Distance/near Differences in Intermittent Exotropia

Background: Burian's classification of exotropia based on the difference between the distance deviation and near deviation (distance/near differences) leaves some questions unanswered. Controversy exists concerning whether the divergence excess pattern is caused by an excess of divergence or by excessive accommodative convergence. Much of the literature on this subject has been confusing because investigators did not eliminate tenacious proximal fusion as an artifact in calculating the ratio of accommodative convergence to accommodation (AC/A ratio). Previously, one of us (B.J.K.) proposed a classification system that respected this artifact and subdivided the classification system proposed by Burian.

Methods: A total of 202 consecutive patients with an exotropia underwent a series of measurements to determine the respective role of accommodative convergence and tenacious proximal fusion as a cause for their distance/near differences. In addition, the value obtained by a rapid prism adaptation test as a possible substitute for 1 hour of monocular occlusion was studied.

Results: In 98 patients, the initial distance deviation exceeded the near deviation. In 10 patients, the distance/near differences were caused by a high AC/A ratio, which would have been mislabeled by Burian's classification system. Brown's recommendation of using +3.00-diopter lenses at near to diagnose simulated divergence excess would have led to the misdiagnosis of a high AC/A ratio in 61 of these patients. In 26 patients, the near deviation exceeded the distance deviation. Burian's classification would have incorrectly labeled 2 patients as having convergence insufficiency when, in fact, they had pseudoconvergence insufficiency. The new proposed classification system proved 100% sensitive and 100% specific (6 of 6 patients for both parameters) for identifying preoperatively exotropic patients who postoperatively developed an esotropia at near with a high AC/A ratio. Rapid prism adaptation tests at near proved useful for identifying the presence of tenacious proximal fusion, but were not accurate in its quantification.

Conclusions: The validity and utility of the new classification system was confirmed. Identification of exotropic patients with a high AC/A ratio and consideration of nonsurgical treatment is important. The rapid prism adaptation test is qualitatively, but not quantitatively, the same as 1 hour of monocular occlusion.