The long-term result of surgery in intermittent exotropia is often an undercorrection and relapse into tropia. This may be due to an underestimation of the horizontal angle and/or an undetected vertical component. The aim of the present study was to test which method, prism neutralization or diagnostic occlusion was best suited to attain the maximal angle of horizontal and vertical deviation.

Twenty-two patients with intermittent exotropia but without initial vertical or incomitant components participated in the study. The horizontal deviation was neutralized with prisms for periods of 14 days on four different occasions with diagnostic occlusion three times for periods of one, three or ten days in between. Measurements of sensory functions and the amount of deviations were performed before and after the procedures. It was shown that the largest horizontal deviation was elicited by prism neutralization without previous occlusion and the largest vertical deviation by diagnostic occlusion for ten days. Conditions of initial divergence excess and lateral incomitances usually were restored by both procedures.