Binocular Vision &

Eye Muscle Surgery Qtrly°

response (48). A sudden disparity vergence stimuli,

induced by a change in fixation or by prism, results in an

initial fast fusional vergence response. Ogle & Prager

(43) pointed out the purpose of the slow adaptive

response is to eliminate the stress of the large demand

on the fast fusional vergence system. The reduction in

the demand on the fast system occurs as a result of a

negative feedback loop in the fusional system. An

increase in the slow adaptive system results in a decrease

in demand on the fast vergence system making it easier

for the fast system to respond to subsequent disparities

(49).

Slow vergence or vergence aftereffects, extensively

described by Schor (48) and Sethi (49), are not only

important to the normal person in reducing the

oculomotor error but are extremely important in the

DEX(T) and somewhat less important in the basic X(T).

This mechanism is most likely responsible for the

decrease in the apparent XT at near (both temporally

and spatially) where both enhanced stereopsis and

fusional detail result in sustained binocularity. Slow

vergence with its long time constant is most likely

responsible for alignment after blinking, thus eliminating

diplopia in patients with significant phorias, i.e., DEX(T)

(50).

Disruption or elimination of slow fusion or vergence

aftereffects may be achieved by sustained occlusion of

one eye, since the slow vergence receives its input

(negative feedback) information from the fast (disparity

driver) or fusional vergence system. Slow vergence or

vergence adaptation has been measured and found to be

either complete or incomplete. Sustained or repeated

occlusion results in a significant increase in the size and

temporal characteristics of the deviation in those pa-

tients who have strong slow vergence systems. Slow ver-

gence reduces the deviation in DEX(T) more at near,

since binocular stimuli, i.e., size, complexity and disparity

cues, are stronger, resulting in stronger disparity vergence

and slow vergence. Sethi & Henson (51) have shown

that the slow vergence system responds differently at dif-

ferent viewing distances to maintain a consistent oculo-

motor phoria. These vergence aftereffects tend to be lar-

ger at near and in downgaze. Slow vergence movements,

also, result in orthophorization with an antisometropic

prescription across the whole oculomotor field, i.e., with

induced spectacle prism there is no alteration in the

measured phoria in different position of gazes (51,52).

Sethi & Henson (51) postulated that there is a

cortical memory map for each position in the motor field

which is associated with a specific amount of innervation

to maintain binocular vision. In patients where the slow

adaptive vergence system is weak or incomplete, occlu-

sion will have a minimum effect on the angle of devia-

tion. Thus, one would predict that the more often Ihe

exotropic patient deviates, the greater the propensity of

the deviation to appear as a basic XT and the weaker

the slow vergence system. Inclusion of slow vergence into

a block design control system analysis as described by

Hung (53) and Cooper (50) is presented in Figure 5,

next page.

Scobee (54) in 1952 reported that 24 hours of

occlusion increases the near deviation in a substantial

Major Review: Intermittent Exotropia;

Basic and Divergence Excess Type

J.

Cooper, MS, OD and N. Medow, MD

Summer of 1993

Volume 8 (No.3): 185.216

Figure 3 (Cooper & Medow): Simnultaneous recordings with an infrared

optometer and eye movement monitor as DEX(T) changes fixation from a 1.5

D stimnulus to a 3.0 D stimnulus. Top three traces are from a true DEX(T),

e acc is the accommodative recording and e em is the eye movement position.

The bottomn three recordings are from a simnulated DEX(T); note the dynamic

overshoots present during response. (Reprinted with permission from cooper

J, Ciuffreda K, Kruger P: Stimulus and response AC/A ratios in intermittent

exotropia of the divergence excess type, Br J Ophthahnol 1982; 66:398-404.

Copyright 1982, BMJ,).

number of X(Y)s. Burian (10) used

this principle of occlusion to classify

DEX(T) patients into two groups,

simulated and true. He defined a

simulated DEX(T) as a DEX(T)

whose near deviation increased

after 30 minutes of occlusion so

that the distance and near devia-

tions approximated each other. In

other words, the deviation changed

from a DEX(T) to a basic X(1)

and the calculated distance near

AC/A decreased. But in reality

monocular occlusion eliminated

vergence aftereffects which

artificially altered the AC/A.

Burian & Franceschetti (24)

reported that the change in devi-

ation with occlusion occurred in

approximately 60% of the DEX(T)

population. The other 40% who are

unaffected by occlusion are known

as true DEX(T), i.e., distance/near

relationship is unaffected by

occlusion. Thus, distance/near

AC/AS are high before and after

occlusion in the true DEX(T).

Since both objective and gradient

AC/As in the true DEX(T) have

been shown to be normal, Cooper

et al (40) postulated that proximal

convergence is responsible for the

discrepancy between post occlusion

distance/near AC/A and gradient

AC/A in true DE The calculated

proximal convergence factor for the

true DEX(T) is 3.0/D (54), while

the normal person has a smaller

proximal vergence finding of

approximately 1.8/1 ±1.6 (55,56).

Ogle et al also found higher than

normal proximal convergence values

191