Real-time rapid acuity assessment using VEPs: development and validation of the step VEP technique

PURPOSE: To develop a reference range of visual acuities corresponding to thresholds found using the step VEP method of rapid, objective visual acuity assessment by using steady state (ss)VEPs in normal adults. METHODS: Sixteen normal adults had visual acuity assessed five times with both the step VEP and with Glasgow Acuity Cards (GAC). Subjects were tested once without filters and with four different levels of optical filtering provided by Bangerter neutral-density filters. Acuity outcomes were compared by linear regression and Bland-Altman analysis. RESULTS: Step VEP and GAC acuities correlated highly (r(2) = 0.60, P = 0.000). GAC scores were predicted with the equation: acuity(GAC) = (0.9 x acuity(step VEP)) - 0.37. Step VEP acuity was 0.46 (95% CI: -0.13 to 1.06) logMAR units greater (poorer) than GAC acuities in these normal subjects. The disparity between test results did not vary with visual acuity. CONCLUSIONS: The step VEP provides a rapid, objective means of estimating visual acuity that can be related to acuity derived from a commonly used letter test.