Variability of angle of deviation measurements in children with intermittent exotropia

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.

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.

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).

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.