S105. Effect of change in pattern stimulation on the response latencies and amplitude of visual evoked potential in migraneurs and normal healthy adults

Abstract

Visual Evoked Potentials (VEPs) are non- invasive studies which measure the electrophysiological activity at visual cortex. It represents the functional activity of optic pathway. It represents the functional activity of the optic pathway including anterior segment, retina, optic nerve, optic chiasm, lateral geniculate nucleus and visual cortex. The visual function in migraneurs differs from normal subjects and studies of VEP in migraneurs have yielded varied results. Several investigators did not find differences between migraneurs and healthy controls. while others reported changes of VEP amplitudes, latencies and habituation response. Thus present study is being conducted to see the variations in VEP latencies and amplitude with different spatial stimulation on pattern reversal VEP in migraineurs and healthy adults. A case control study is being conducted enrolling 250 migraine patients and 250 controls of either gender after having a written informed consent. VEP recording on a 6 channel EMG/EP measuring system of Nihon kohden is being done on all participants. Responses are recorded from scalp by electrodes placed according to the International 10/20 system. Visual stimulation is done with checkerboard pattern generated on the monitor consisting of black and white checks whose phase reverse with spatial frequency rates 0.5, 1, 2 and 4 cycles per degree and pattern size 16 × 16, 32 × 32, 64 × 64, 128 × 128, 256 × 256. 100 stimuli are given for each combination of pattern and frequency and an interval of at least 3 min is allowed between two patterns of stimulations. Preliminary analysis of data of 60 patients and 60 controls collected till now was analysed by clubbing the waveforms having the similar frequencies but different checkerboard size. It was seen that the onset latency, N75 was significantly different with smaller check size, i.e. 64 × 64 and 128 × 128 at all the frequencies 0.5,Hz,1 Hz, 2 Hz and 4 Hz. N75 latency was significantly different also at 4 Hz frequency and checkerboard 32 × 32. P100 latency showed significant differences only with smallest check size (128 × 128) at extremes of frequencies used i.e. 0.5 Hz and 4 Hz. While N145 latencies did not show any difference with any combinations of frequencies and checkerboard size. In the light of initial observations and preliminary analysis, it is prudent to include more participants in the study for increasing the power especially for the results which are showing borderline significance. This will assign more objectivity to finding. Probably migraineurs have different response to extreme visual stimuli than non-migraineurs thus we plan to summarize the data with mean as measure of central tendency and standard deviation as measure of dispersion. The clubbed data will be compared for specified latency and amplitude by applying the repeated measure ANOVA (RANOVA). In the next step we intend to do a linkage analysis for two groups for identifying the pattern (if any).