Myopic shift during control of the deviation in intermittent exotropia: a study with the Plusoptix 12R photoscreener

Purpose: To assess refraction changes under binocular viewing conditions dur-ing the control of the deviation for near fixation in intermittent exotropia (XT) patients. Methods: 20 (XT) patients aged 4 to 26 years were enrolled in the study. The four patterns of divergent strabismus were present in the study group: divergence excess, basic exodeviation, convergence insufficiency, and simulated divergence excess. All the patients exhibited random dot stereopsis. The refraction and pupillary diameter were measured by infrared video retinos-copy and pupillometry with the Plusoptix A12 photoscreener (Nuernberg, Germany) under ei-ther monocular or binocular viewing conditions at 1 meter distance. The accommodation con-vergence to accommodation (AC/A) ratio was measured in all patients with the gradient method. Results: A myopic shift (mean 0.44, SD 0.422) was recorded under binocular viewing condi-tions during the control of the deviation. No hyperopic shift was detected turning from monocu-lar to binocular viewing conditions in any patient. No change in pupil diameter was recorded passing from monocular to binocular fixation. The mean AC/A ratio resulted 5.61. When con-sidering the sub-groups, it ranged from 8.7 in the divergence excess group, to 5.6 in the basic exodeviation group and 3.4 in the convergence insufficiency group. Conclusions: The myopic shift occurring under binocular viewing conditions during the con-trol of the deviation at near fixation could be explained by the increased vergence demand in (XT) patients. Fusional vergence through convergence accommodation to convergence ratio ra-ther than accommodation through AC/A ratio seems to be the main compensatory mechanism for the deviation in (XT).