Reliability and consistency of visual acuity and contrast sensitivity measures in advanced eye disease

Purpose. The purpose of this article is to establish reliability of standard visual function measures in patients with severe vision loss. Methods. Legally blind subjects were enrolled in a repeated-measures study to determine the reproducibility of a battery of psychophysical vision measures. Tests reported here include ETDRS visual acuity (VA) under regular and dim illumination and Pelli-Robson letter contrast sensitivity (CS) under regular, dim, and glare conditions. Two to five test repetitions were performed at monthly intervals. Subject groups included retinitis pigmentosa (RP) (n = 29), macular disease (MD) (n = 17), optic nerve disease (ON) (n = 4), diabetic retinopathy (DR) (n = 5), and other retinal diseases (OR) (n = 10). Results. For most subject groups, VA measures had mean 95% coefficient of repeatability (CR.95) of 0.20 (group means between 0.13 and 0.36) log units, and CS measures had mean CR.95 of 0.35 (group means between 0.19 and 0.48) log units. Conclusions. The established reliability of VA and CS tests in subjects with severe visual impairments will allow investigators to determine whether a significant change in vision has occurred during future clinical trials to reverse, halt, or slow vision loss. The criterion for change needs to be larger than for normally sighted controls, i.e., approximately two to three lines for VA and between eight to 11 letters for CS.