Contrast Sensitivity Following Amblyopia Treatment in Children

Prior studies have found a reduction in contrast sensitivity in eyes with amblyopia using sinusoidal gratings,^1-3 whereas minimal loss has been reported with Pelli-Robson charts.^3,4 Most studies have evaluated contrast sensitivity at the time of diagnosis of amblyopia or after short-term treatment. A follow-up study of an earlier randomized trial provided us the opportunity to evaluate contrast sensitivity using Pelli-Robson low-contrast letter charts at age 10 years, several years after treatment of amblyopia.⁵

Contrast sensitivity was measured using Pelli-Robson charts⁵ (Richmond Products, Albuquerque, New Mexico) in 86 subjects (mean age, 10.3 years) who at ages 3 to 6 years had participated in a randomized treatment trial comparing atropine vs patching for moderate amblyopia. Institutional review boards approved the study, and written consent was obtained from parents. Details of the randomized trial and the examination results at age 10 years have been reported.⁶ The low-contrast letter identification score for each eye was the lowest-contrast triplet for which the child correctly identified at least 2 of 3 letters on the first attempt, reading from highest to lowest contrast. Monocular visual acuity was measured with the Electronic Early Treatment of Diabetic Retinopathy Study visual acuity testing protocol.⁷

Mean visual acuities in the amblyopic and fellow eyes at the time of the examination were 0.17 logMAR (approximately 20/32) and −0.04 logMAR (approximately 20/20), respectively. The mean log low-contrast letter identification score was slightly worse in the amblyopic eye than in the fellow eye (1.75 vs 1.78, respectively; P = .04 with a paired samples t test) (Table 1). There was a weak correlation between the interocular contrast sensitivity difference (Table 2) and the interocular visual acuity difference (Spearman correlation = 0.27; 95% confidence interval, 0.08 to 0.47). Contrast sensitivity scores in the amblyopic and fellow eyes did not differ with respect to original treatment group assignment (atropine vs patching, P = .77 for the amblyopic eye and P = .95 for the fellow eye with an independent samples t test). There was a suggestion that younger age at the time of entry into the randomized trial was associated with better contrast sensitivity in the amblyopic eye (Spearman correlation = −0.21; 95% confidence interval, −0.42 to −0.01).

Our results confirm the finding of other studies that the loss of contrast sensitivity after treatment of strabismic and anisometropic amblyopia is slight in the intermediate spatial frequencies tested with the low-contrast letters of the Pelli-Robson charts.^3,4 The distribution of contrast sensitivity in the amblyopic eye nevertheless was similar to that reported for monocular testing of healthy children aged 10 years.⁸

The suggestion that patients who were younger at enrollment into the randomized trial (aged 3 to <5 years compared with those aged 5 to <7 years) were more likely to have slightly better contrast sensitivity in the amblyopic eye at age 10 years is consistent with what we have reported for visual acuity.⁶ This effect, if substantiated, could be due to a number of factors. One possibility is a younger age at treatment allowing more complete cortical development, and another is a shorter duration of the vision deficit. Each of these circumstances might allow a more complete treatment effect or alternatively a shorter and thus less profound insult to the developing visual sensory system.

The Pelli-Robson chart is limited in that it is a measure of low-contrast letter identification that correlates with contrast sensitivity in the medium spatial frequency range.⁵ Losses at high spatial frequency have been reported for all types of amblyopia.⁹ Thus, although we found a slight reduction in Pelli-Robson chart scores, there could be greater contrast sensitivity loss at other spatial frequencies or prior to treatment. Nevertheless, it seems likely that mild residual amblyopia, which was present in most cases, is associated with only a mild reduction in contrast sensitivity after treatment of moderate amblyopia from strabismus, anisometropia, or both combined.

Correspondence: Dr Repka, c/o Jaeb Center for Health Research, 15310 Amberly Dr, Ste 350, Tampa, FL 33647 (pedig@jaeb.org).

Financial Disclosure: None reported.

Funding/Support: This study was supported through cooperative agreement EY11751 from the National Eye Institute.

Group Information: A list of the Pediatric Eye Disease Investigator Group members participating in the study was published in Arch Ophthalmol. 2008;126(8):1039-1044.