Binocular Vision &

Summer of 1993

Eye Muscle Surgeiy Qtrly°

Volume 8 (No.3): 185-216

Ogle & Dyer (38) used fixation disparity methods to

calculate stimulus AC/As. They reported an average

AC/A of 3-4/1 in the DEX(T) (see Figure 2, right).

Interestingly, fixation disparity measured AC/As do not

change significantly after surgery while distance near

stimulus AC/AS do. Cooper, Ciuffreda & Kruger (40)

simultaneously recorded accommodation and vergence

responses using an infrared optometer and an infrared

eye movement monitor. From their recordings they

calculated response AC/As in both true and simulated

DEX(T) patients. They found the response AC/A ratios

to range from normal to slightly high. The mean

response AC/A was 5.9/1 (with a range 4.5-8/1) (see

Figure 3, top, next page >>>). Furthermore, they

found no difference between response AC/As for

simulated and true DEX(l’). They postulated that

proximal convergence in true DEX(T) and vergence

aftereffects (slow vergence) in simulated DEX(T) were

responsible for the discrepancy between distance/near

AC/A ratios and gradient AC/A ratios in these patients.

2.

Results from occlusion:

Any discussion of the effects of occlusion on X(T)s

must address the concept of vergence aftereffect.

Vergence aftereffects or slow vergence are a temporary

change in oculomotor position which result from any

sustained fusional induced vergence (42). For example,

Ogle & Pragen (43) have shown that if 6 diopters of

vertical prism is slowly added with a Risley prism, e.g.,

1 pd per minute, in front of some subject’s eyes and those

subjects completely adapt, both cover testing and forced

fixation disparity curve before and while wearing the

prism will be identical, e.g., orthophoria with and without

the prism. Significant adaptation to the prism occurs

within a few minutes. Abrupt removal of the prism

usually results in diplopia with a slow recovery of fusion.

The rate of recovery depends upon the duration of the

vergence stimulus (44). Recovery occurs when the

original phoria is attained (see Figure 4, right below).

This phenomenon, described as vergence or prism

adaptation, may occur if an orthophoric patient wears

horizontal prism, such as 10 pd base Out, for an hour.

Complete vergence adaptation occurs when cover testing

with and without prism does not change, i.e. orthophoria.

Upon immediate removal of the prism, cover testing will

reveal a moderate esophoria.

These findings demonstrate that elimination of

fusional impulses by occlusion, such as cover testing does

not result in an immediate loss of vergence related

impulses. The slow change in oculomotor position over

time secondary to disruption of fusional impulses has

been called prism adaptation, vergence aftereffect, or

slow vergence (45). These vergence aftereffects are time

dependent and receive their input from the fast, fusional,

disparity vergence system (46). This is supported by the

observation that the amount of diplopia, which is induced

by vergence aftereffects, is not affected by either

darkness or sleep (47). The position that the eyes are in

just prior to going to sleep is the same position that one

awakes with. Thus, the oculomotor position of the eyes

is not altered by darkness or sleep.

The total disparity induced vergence response is made

up of the sum of the fast fusional and slow adaptive

190

Major Review: Intermittent Exotropia;

Basic and Divergence Excess Type

J.

Cooper, MS, OD and N. Medow, MD

Figure 2 (Cooper & Medow): Typical fixation disparity curve at near and far,

and derived AC/A ratios before and after surgery for DEX(T). The presurgical

measurements were 30t pd X(T) and 14A X’ (AC/A = 12/1). The fixation

disparity curve before and after surgery was not changed. The derived fixation

disparity AC/A was 5/1 both before and after surgery. (‘Reprinted with

permission from Ogle K Dyer TA: Observations on Intermittent Exotropia

Arch Ophthalmol 1965; 73:58-73. Copyright A,n Med Assn 1965).

Figure 4 (Cooper & Medow): Eye movement recordings for three different

subjects demonstrating the slow decay of signals after occlusion. At point A

a step vergence stimulus is introduced, i,e,. wedge prism. All three subjects

demonstrate a rapid fast fusional response to eliminate disparity (closed loop).

At B 15 seconds and B 160 seconds the loop is reopened by occlusion. The

rate of relaxation of fusional vergence varies from individual to individual

(RC greatest; RK least) and with the time of the adapting prism, 4e,. 15

seconds versus 160 seconds. (Reprinted from Vision Research Vol. 19, Schor

CM The relationship between fusional vergence eye movement and fixation

disparity 1359-67, copyright 1979 with permission from Pergamnon Press Ltd.,

Headingham Hill Hall, Oxford (X30BW, UK).