Binocular Vision &

Eye Muscle Surgery Qtrly°

The cover test should be measured while the patient views an accommodative target at 6 meters and at 40 cm. The average deviation when the patient fixates at 6M is 29A and at 40 cm it is 9 pd (31).

The distribution of angular measurements is leptokurvic resulting in few XTs being larger than 50 pd or smaller than 10 pd White (61) has noted that the greatest deviation occurs when viewing at 200 ft. Intermittent values occur while viewing at 60 ft.

Most X(l)s, who have never been treated, think that they know or “feel” when their eye is deviating or straight. However, casual observation demonstrates that their perception of alignment or deviation is inaccurate.

Ogle & Dyer (38) used fixation disparity methods to provide accurate information about oculomotor coordination while fusion was being maintained. According to their studies, the oculomotor balance obtained during dissociation testing, i.e., cover test, does not indicate true binocular imbalances. They reported that during fusion the oculomotor imbalances found in DEX(T) were often normal and/or the associated phoria was often an esophoria. In no case was the associated phoria close to the magnitude of a dissociated phoria/tropia.

Surgery, according to Ogle & Dyer (38) did not alter the fixation disparity curve but did affect the measurement obtained by dissociated means. They concluded that:

‘The tropia found by clinical disassociation tests are a manifestation of an innervation or oculomotor imbalance not present when fusion is maintained”.

Although most X(T)s demonstrate relative comitancy, Moore et al (62) reported about 24% of their patients had lateral gaze incomitances, i.e., limitation in abduction. According to Moore et al, those patients with lateral gaze incomitance were eight times more likely to have a surgical overcorrection.

Kushner (41), who cites Morton, states that only 5% have lateral incomitance. Furthermore, Repka & Arnoldi (63) suggest that many lateral incomitancies are due to errors in measurements from improper prism positioning.

A vertical deviation in the tropic eye has been found in 45-55% of all DEX(T)s during distance fixation (64,65). This vertical deviation is often similar to Bell’s phenomenon in that it does not occur upon initial deviation, but only after the

Major Review: Intermittent Exotropia; Basic and Divergence Excess Type

fully manifested deviation has occurred. Jampolsky (66) feels that the superior rectus (SR) muscle is a stronger elevating muscle than the inferior oblique (10) muscle. Thus, during manifest XT, the adducted SR might cause an elevation or hyperdeviation of the deviating eye. The incidence of overaction of the inferior oblique muscles (IOOA) in DEX(T), according to Davies (67), is 30%. Similar findings have been reported by Wilson & Parks (68) who noted that 32% of all X(T)s have IOOA which they first noted around 5 years of age. Davies (67) reported that the vertical deviation was more often found in distance testing and absent in near testing. Of the patients having a vertical deviation as identified by Davies, 62.8% had a primary vertical deviation without IOOA 32.5% had a primary vertical deviation with IOOA, while 4.6% had non-dissociated vertical deviation.

Most exotropic deviations are intermittent and alternators (of fixation). Alternation takes approximately 80 msec (69). This is approximately the same time for a large saccade.

Schlossman & Boruchoff (32) reported that 85% of all XTs are intermittent, 9% are constant alternators, and 6% are amblyopic. They were the first to use the term intermittent exotropia to include the majority of DEX(T)s and basic X(T)s.

The first author (1) has suggested that many of the previously classified constant, alternating or minimally amblyopic XTs were most likely intermittent if examined carefully. One must note the position of the eyes before disassociating with a cover test, since an X(T) may be inadvertently broken down by occluding the eye during visual acuity measurements. They will be incorrectly classified as a constant XT.

Amblyopia associated with X(Y) is usually secondary to anisometropia (not strahismu.s) and minimal in amount (65).

The deviation is usually more latent at near than at distance. Burian & Smith (70) reported that the angle of deviation increases by 25% when changing fixation from 20 to 100 feet. Also, approximately 10% dramatically change their temporal characteristic, e.g., become constant. The difference in frequency noted between distance and near fixation has been ascribed to: high AC/A, proximal convergence, slow vergence (vergence aftereffects), larger retinal disparity, enhanced binocular visual acuity and/or greater angle subtended by fusional stimuli (1). As previously mentioned, the AC/A probably has minimal

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

effect on either the magnitude or frequency of the deviation.

The deviation can be triggered by a myriad of situations and stimuli, such as inattention, daydreaming, distance viewing, fatigue, ifiness or bright sunlight. Deviating in sunlight, resulting in closure of an eye is so common in XT, that any child with a history of monocular closure of an eye must be assumed to have an XT until otherwise proven.

This photophobia from a photic stimulus leading to eye closure has been assumed to be due to a dazzling of the retina so that fusion is somehow lost (17). Wirtshafter & von Noorden (71) have shown that light adversely affects fusional convergence in X(T). Additionally, monocular closure has been induced by artificially increasing illumination. Eustace & Wesson (17) postulated that sunlight causes an X(1) since, statistically, the closer to the equator one lives the greater the incidence of X(T). Romano (18) has suggested that elimination of sunlight by hats with brims might eliminate or reduce the deviation. Wang & Chiyssanthau (72) investigated the relationship between anomalous retinal correspondence (ARC)! normal retinal correspondence (NRC) and monocular eye closure. Monocular closure in X(T) occurred in 90% of the patients with NRC and only 35% of the patients with ARC. They concluded that this is evidence that eye closure is performed to avoid diplopia and confusion. However, this is conjecture since DEX(T) patients with NRC do not complain of diplopia or confusion. (Also, high correlations do not prove cause and effect.)

Recently Wiggins & von Noorden (73) evaluated the report of diplopia as the reason for monocular eye closure in DEX(T). They also video-recorded eye closure responses to bright light in an attempt to determine if the deviation occurred prior to closure of the eye. They reported that closure occurs before deviation and that no subject perceived diplopia. Contrary to the report of Wang & Chryssanthau (72), they found no relationship between retinal correspondence and eye closure. Wiggins & von Noorden also did not find any effect on fusional convergence with increased illumination. Wiggins & von Noorden (73) concludeth “No satisfactory explanation can be offered at this time to explain the high prevalence of monocular eye closure in intermittent exotropia”.

An alternate explanation might be that bright light decreases contrast and therefore fusional detail resulting in an

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