Variability of visual threshold in multiple sclerosis: effect of background luminance on frequency of seeing.

Visual thresholds were measured at four different background luminance levels in patients with multiple sclerosis (MS) and in control subjects by means of frequency-of-seeing curves. Results were examined by probit analysis and measures of threshold and threshold variability were obtained. Comparison of patient and control groups showed that the patient's threshold was significantly raised only at the highest background luminance level, but that threshold variability was greater at all three non-zero background luminance levels tested. In addition, threshold variability increased with background luminance in the MS patients but not in the control subjects. Possible underlying pathophysiological mechanisms are discussed, and it is suggested that this luminance-dependent variability in visual threshold shown by patients with multiple sclerosis may be due to intermittent conduction block or ephaptic transmission occurring within the demyelinated visual pathway.