Influence of pupil size and other test variables on visual function assessment using visual evoked potentials in normal subjects.

The purpose of this study was to assess the influence of pupil size and optical blur on measurements obtained with isolated-check visual evoked potential (icVEP). Two stimulus conditions of icVEP, +15 and -15% contrasts, were studied in normal subjects with normal (N), miotic (M), and dilated (D) pupils. The effects of optical blur were studied in subjects with normal pupil. Response to visual stimuli was quantified by a signal-to-noise (SNR) ratio. In 30 normal subjects, the mean age was 26.0 +/- 3.4 years. Mean pupil diameters were N = 4.2 +/- 0.6 mm, M = 2.7 +/- 0.6 mm, and D = 7.3 +/- 0.9 mm. For both +15 and -15% contrast levels, mean SNR values were reduced for dilated and constricted pupils when compared with normal pupils. Mean SNR values for optical blur with a +2 or +3 diopter lens placed over the distance correction were reduced when compared with SNR measurements obtained with best-corrected visual acuity under both +15 and -15% contrast levels. Statistical significance was found in comparisons of N versus M (P < 0.001) and N versus D (P = 0.002) for +15 and -15% contrast conditions, respectively. No statistical difference was seen for M versus D (P = -0.435). The effect of optical blur was statistically significant when compared to the normal pupils with best-corrected vision (P < 0.001). No statistically significant difference was found comparing +2 and +3 diopters lenses for optical blur testing. Visual evoked potential values are influenced by pupillary constriction and dilation, as well as optical blur. When obtaining icVEP measurements, the influence of pupil size and optical blur should be kept in mind for accurate interpretations.