ON-Pathway visual acuity deficits in 8–12 years old children with unilateral amblyopia

Conventional visual acuity (VA) tests, using black optotypes on white background, may not fully detect subtle ON-pathway amblyopia deficits or increases in visual dark dominance. This pilot study tests the hypothesis that VA testing with reverse contrast polarity, white optotypes on black background, is more sensitive in detecting amblyopia than conventional VA testing. Two groups of children aged 8–12 were enrolled. The amblyopic group (N = 13) had a best-corrected VA of 20/32 or worse in the amblyopic eye and an interocular VA difference of ≥ 0.2 logMAR. The normal control group (N = 16) had best-corrected VA equal to or better than 20/25 and an interocular VA difference of ≤ 0.1 logMAR. Monocular VA was tested for each polarity using an ETDRS program using the Amblyopia Treatment Study protocol, with results reported as a Score. Two optotype polarities were tested in randomized order, and tests were repeated after a 30-minute interval. VA was compared among amblyopic, fellow, and the right eyes of the control group for both polarities. For amblyopic eyes, the mean VA Score was significantly lower for white optotypes than for black optotypes by 3.6 letters or approximately 0.07 logMAR, indicating that amblyopic eyes had more difficulty seeing white than black optotypes. For fellow eyes, the mean Score was also significantly lower for white than for black optotypes. No significant differences in mean Scores between two polarities were found within the control eyes. Our findings suggest that visual acuity testing with reverse polarity could be more sensitive to detect amblyopia in children.