Hyperexcitability of the primary somatosensory cortex in migraine--a magnetoencephalographic study

The excitability of the cerebral cortex in the interictal state of migraine appears to be fundamental in the brain's susceptibility to migraine attacks. Subpopulations of cortical neurons are reported to have different physiological response properties to different interstimulus intervals (ISIs) and, hence, may be differentially altered or modulated in migraine. The aim of this study therefore was to evaluate response characteristics of temporally and spatially defined neuronal subpopulations in the cortex of migraineurs. To this end, we measured, by means of magnetoencephalography (37-channel neuromagnetometer), the response properties of the early components of the somatosensory evoked magnetic fields following electrical stimulation of the median nerve, the N20m and P35m, at ISIs ranging between 0.3 and 6 s. As a measure of the number of excited neurons underlying the N20m and P35m, we evaluated the root mean square (r.m.s.) of the deflections across all 37 channels at the corresponding latencies and the corresponding dipole moment of the equivalent current dipole (ECD strength). Twenty consecutive women with at least three migraine attacks/month (range 3-8/month) fulfilling the International Headache Society criteria and 20 age-matched healthy women were included in the study. In migraineurs, the r.m.s. and ECD strength of N20m was increased at all ISIs (r.m.s., P < 0.05; ECD strength, P < 0.01) and positively related to the mean attack frequency (r.m.s., R(s) = 0.6, P < 0.01; ECD strength, R(s) = 0.5, P < 0.05). In contrast, the r.m.s. and ECD strength of P35m did not differ significantly between migraineurs and control subjects and did not correlate significantly with the frequency of migraine attacks. Responses to different ISIs did not differ significantly between migraineurs and control subjects. The r.m.s. of N20m was stable for ISIs between 0.5 and 6 s and decreased significantly at an ISI of 0.3 s. In contrast, the r.m.s. of P35m decreased continuously as the ISI was decreased below 6 s and this reached significance for an ISI of < or =1 s. Habituation of N20m or P35m, i.e. a decrease in response magnitude following repetitive stimulation over time, was not found in either the control subjects or in the migraineurs. It is concluded that the population of neurons in the primary somatosensory cortex underlying the N20m are hyperexcitable and that this hyperexcitability is linked to the frequency of migraine attacks. This hyperexcitability appears not to be related to habituation since habituation was not found in the control subjects. In contrast, the magnitude of P35m is not pathophysiologically linked to the interictal state of migraine. Furthermore, the cellular mechanisms causing ISI-dependent depression of N20m and P35m are not altered in migraine.