Associations of anisometropia with unilateral amblyopia, interocular acuity difference, and stereoacuity in preschoolers.

PURPOSE To evaluate the relationship of anisometropia with unilateral amblyopia, interocular acuity difference (IAD), and stereoacuity among Head Start preschoolers using both clinical notation and vector notation analyses. DESIGN Multicenter, cross-sectional study. PARTICIPANTS Three- to 5-year-old participants in the Vision in Preschoolers (VIP) study (n = 4040). METHODS Secondary analysis of VIP data from participants who underwent comprehensive eye examinations, including monocular visual acuity testing, stereoacuity testing, and cycloplegic refraction. Visual acuity was retested with full cycloplegic correction when retest criteria were met. Unilateral amblyopia was defined as IAD of 2 lines or more in logarithm of the minimum angle of resolution (logMAR) units. Anisometropia was defined as a 0.25-diopter (D) or more difference in spherical equivalent (SE) or in cylinder power and 2 approaches using power vector notation. The percentage with unilateral amblyopia, mean IAD, and mean stereoacuity were compared between anisometropic and isometropic children. MAIN OUTCOMES MEASURES The percentage with unilateral amblyopia, mean IAD, and mean stereoacuity. RESULTS Compared with isometropic children, anisometropic children had a higher percentage of unilateral amblyopia (8% vs. 2%), larger mean IAD (0.07 vs. 0.05 logMAR), and worse mean stereoacuity (145 vs. 117 arc sec; all P<0.0001). Larger amounts of anisometropia were associated with higher percentages of unilateral amblyopia, larger IAD, and worse stereoacuity (P<0.001 for trend). The percentage of unilateral amblyopia increased significantly with SE anisometropia of more than 0.5 D, cylindrical anisometropia of more than 0.25 D, vertical and horizontal meridian (J0) or oblique meridian (J45) of more than 0.125 D, or vector dioptric distance of more than 0.35 D (all P<0.001). Vector dioptric distance had greater ability to detect unilateral amblyopia than cylinder, SE, J0, or J45 (P<0.001). CONCLUSIONS The presence and amount of anisometropia were associated with the presence of unilateral amblyopia, larger IAD, and worse stereoacuity. The threshold level of anisometropia at which unilateral amblyopia became significant was lower than current guidelines. Vector dioptric distance is more accurate than spherical equivalent anisometropia or cylindrical anisometropia in identifying preschoolers with unilateral amblyopia.