Objective evaluation of visual fatigue in patients with intermittent exotropia.

PURPOSE: The purpose of this study was to evaluate the degree of visual fatigue in patients with intermittent exotropia (IXT) using the binocular fusion maintenance (BFM) test. METHODS: Fourteen patients with IXT (32.1 +/- 16.4 years) and 15 age-matched healthy volunteers (31.2 +/- 9.3 years) participated in the study. BFM was assessed by measuring the transmittance of liquid crystals placed in front of the subject's nondominant eye at the instance when binocular fusion was broken and vergence eye movement was induced. A questionnaire on subjective symptoms was administered to the subjects before and after the visual task. The visual task consisted of a reciprocal movement between 67 and 40 cm. RESULTS: The change [post-pre] of BFM was significantly lower in the IXT group (-0.185 +/- 0.187) than in the control group (-0.030 +/- 0.070) (P = 0.010). The change of total subjective eye symptom score was significantly greater in the IXT group (2.28 +/- 1.43) than in the control group (0.93 +/- 1.27) (P = 0.018). The reduction in BFM rate with increasing total subjective eye symptom score was significantly greater in the IXT group (-0.106 +/- 0.017) than in the control group (-0.030 +/- 0.013) (P = 0.006). CONCLUSION: The present findings objectively showed that patients with IXT are at a greater risk of visual fatigue in comparison with healthy individuals.