Landolt C-Tests With "Fixed" Arcmin Separations Detect Amblyopia But Underestimate Crowding in Moderate-to-Severe Amblyopic Children and Adults.

Purpose: Crowding is exaggerated in central vision of strabismic amblyopia, impacting on reading ability. Crowding magnitude and interocular differences (IODs) in acuity are indicators for detection, assessment, and monitoring of treatment. Lateral masking (including contour interaction) also affects acuity and can mimic or ameliorate crowding. We investigated lateral masking/contour interaction and crowding impact on crowding magnitude and IOD measures in healthy and amblyopic pediatric and juvenile/adult groups using two Landolt C-tests with "fixed" arcmin separations.

Methods: Acuity (logMAR) was measured with Landolt C-tests with specified 2.6' ("crowded") and 35' ("uncrowded") separations. Crowding magnitudes (crowded - uncrowded acuities) and IODs were calculated. Participants were 69 subjects with strabismic amblyopia (n = 39 pediatric, i.e. children ≤8 years of age), 31 subjects with anisometropic amblyopia (n = 14 pediatric), and 76 healthy controls (n = 36 pediatric). Subjects with amblyopia were subgrouped by acuity as low severity (<0.4 logMAR) or high severity (≥0.4 logMAR) using the 35' separation C-test.

Results: Crowding magnitudes were greater in strabismic than in anisometropic amblyopia and control/fellow eyes. They were higher in pediatric control/fellow eyes than in juvenile/adult eyes. In high severity strabismic amblyopia, crowding magnitudes progressively and significantly reduced (slope = -0.17 ± 0.07, P < 0.05) with worsening acuity. IODs for this group were higher on the 2.6' C-test, but lower than expected. In high severity pediatric subjects with anisometropic amblyopia, seven of eight had lower IODs measured with the "crowded" than the "uncrowded" C-tests.

Conclusions: These C-tests detect amblyopia but underestimate crowding in children and adults with high severity strabismic amblyopia. Separate isolated optotype acuity and crowding distance tests may better target specific functions, while minimizing the impact of masking.