Considerations of modified vision and crowding tests, for improved detection of amblyopia

Amblyopia is a neurodevelopmental disorder characterised by deficits of visual acuity. It demonstrates greater sensitivity to visual crowding; the phenomena whereby accurate object identification is reduced due to the presence of nearby or flanking objects. Recent research suggests that the current visual acuity tests may not contain sufficiently crowded optotypes, potentially allowing amblyopes to avoid detection. Increasing crowding in acuity tests by using an optimised test arrangement may improve sensitivity to the detection of amblyopia by maximising interocular acuity differences (IOD). In the first experiment of this study, acuity is examined in both visually healthy and amblyopic children using an optimised arrangement and the acuities obtained compared with the Sonksen logMAR test (SLT). This ‘Enhanced Cambridge crowding’ (L-ECC) test demonstrated significantly increased crowding magnitudes for all groups examined (p<.001). L-ECC IODs were significantly larger for strabismic/mixed amblyopes, compared with the SLT (p<.005), but significantly smaller for anisometropic amblyopes (p<.005). Research with contrast-modulated stimuli has demonstrated increased crowding in adults, compared with luminance-generated stimuli. The second experiment examined the same paediatric cohort with a contrast-modulated ‘Enhanced Cambridge crowding’ CM-ECC test. While the CM-ECC yielded significantly higher acuities (p<.001), IODs were not significantly different from the SLT (p=.306). The third and final experiment in this study establishes the foveal crowding distances of amblyopic children, using the new ‘Pelli’ Optotype, for the first time. Pelli foveal crowding distances measured in both trigram and repeated formats showed that strabismic/mixed amblyopic eyes had significantly larger crowding distances than controls (p<.005) and anisometropic amblyopic eyes. (p<.05). Crowding distance IODs were also 20-30 times larger in strabismic/mixed amblyopes than controls. Crowding distance ratios (amblyopic eye/fellow eye) were larger than acuity ratios (p<.001) for strabismic/mixed amblyopes, but not anisometropic amblyopes. Crowding distance tests may therefore be more sensitive than acuity tests for detecting strabismic amblyopia.