Clinical Characteristics of Patients with Intermittent Exotropia According to the Response to Short-term Prism Adaptation Test

Intermittent exotropia (IXT) is the most common form of childhood strabismus in Asian countries [1], character-ized by gradual and progressive exodeviation that presents primarily during distance fixation or when the patient is inattentive [2,3].IXT has been classified based on distance and near exodeviations, and assessment of true exodeviation is a critical part of classifying IXT and deciding on a surgical plan.However, evaluation of true deviation in IXT patients is difficult because some of the patients exhibit a vergence aftereffect.Purpose: To evaluate the prevalence of the prism adaptation response in patients with intermittent exotropia (IXT) using the short-term prism adaptation test (PAT) and to assess factors associated with prism adaptation response in IXT patients.Methods: A case-controlled retrospective analysis was performed on 113 patients with IXT without prior surgical treatment.Age, sex, visual acuity, refraction, stereoacuity, control scale, type of exotropia, history of occlusion, and presence of accompanying visual symptoms were recorded.Prism alternate cover test (PACT) was performed with fixation targets at 6 m and 1/3 m.All patients underwent short-term PAT wearing prism glasses that offset the exodeviation previously measured by PACT.After 30 minutes, angle deviation was measured, and patients were classified into either an increase group, which had an increase in deviation ≥5 prism diopters after short-term PAT, or a no-change group.Analysis was performed to investigate the clinical factors influencing the increase in exodeviation after short-term PAT.Results: Fifty patients (44.2%) showed an increase ≥5 prism diopters during distance or near fixation after short-term PAT compared to the previous PACT: 12 patients (10.6%) showed an increment at distance fixation, and 45 patients (39.8%) showed an increase at near fixation.At distance fixation, the increase-group had a significantly smaller maximum angle measured by PACT.At near distance, age at PAT, maximum distance angle, minimum distance angle, maximum near angle, minimum near angle, angle fluctuation at near, and IXT type showed significant associations with positive short-term PAT response.In the multivariate analysis, older age and smaller maximum near angle were significantly associated with positive short-term PAT response at near fixation.Conclusions: Short-term PAT could be helpful in older IXT patients with a small maximum angle of deviation at near fixation to mitigate the vergence aftereffect and show the maximum angle of deviation.