The role of diagnostic monocular occlusion test in basic type intermittent exotropia

To assess the response to monocular occlusion test in basic type intermittent exotropia (IXT) and to evaluate the surgical outcomes of titrated surgery based on the test's result. Medical records were retrospectively reviewed for patients who underwent bilateral lateral rectus recession for basic type IXT. Patients were categorized into two groups: those who underwent a preoperative diagnostic monocular occlusion test (occlusion group) and those who did not (no occlusion group). In the occlusion group, patients exhibiting a change in deviation angle of ≥ 5 prism diopters (PD), either at distance or near fixation following occlusion therapy, were classified as responders, and augmented surgery was performed for patients with an increased deviation. A total of 215 patients were included in this study, with 79 patients (36.7%) in the no occlusion group and 136 patients (63.3%) in the occlusion group. In the occlusion group, while the mean distance deviation did not significantly change, the mean near deviation significantly increased from 27.2 ± 7.7 PD to 32.5 ± 8.6 PD after diagnostic monocular occlusion (p < 0.001). 58% (58%) of patients showed an increase in near deviation angle of 5 PD or more. Patients with good fusional control at near fixation were more likely to show a change in their deviation after the occlusion (odds ratio = 1.722, p = 0.028). The success rate of the strabismus surgery was significantly higher, and recurrence rate was significantly lower in the occlusion group compared to the no occlusion group (p = 0.025 and p = 0.030), while overcorrection rate was not significantly different between the two groups (p = 1.000). Over half of the patients with basic type IXT demonstrated a significant increase in near deviation angle after diagnostic monocular occlusion. Diagnostic monocular occlusion may be useful for revealing the maximum deviation angle at near fixation and may help determine the optimal surgical dosage in basic type IXT.