Assessing cortical excitability in migraine: Reliability of magnetic suppression of perceptual accuracy technique over time

Objectives.-To examine test-retest reliability of magnetic suppression of perceptual accuracy (MSPA) prior to its use as a marker of cortical excitability in a trial of migraine prophylactic agents. Background.-MSPA is a relatively novel avenue of research in headache, providing an opportunity to study cortical responsiveness objectively and noninvasively. However, little is known about the reliability of magnetic stimulation protocols such as MSPA in longitudinal research designs. Methods.-We tested 10 healthy headache-free volunteers who had no family history of migraine. In 54 trials, they were briefly presented different three-letter combinations, flashed on a computer screen for 24 ms (target). After a brief interval, each target was followed by a single magnetic pulse through a 90-mm circular coil centered 7 cm above inion in the midline. The interval between target and magnetic pulse was systematically varied. Volunteers were requested to report as many letters as they had possibly identified. After 2 weeks, all volunteers were retested using identical methods. Results.-MSPA performance is expressed as a profile of response accuracy (ie, percentage of correctly identified letters) across target-pulse intervals. Profiles were characteristic of normal headache-free subjects at the first test. Analysis of variance revealed no significant difference in profiles between test and retest (F= 2.05; P= .136): the retest profiles are almost coincidental with the test profiles. Conclusions.-MSPA is a safe and objective measure of cortical excitability, which is reliable over time. MSPA, therefore, shows excellent promise as a biological marker of cortical response in trials of migraine prophylactics.