We recorded median-nerve SEPs in 28 patients (8 cranial, 4 cervical, 1 cranial-cervical dystonia, 15 HSF) and 18 age-matched controls. We measured N20-P25 amplitudes from 3 blocks of 200 sweeps, and assessed amplitude of block 1, and habituation from amplitude changes between the 3 sequential blocks.
ANOVA showed a main effect of factor group (F = 4.4 p = 0.015). In patients with dystonia, SEP amplitudes were normal in block 1, but thereafter failed to habituate (p = 0.03). Patients with hemifacial spasm had normal SEP amplitudes and habituation (p = 0.98).
SEP habituation is altered in dystonia indicating that somatosensory cortex becomes hyperresponsive. This abnormality is not the result of involuntary movements because in patients with HSF SEPs habituate normally.