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 increase the angle of deviation with prolonged, repeated 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 increase 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 stereoscopic cues and less fusible detail, thus decreasing reflex fusional vergence and subsequent slow fusional 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 preocclusion hyperdeviation showed a significant increase in their deviation, 5 of 8 orthophoric patients demonstrated a post occlusion hyperphoria; and the final 5 showed minimal changes with occlusion. Beisbarth suggested that prismatic correction based upon prolonged occlusion would result in over correction of most vertical deviations. Prolonged 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 controversy whether prolonged occlusion produces a true deviation or just an exaggeration of Bell’s phenomenon. 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.” Elimination of all fusion-related impulses requires prolonged occlusion. The “latent deviation” identified may in some cases be the elusive cause of asthenopia 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 finding with occlusion is important in understanding the physiological mechanisms responsible for sensory-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 decreased the ACA ratio to approximately 6/1. Occlusion should not have altered the true response ACA ratio.

Cooper et al.,28 using an infrared measurement system, measured accommodation and vergence simultaneously to determine objective, response ACA’s. They demonstrated that response ACA’s in patients with intermittent exotropia for both simulated and true DE were normal (mean = 4.9) and did not change with occlusion. It should be remembered 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 postulated that most patients with simulated DE have a robust slow vergence system, whereas most true DE patients use excessive proximal vergence. Obviously, 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