Classification and Misclassification of Sensory Monofixation in Intermittent Exotropia

Purpose: The course of intermittent exotropia and response to surgery may depend on whether there is underlying monofixation. The purpose of this study was to report the prevalence of sensory monofixation in intermittent exotropia using different stereotests and to determine the risk of misclassifying monofixation based on a single administration of each test. Design: Retrospective case review of children with intermittent exotropia. Methods: Forty-four children were identified for whom Preschool Randot, Frisby, and Titmus stereoacuity were measured at a single examination. Ninety-two children were identified with near stereoacuity measured on 2 sequential visits (Preschool Randot, n = 73; Frisby, n = 66; and Titmus, n = 40). Monofixation was defined as stereoacuity worse than previously published age-referenced normal thresholds, bifixation was defined as at least 40 arc seconds, and uncertain was defined as within normal range for age but worse than 40 arc seconds. Results: In children measured by all 3 tests on the same visit, sensory monofixation occurred in 36% using Preschool Randot, in 48% using Titmus, and in 55% using Frisby (P > .1 for each comparison). There was poor agreement between Frisby and Preschool Randot when classifying monofixation in individual patients (P = .05). In children measured on sequential visits, misclassification occurred in 5% with Preschool Randot, in 13% with Titmus, and in 23% with Frisby (Preschool Randot vs Frisby, P = .005). Conclusions: Classification of monofixation depends on the stereotest used. Regardless of the stereotest, there is a risk of misclassifying monofixation on a single assessment. Potential misclassification needs to be considered in clinical practice and in study design.