopic vision. The evolution of man, which favors manipulation to make tools, has eliminated the need for panoramic viewing preferring overlapping fields of view which give rise to stereopsis.

DEX(T) seems to be a functional compromise between the two visual systems. They have both stereoscopic vision when it is advantageous and panoramic viewing during exodeviation when there are few stereoscopic cues present (1). Both stereopsis and panoramic viewing are thus attainable in the same species.

During near vision, where retinal disparity cues are plentiful and the stimuli have a lot of detail, the eyes are aligned. An experienced clinician will attest to the fact that DEX(T) patients rarely deviate during stereopsis testing, i.e., Titmus Stereo Test. Their visual systems can identify disparity targets which elicit bifoveal fixation (1). Bifoveal fixation eliminates disparity vergence and stimulates the slow vergence system (vergence aftereffect) through a feedback loop proposed by Schor (45,46,48). Sustained utilization of slow vergence in simulated DEX(T) reduces the load on the disparity vergence system and thus reduces the apparent phoria.

True DEX(T) has a weaker slow vergence system and substitutes abnormally strong proximal convergence which eliminates the need for sustained fusional vergence. Only a small percentage of true DEX(T) cases, who have normal proximal vergence and vergence adaptation, use their high AC/A to reduce the near deviation. Basic XT’s have normal stereopsis at near and also align normally with appropriate stimuli. However, since slow vergence is less effective in this population, they are more apt not to align binocularly under ideal stimulus conditions, i.e., stereo- acuity.

Orthoptic treatment emphasizing convergence amplitudes increases vergence adaptation and results in many basic XTs becoming DEXçI)s, i.e., the near deviation decreases in size and amount of time exodeviated.

The system is spatially dependent (1) relying on both proximal (near) and stereoscopic cues. If motor fusion is

measured in DEX(T) using flat fusion targets, intermittent suppression with poor fusion ranges is usually noted. If simultaneous perception targets are presented, exodeviation often occurs with resultant suppression and/or ARC. On the other hand, if digital manipulation, proprioception, and/or attention are utilized by the DEX(T) patient, binocular alignment ensues. These findings are the opposite of ET which responds more vigorously to first degree targets.

Upon distance viewing or during relatively passive viewing there is no need for stereoscopic information, thus an eye exodeviates. During exodeviation there is an increase in the binocular field of view equal to the angle of deviation and fine stereopsis is lost. The system during exodeviation has a rudimentary binocular system evidenced by measurement of a horopter (93). Specific sensorial binocular functioning in X(Y) during deviation is controversial.

It seems that DEX(T) has a dual retinal correspondence system; ARC when deviated, and NRC when straight. Retinal correspondence varies with motor position so that the angle of anomaly covaries with the exodeviation. ARC also changes with stimulus conditions. Morgan (103) has shown that this alteration between ARC and NRC might be explained on the basis of motor induced sensory changes resulting in covariance.

Treatment for X(T) may be divided into surgical and orthoptic (i.e., non- surgical). Which form of therapy is advised is biased by different academic training based on different bodies of information or differing interpretations of same and the different treatment capabilities and options imposed by education, training and licensing authorities.

Ophthalmologists, in the United States, u.sing a medical model, are biased towards pharmaceutical and surgical intervention.

Orthoptists advocate both orthoptics and/or surgery. Orthoptists tend to employ mostly home-based orthoptics.

Optometrists tend to use in office orthoptic therapy supplemented with home orthoptic therapy.

None of the therapeutic regimens used have undergone the scrutiny of a prospective doubly masked clinical trial.

Additionally, specific techniques used in