Binocular coordination and reading performance during smartphone reading in intermittent exotropia

Purpose: This study aimed to evaluate binocular coordination using video-oculography during smartphone reading in patients with intermittent exotropia compared to individuals with normal vision. Patients and methods: Eleven youth and adult patients with intermittent exotropia (21.9±9.3 years) and 15 control subjects (26.6±4.3 years) were examined. Eye movements were recorded during smartphone reading at 50, 30, and 20 cm using video-oculography. The loss of binocular coordination was tentatively defined as a horizontal disparity greater than 2°. The proportion of monocular viewing was the percentage of time for which binocularity was lost during smartphone reading. The proportion of monocular viewing, the reading speed, and the correlation between proportion of monocular viewing and reading speed were analyzed. Results: The proportion of monocular viewing during smartphone reading was significantly higher in the intermittent exotropia group than in the control group (P<0.001). It was significantly more frequent at 20 cm than at 50 cm in the intermittent exotropia group (P<0.05). The reading speed was significantly negatively correlated with the proportion of monocular viewing at 30 and 20 cm in the intermittent exotropia group (P<0.05). Conclusion: A significant increase in the proportion of monocular viewing in the intermittent exotropia group suggests that an appropriate viewing distance should be advised so that users can maintain binocular coordination when viewing a smartphone.