Binocular Vision &

Eye Muscle Surgery Qtrly°

normal average 60 mm (inter)pupillaiy distance (PD) and a 10 pd difference between distance and near (40 cm), the distance near AC/A has to be at least 10/1:

D/N AC/A = (PD cm +[XT - XT]/D)

D/N AC/A = (6 cm +[10]/2.5)= 10.

Brown (33) reported that the mean distance near AC/A in DEX(T) was 13/1. Similar findings have been reported by Jampolsky (34) and Parks (35). Basic XT’s have similar near distance measurements, and thus are assumed to have normal AC/A ratios, i.e., 6/1.

Clinical measurement of the distance/near AC/A is confounded by various

Major Review: Intermittent Exotropia; Basic and Divergence Excess Type

factors. It is often assumed that the accommodative response is the same as the stimulus to accommodation. For example, if a fixation target is placed at 40 cm a 2.5 D accommodative response is

assumed. But the accommodative response is usually less than the accommodative stimulus at near and more at distance (36). Variability in distance/near AC/A may also result from alterations in pupil size which is smaller at near, thus decreasing the demand for accommodation, the demand for an accommodative response and the resultant AC/A. On the other hand, proximal convergence and ver

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

gence aftereffects (prism adaptation, slow vergence response) may increase the AC/A (37) (See Section III C.)

Though these factors at first glance may seem to be inconsequential, Ogle & Dyer (38), von Noorden (39), Cooper et al (40), and Kushner (41) have all shown that stimulus-gradient AC/As in DEX(T) patients are not excessively high but are only slightly above normal (range 3.0-9.0). Figure 1 (below) presents stimulus-gradient AC/A ratios derived in a synoptophore and with a cover test. It is apparent that the gradient AC/As in DEX(Y), unlike normals, are not linear.

Figure 1 (Cooper & Medow): Gradient stimulus AC/A ratios were determined in 3 subjects with a synoptophore and at near

using alternate occlusion; i,e,. alternate cover test. These patients had a an average distance/near stimulus AC/A of 11/1; and an average objective AC/A of 5.9/1. Dashes represent repeat AC/A ratios 1 week later. It is readily apparent that the AC/A is not linear nor exactly repeatable for DEX(T). The mean derived gradient stimulus AC/A determined by a best fitting straight line was 5.0 in a 3ynoptophore and 7.5 with cover test. (Original, unpublished data)

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