Variability of angle of deviation measurements in children with intermittent exotropia

Abstract

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.

FIG 1

Test–retest variability of prism and alternate cover test measurements. A, at distance fixation. B, at near fixation. Bland-Altman plot showing 95% repeatability coefficient (equivalent to 95% limits of agreement for the difference between 2 measures). Middle dotted line represents the mean of test–retest differences (0). There was significantly greater variability with larger versus smaller deviations at near (P < 0.0001) with borderline greater variability with larger angles at distance (P = 0.09).