Abnormalities of the balance between inhibition and excitation in the motor cortex of patients with cortical myoclonus.

Patients with cortical myoclonus may have purely focal or multifocal jerks, or they may have additional bilateral or generalized jerks, suggesting the spread of excitatory myoclonic activity between the cerebral hemispheres and across the sensorimotor cortex. The factors contributing to this spread of activity were investigated in 10 patients with multifocal cortical myoclonus and eight patients with multifocal and bilateral or generalized cortical myoclonus. The two groups were termed 'non- spreaders' and 'spreaders' respectively. Eight of the patients were also epileptic. Motor thresholds to single transcranial magnetic shocks at rest were higher in 'non- spreaders' (median 88%, range 45-100% of stimulator output) than either 'spreaders' (50%, range 26-90%, P=0.023) or health controls (38%, range 28-53%, P<0.001). This pathological elevation in motor threshold was not simply an effect of treatment with antiepileptic drugs. Paired transcranial magnetic stimuli were used to investigate ipsilateral cortico-cortical and transcallosal inhibition, There was less (MANOVA, P<0.05) ipsilateral inhibition at interstimulus intervals (ISIs) of 1-6 ms in 'spreaders' (mean 107+/-SEM 23% of control) compared with 'non- spreaders' (75+/-15%) or healthy subjects (59+/-10%). There was also less (P<0.05) transcallosal inhibition across inhibitory timings (10, 12 and 14 ms) in the 'spreaders' (98+/-6% of control) compared with the 'non-spreaders' (64+/-8%) or healthy subjects (59+/-6%). There was no relationship between ipsilateral cortico-cortical and transcallosal inhibition and the presence or absence of epilepsy, although non-epileptic patients did have higher motor thresholds (median 85%, range 32-100% of stimulator output) than either epileptic patients (50%, range 26-90%, P<0.001) or healthy controls (38%, range 28-53%, P=0.002). Abnormalities in ipsilateral and transcallosal inhibition appear to facilitate the spread of the cortical myoclonic activity responsible for bilateral and generalized jerks. However, these abnormalities in inhibition do not play a major role in the development of generalized seizures in patients with cortical myoclonus.