Efficacy of the Pelli-Levi Dual Acuity Chart in diagnosing amblyopia

Purpose: Amblyopia is a developmental disorder of spatial vision for which there is no positive diagnostic test. The present study evaluates the recently developed “Pelli-Levi Dual Acuity Chart” as a positive diagnostic test for amblyopia (Pelli, Levi & Chung, 2004). This chart consists of letters, as usual, on one side and letters in noise on the other. This exploits the finding that, for letters near acuity threshold, noise raises the contrast threshold in the amblyopic eye much more than in the fellow eye. By design, normal observers will have similar acuity when tested with either side of the chart (dual acuity log ratio about zero), whereas an observer with lower-than-normal efficiency (as in amblyopia) will show a marked difference in the dual acuity log ratio. Methods: In this study, we use the Pelli-Levi Dual Acuity Chart to measure acuity thresholds in 64 normal eyes and 28 amblyopic eyes. We also measure the effects of viewing distance, defocus, and the use of a pinhole. Results: Without a pinhole, the Pelli-Levi Dual Acuity Chart has high specificity but poor sensitivity in distinguishing between amblyopic and normal eyes. However, the use of a 1.5 mm artificial pupil retained the high specificity (95%) and greatly improved the sensitivity (to 90%). Conclusions: When used in conjunction with a 1.5 mm pupil, the dual acuity chart is a useful positive test for amblyopia.