Transcranial magnetic stimulation in migraine: a review of facts and controversies.

There is compelling evidence that cortical excitability is modified in migraine patients between attacks. Transcranial magnetic stimulation (TMS) is a non-invasive tool to investigate this abnormality. Repetitive transcranial magnetic stimulation (rTMS) activates the underlying cortex at high, but inhibits it at low stimulation frequencies. This is a review of published results obtained in migraineurs with TMS and rTMS over motor or visual cortices. Prevalence and/or threshold data of phosphenes induced by single pulse TMS of the visual cortex are contradictory, some favouring increased, others decreased interictal excitability. The discrepancies may be due to differences in methodology and poor reliability of phosphene reporting. In a recent rTMS study of the occipital cortex we have found evidence in favour of an interictal decrease of the preactivation excitability level by using amplitude of visual evoked potentials and its habituation during sustained stimulation as indices of cortical excitability. The hypothesis of increased cortical excitability, taken in its strict physiological sense of a decreased response threshold and/or an increased response to a single suprathreshold stimulus, may thus not be any longer tenable. The long lasting effects of rTMS allow in future studies to assess metabolic changes of the cortex and subcortical structures with functional imaging methods and to explore novel therapeutic strategies for migraine.