Short-term cortical synaptic depression/potentiation mechanisms in chronic migraine patients with or without medication overuse.

To study the effects of trains of repetitive transcranial magnetic stimulation (rTMS) over the motor cortex in patients with chronic migraine (CM) with or without medication overuse (MOH). Thirty-two patients (CM [n = 16]; MOH [n = 16]) and 16 healthy volunteers (HVs) underwent rTMS recording. Ten trains of 10 stimuli each (120% resting motor threshold) were applied over the left motor cortex at 1 Hz or 5 Hz in random order. The amplitude of motor evoked potential (MEP) was evaluated from electromyographic recording in the first dorsal interosseous muscle. The slope of the linear regression line for the 10 stimuli for each participant was calculated using normalized data. rTMS-1 Hz had a normal depressive effect on MEP amplitude in all groups. rTMS-5 Hz depressed instead of potentiating MEP amplitudes in MOH patients, with a significantly different response from that in HVs and CM patients. The slope of the linear regression of MEP amplitudes was negatively correlated with pain intensity in CM patients, and with the duration of overuse headache in MOH patients. This different plastic behaviour suggests that MOH and CM, despite exhibiting a similar clinical phenotype, have different neurophysiological learning processes, probably related to different pathophysiological mechanisms of migraine chronification.