Diagnosis of pseudo-divergence excess exotropia secondary to high accommodative convergence to accommodation ratio

Calculation of the AC/A ratio in divergence excess exotropia is accomplished by measurement at near through convex lenses following a period of monocular occlusion to eliminate tenacious proximal fusion (TPF). The purpose of this study was to determine if the AC/A ratio could be accurately calculated using minus lenses at distance fixation, thereby eliminating the need for occlusion.

This was a prospective study of 33 consecutive patients with exotropia who had been judged to be surgical candidates. All had a sensorimotor exam in addition to measurement at distance through -2.00 lenses, a 60-minute Patch Test, and remeasurement through +3.00 lenses at near. Measurement of the AC/A using plus lenses at near following occlusion was accepted as the standard to which the results of the minus lens AC/A measurement were compared.

Nineteen patients (58%) had an increase in the near exotropia by more than 5(Δ) with occlusion (mean = 8.6 ± 1.4(Δ)). There was no statistical difference between the AC/A calculated with the plus lens method and that calculated using the minus lens method (P > 0.7). Seven patients (21%) had a high AC/A ratio using the plus lens method. There were two false negatives with the minus lens method.

Testing with minus lenses at distance is effective in identifying a high AC/A, and may be used to determine the risk of surgical over-correction at near, or identify patients who may benefit from optical rather than surgical management.