usually larger than the amount measured with the

unilateral cover test. The former is a result of the

sum of the fast and slow fusional vergence system,

whereas the latter is a measurement of the fast

fusion system. Patients who show strong vergence

adaptation and have high latent phorias will in-

crease the angle of deviation with prolonged, re-

peated alternate cover testing’, patients with large

phorias and weak slow vergence systems will not.

Thus, patients with robust vergence adaption are

less likely to be symptomatic, inasmuch as the slow

fusional vergence system eliminates the constant

demand on the fast fusional vergence system.

Burian and Smith 20 noted that both intermittent

exotropes and normals often show significant in-

crease in the measurement of their deviations when

fixation changes from 6 to 30 m (20 to 100 ft). At

greater viewing distances there are fewer stereo-

scopic cues and less fusible detail, thus decreasing

reflex fusional vergence and subsequent slow fu-

sional vergence. This predictably results in a larger

deviation at a farther distance.

Prolonged Occlusion

Marlow 21 popularized a technique of prolonged

occlusion to uncover a latent heterophoria. He used

prolonged occlusion when traditional methods of

refractive or prismatic correction failed to provide

relief of asthenopia. The test consisted of prolonged

occlusion followed by. measurement of a phoria with

a Maddox rod. Care was taken not to allow any

opportunity for fusion during testing.

Beisbarth22 studied 29 patients with prolonged

occlusion (1 h to 9 days). Thirteen of 16 eyes with

preocciusion hyperdeviation showed a significant

increase in their deviation, 5 of 8 orthophoric pa-

tients demonstrated a postocclusion hyperphoria;

and the final 5 showed minimal changes with occlu-

sion. Beisbarth suggested that prismatic correction

based upon prolonged occlusion would result in

overcorrection of most vertical deviations. Pro-

longed occlusion eliminates vergence aftereffects,

resulting in an increase in both the number of

patients with a vertical deviation and in the

amount. However, robust vergence aftereffects

eliminate the load on the fast fusion system and

reduce asthenopia and diplopia. There is contro-

versy whether prolonged occlusion produces a true

deviation or just an exaggeration of Bell’s phenom-

enon. For a comprehensive review of this topic see

Amos and Rutstein.23

The important point to remember is that the

cover test does not measure the “true phoria.” Elim-

ination of all fusion-related impulses requires pro-

longed occlusion. The “latent deviation” identified

may in some cases be the elusive cause of astheno-

pia in patients. Thus, prolonged occlusion may be

used to identify binocular-induced symptoms by the

elimination of binocular fusion impulses and to

measure vergence aftereffects.

In 1952 Scobee24 occluded intermittent exotropes

of the divergence excess type for 1 h; he reported

that a significant percentage of them increased

their angular measurement at distance and near

after occlusion. Burian,25 based upon the results of

occlusion, classified the divergence excess type of

intermittent exotropia (DE) into two groups. One

group, which he called simulated DE, responded to

occlusion by increasing the angle of deviation at

near so that it approximated the distance deviation

(this represented 60% of the cases). The other 40%,

which were not affected by occlusion, were called

true DE.

ACA Ratio

Burian25 suggested that differentiation between

simulated and true DE was important because each

required a different surgical procedure. This finding

was confirmed by von Noorden26 though it has been

subsequently denied by other surgeons. This find-

ing with occlusion is important in understanding

the physiological mechanisms responsible for sen-

sory-motor functioning in DE. Before this finding,

most authorities reported that DE was associated

with a high ACA ratio. Using the distance near

formula, the minimum calculated ACA for a DE

patient, where the distance deviation is at least 10

greater than the near deviation, has to be at least

10/1.27 However, Scobee’s24 and Burian’s25 obser-’

vations with occlusion suggested that occlusion de-

creased the ACA ratio to approximately 6/1. Occlu-

sion should not have altered the true response ACA

ratio.

Cooper et al.,28 using an infrared measurement

system, measured accommodation and vergence si-

multaneously to determine objective, response

ACA’s. They demonstrated that response ACA’s in

patients with intermittent exotropia for both sim-

ulated and true DE were normal (mean = 4.9) and

did not change with occlusion. It should be remem-

bered that all response ACA measurements, by the

nature of testing, use prolonged occlusion and

therefore eliminate vergence aftereffects as opposed

to clinical stimulus ACA measurements.

Cooper et al.28 postulated that the difference in

ACA ratios between objective, response ACA’s and

subjective, stimulus, distance-near ACA ratios in

patients with DE was due to the additive effects of

both vergence adaptation (slow fusional vergence)

and proximal convergence findings. They postu-

lated that most patients with simulated DE have a

robust slow vergence system, whereas most true DE

patients use excessive proximal vergence. Ob-

viously, some DE patients have mixed systems.

Kushner29 substantiated the findings of Cooper

et al. by studying gradient ACA’s in intermittent

exotropes. He reported that the majority (93%) had

normal gradient ACA’s. Only 7% of all DE patients

had a true high ACA. In those few DE patients

who had both high gradient and distance-near ACA

ratios, Kushner reported ,that surgical correction

resulted in a near esotropia. Thus gradient ACA’s,

near-far ACA’s, and occlusion testing should be

done on intermittent exotropes to determine the

Implications of Vergence Adaptation—Cooper. 303