Effect of test chart configuration on crowding in strabismic amblyopia

Title Effect of test chart configuration on crowding in strabismic amblyopia
Author, Co-Author Yvonne Norgett, John Siderov
Topic Functional Vision/Pediatrics
Program Number
Ballroom A-B
Anglia Ruskin University

Purpose: Crowded visual acuity tests have been produced to increase sensitivity to amblyopia detection, yet there is no standardization of crowding features. This study used custom designed tests in a clinical setting to investigate crowded letter recognition in strabismic amblyopes in a variety of conditions.

Methods: LogMAR was measured monocularly in both the amblyopic and non-amblyopic eyes of 11 adult strabismic or mixed strabismic/anisometropic amblyopes using custom designed letter tests employing the Sloan letter set. The tests were designed in single letter and linear formats using either bar or letter flankers to introduce crowding. An unflanked, single letter test condition was also used. Edge-to-edge letter-flanker separation for each crowded test was 0.5 letter widths. Tests were presented monocularly on a high resolution Apple Macintosh display at 4m test distance, using standardized instructions. For each condition, 5 letters of each size were shown and testing continued until 3 letters of a given size were named incorrectly. Uncrowded logMAR was subtracted from logMAR in each of the crowded tests to highlight the crowding effect.

Results: Repeated measures ANOVA showed that letter flankers and linear presentation individually resulted in poorer performance in the amblyopic eyes (mean normalized logMAR 0.29, SE 0.07 and 0.27, SE 0.07 respectively, p < 0.05) and together had an additive effect (mean 0.42, SE 0.09, p < 0.001). There was no difference across the tests in the fellow eyes (p > 0.05).

Conclusions: The elevated crowding seen in the amblyopic eyes of strabismic amblyopes is dependent on stimulus and task demands. Both linear presentation and letter rather than bar flankers increase crowding in this group with most crowding seen in linear letter tests with letter flankers. These results have a potential application in screening for amblyopia. 

Affiliation of Co-Authors Anglia Ruskin University