Variability of angle of deviation measurements in children with intermittent exotropia

Background: A change in the angle of deviation is often used to monitor the change in severity of intermittent exotropia over time; nevertheless, thresholds for a clinically significant change in angle have not been determined. We analyzed variability associated with test-retest differences and short-term variability in the condition, to provide thresholds for assessing clinically significant, long-term change in angle of intermittent exotropia. Methods: Twenty-six children with intermittent exotropia (median age, 7; range, 1-13 years) underwent repeat prism and alternate cover test measures during 3 or 4 examinations (2 hours apart) over the course of a day; 95% repeatability coefficients were derived to determine test-retest differences at distance and near fixation. Results: Derived 95% repeatability coefficients at distance were 3.4 ^Δ (95% CI, 0.7 ^Δ-6.2 ^Δ) for angles ≤20 ^Δ and 7.2 ^Δ (95% CI, 4.4 ^Δ-9.9 ^Δ) for angles >20 ^Δ; at near, 6.6 ^Δ (95% CI, 3.7 ^Δ-9.6 ^Δ) for angles ≤20 ^Δ and 12.8 ^Δ (95% CI, 5.3 ^Δ-20.3 ^Δ) for angles >20 ^Δ. Conclusions: Test-retest reliability data in this study provide thresholds to help determine clinically significant change in angle of strabismus in children with intermittent exotropia. These data should facilitate evidence-based assessment of long-term change in intermittent exotropia over time.