Small eye movements during convergence and divergence in individuals with intermittent exotropia

We explored eye movement disorganization during convergence and divergence in individuals with intermittent exotropia (iXT), using a 300-Hz Tobii Pro Spectrum eye-tracking system. The participants comprised 20 patients with iXT, who were evaluated both before and three months after strabismus surgery, aged 15-40 years and 20 controls with orthophoria or horizontal exophorias of ≤ 8 prism diopters, exhibiting latent exo-deviations under dissociated viewing conditions. The participants alternated their gaze between a target at 30 cm and a monitor at 60 cm every 5 s. The convergence time was significantly longer in the iXT group than in the control group. We identified three types of small, conjugate eye movements in the iXT group that occurred less frequently in the control group: small, back-and-forth movements during convergence or divergence ("notch events"); transient, brief movements that occurred immediately before the initiation of convergence or divergence ("static notch events"); and overshooting of the target followed by correction at the end of the vergence response ("overshoot events"). The iXT group showed significantly more static notch events during convergence, as well as more notch and overshoot events during divergence. The static notch events during convergence significantly decreased after strabismus surgery. Small eye movements were associated with preoperative symptoms and postoperative Adult Strabismus-20 Questionnaire (AS-20) improvement. Frequent vergence-related movements in iXT may impair daily activities.