The age-dependent effect of anisometropia magnitude on anisometropic amblyopia severity.

INTRODUCTION: Anisometropia is an important cause of amblyopia. The relationship between anisometropia depth and amblyopia magnitude is not well characterized, as previous studies have been limited to patients identified because of their amblyopia. We analyzed results from anisometropic patients identified with photoscreening to eliminate this selection bias.
METHODS: We performed a retrospective observational study of preschool children with anisometropia >1.0 D identified during a statewide photoscreening program. Nine hundred seventy-four children with anisometropia were detected over a 9-year period. Visual acuity, cycloplegic refraction data, and patient age from a formal follow-up examination were analyzed. Effect of anisometropia magnitude on amblyopia was measured by ordinal logistic regression, taking age into account.
RESULTS: Six hundred forty (65.7%) children had amblyopia > or =2 lines. Three hundred sixty-four (37.4%) had > or =4 lines amblyopia. There was a statistically significant increase in risk of amblyopia with increasing magnitude of anisometropia. Calculated odds ratios for amblyopia with maximal meridional anisometropia of > or =2 to <4 D compared with >1 to <2 D was 2.13 (95% CI [1.63, 2.78], p < 1 x 10(-7)), and 2.34 (95% CI [1.67, 3.28], p < 1 x 10(-6)) when comparing > or =4 D to > or =2 to <4 D. Odds ratios for spherical equivalent anisometropia were also highly statistically significant.
CONCLUSIONS: Children with higher magnitudes of anisometropia had higher prevalence and greater depth of amblyopia. Older children had an increased risk of amblyopia compared with younger children for moderate levels of anisometropia. Low magnitude anisometropia in young children may not predispose to amblyopia; these findings have implications for vision screening criteria at various ages.