Visual Recognition Time in Strabismus: Small-Angle versus Large-Angle Deviation

Abstract

Purpose To measure the possible differences in monocular detection time of a threshold visual acuity stimulus (recognition time [RT]) between patients with small-angle and large-angle strabismus. METHODS Ten patients with free alternating esotropia were tested (10 to 18 years old): five with small-angle esotropia (≤7°), five with large-angle esotropia (15° to 20°). Six age-matched normal subjects served as controls. The RT of the threshold stimulus was measured in both eyes sequentially for stimuli presented in the center of a computer monitor (RT 1). Moreover, we measured the time necessary for identifying the same threshold visual acuity stimulus generated on the computer screen in the moment in which fixation is taken up by one eye after occlusion of the second eye (RT 2). Using the same setting, RT was also measured monocularly in all strabismic and normal subjects who were originally looking at a luminous fixation point positioned horizontally at 6.5 and 15 degrees from the center of the monitor (RT 3). Results The multivariate analysis of variance for repeated measures indicated that there was no statistical difference in RT 1 between groups. The mean RT2 was significantly longer (p<0.001) in large-angle strabismic eyes when compared with that of normal control eyes. The mean RT 2 in small-angle strabismic eyes did not differ significantly from that of normal eyes. Finally, RT 3 (both at 6.5° and 15° of eccentricity) did not show any significant difference in the three different study groups. CONCLUSIONS The authors hypothesized that alternating strabismus patients may have a significant advantage in maintaining a small-angle deviation, as a large-angle deviation would require longer RT in the moment the deviated eye takes up fixation. It can be speculated that the extension of re-fixation movement, obviously shorter in small-angle strabismus patients, is the main factor responsible for longer RT occurring in large-angle strabismus patients.