EXCESS CYCLOVERGENCE IN PATIENTS WITH INTERMITTENT EXOTROPIA

Recently, we developed a model of binocular fixation, This model predi cts the amount of cyclovergence as a function of target elevation and horizontal target vergence. The prediction derives from the assumption that version and vergence add linearly and that the eye positions are constrained in three respects: (1) the foveae of the two eyes are dir ected towards the target, (2) the version component follows Listing's law, i.e. cycloversion, and horizontal and vertical version are not in dependent, (3) the vergence component is restricted to a plane approxi mately perpendicular to Listing's plane, i.e. horizontal, vertical and torsional vergence are not independent. The version and the vergence components are characterized by a common primary direction for the two eyes. We applied this model to data of patients with intermittent exo tropia. In two patients with an amblyopic eye we found that the common primary direction rotates towards the amblyopic eye. In the third pat ient, not suffering from amblyopia, the common primary direction was p ractically straight ahead. In all three patients, cyclovergence angles were larger than those found in normal subjects. We found that the in creased cyclovergence was compatible with our model for normal subject s if an offset on the horizontal vergence was given. This offset repre sents the additional convergence effort required in these patients to overcome the exodeviation of the eyes. According to our model the incr eased horizontal vergence effort results in excess cyclovergence. The relation between horizontal vergence and cyclovergence offers a new me thod for measuring the angle of exotropia.