[Somatosensory evoked potentials and action myoclonus].

The authors studied the somatosensory evoked potentials (SEPs) in 16 cases of myoclonic encephalopathies: 8 cases of dyssynergia cerebellaris myoclonica (DCM); 2 cases of dyssynergia cerebellaris progressiva (DCP); 2 cases of Lafora's disease; 1 case of ceroid lipofuscinosis; 3 unclassifiable myoclonic syndromes. The mean age of the patients was 18 years and the mean duration of pre-study evolution was 10 years. All the patients had been treated by anticonvulsant drugs (phenobarbital, valproic acid, benzodiazepines). The amplitude of the complex P1N2 at the level of the contralateral parietal cortex, with stimulation of the median nerve at the wrist, was found to be enlarged in only 6 cases and giant responses (over 40 microV) were obtained in 2 cases. Only half of the patients with DCM presented a high amplitude response. There was no correlation either with clinical parameters (and in particular, certain patients with marked action myoclonic jerks have a normal SEP), or with the EEG data: on the contrary, the amplitude variations of the SEPs are most often similar to variations of the visual evoked potentials.