Purpose. Cortical visual impairment (CVI) is a leading cause of bilateral vision impairment. Because many patients with CVI cannot perform an optotype test, their acuity is often measured with a grating stimulus using a preferential looking (PL) test or the visual-evoked potential (VEP) recording. The purpose of this study is to determine the relationship among VEP vernier acuity, VEP grating acuity, and behavioral grating acuity in patients with CVI.
Methods. Sweep VEP vernier acuity, sweep VEP grating acuity, and behavioral grating acuity (measured with PL cards) were measured in 29 patients with CVI. The patients ranged in age from 3.2 to 22.7 years (mean: 12.3; SD: 5.3). Because the measures of vernier acuity and grating acuity have different units, the results were expressed as the log deficit (with normal being 30 cycles per degrees and 0.5 arc min, respectively).
Results. VEP grating acuity loss and VEP vernier acuity loss were significantly related (r = 0.70) with a slope of 1.31, indicating that indicating that on average, vernier acuity showed a 0.2 log unit deficit compared with VEP grating acuity. Behavioral grating acuity loss and VEP grating acuity loss were also significantly related (r = 0.64) with a slope of 1.55, indicating that behavioral acuity was more reduced (by ∼0.3 log unit). VEP vernier acuity loss and behavioral grating acuity loss were significantly related (r = 0.66) with a slope of 0.85, indicating that behavioral acuity and VEP vernier acuity showed a similar magnitude of reduction. A Bland-Altman comparison between the VEP vernier acuity method and the behavioral acuity method showed a flat slope (0.30), indicating that the two measures produce similar visual acuity measures across the range of acuity levels.
Conclusions. In patients with CVI, VEP vernier acuity showed greater deficits than VEP grating acuity and was more similar to the behavioral measures of grating acuity.
*OD, PhD, FAAO
School of Optometry, University of California, Berkeley, California.
Received September 10, 2008; accepted January 15, 2009.