Magnetic evoked potential polyphasia in idiopathic/genetic generalized epilepsy: An endophenotype not associated with treatment response

•Increased Motor Evoked Potential (MEP) polyphasia is a fixed feature of idiopathic generalized epilepsy.•A large variety of clinical variables (incl. treatment response) were not associated with MEP polyphasia.•Anti-epileptic treatment with valproic acid may modulate MEP polyphasia. Increased Motor Evoked Potential (MEP) polyphasia was recently described in idiopathic/genetic generalized epilepsy (IGE). Here, we studied the association of MEP polyphasia with treatment response and other clinical characteristics in patients with IGE. MEPs were recorded from the biceps brachii, flexor carpi radialis and interosseus dorsalis muscles bilaterally during tonic contraction in IGE patients (n = 72) and historical controls (n = 54) after single pulse transcranial magnetic stimulation. Detailed clinical data was available for all IGE patients; predefined endpoint was the association of MEP polyphasia with treatment response. The mean number of phases was higher in the interosseus dorsalis muscle (2.33 vs. 2.13, p = 0.002) in IGE patients as compared to normal controls, as was the proportion of MEPs with more than two phases in at least one test (59.4% vs. 30%, p