Effect of nystagmus on VEP-based objective visual acuity estimates

In order to determine the effect of nystagmus on objective visual acuity (VA) estimates, we compared subjective (VApsych) and objective (VEP, VAVEP) VA estimates in participants with nystagmus. For this purpose, 20 participants with nystagmus (NY) caused by idiopathic infantile nystagmus, albinism, achiasma or acquired nystagmus were recruited in this study. Estimates of BCVA (best corrected visual acuity) were determined psychophysically (VApsych; FrACT, Freiburg visual acuity test) and electrophysiologically (VAVEP; EP2000) according to ISCEV (International Society of Clinical Electrophysiology of Vision) guidelines. For each participant the eye with the stronger fixation instability [Nidek microperimeter (MP-1), Nidek Instruments] was included for further analysis. VApsych vs VAVEP were compared via paired t-tests and the correlation of the difference between VApsych and VAVEP (triangle VA) vs the degree of fixation instability was tested with Pearson correlation (r). We found VAVEP to be better than VApsych [by 0.12 Logarithm of the Minimum Angle of Resolution (logMAR); mean +/- standard error (SE) of VAVEP vs VApsych: 0.176 +/- 0.06 vs. 0.299 +/- 0.06, P = 0.017] and triangle VA to be correlated linearly with the degree of fixation instability (r2 = 0.21,p = 0.048). In conclusion, on average we report a small VA overestimation, around 1 line, for VAVEP compared to VApsych in NY. This overestimation depended on the magnitude of the fixation instability. As a rule of thumb, a reduction of the fixation probability in the central 4 degrees from 100 to 50% leads on average to a VAVEP overestimation of around 0.25 logMAR, i.e. 2.5 lines.