Binocular Vision &

Eye Muscle Surgery Qtrly°

normal average 60 mm (inter)pupillaiy

distance (PD) and a 10 pd difference be-

tween distance and near (40 cm), the dis-

tance 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 dis-

tance/near AC/A is confounded by various

Major Review: Intermittent Exotropia;

Basic and Divergence Excess Type

J.

Cooper, MS, OD and N. Medow, MD

factors. It is often assumed that the

accommodative response is the same as

the stimulus to accommodation. For ex-

ample, 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 de-

creasing the demand for accommodation,

the demand for an accommodative re-

sponse 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) pa-

tients are not excessively high but are only

slightly above normal (range 3.0-9.0). Fig-

ure 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 nor-

mals, 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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