Clonic convulsion caused by epileptic discharges arising from the human supplementary motor area as studied by...

In order to help clarify the mechanism of tonic convulsion, which is commonly seen in supplementary motor area (SMA) seizures, we investigated the temporal relationship between ictal discharges arising from the SMA and the associated EMG discharges in the foot, in a patient with SMA seizures, prior to surgical treatment. The patient's intractable seizures consisted of tonic followed by clonic convulsion of the left foot also involving at times, the left hand. Ictal EEGs were investigated by subdural electrodes placed on the SMA-proper and pre-SMA, which were defined by cortical stimulation and by recording cortical-evoked potentials. Interictally, repetitive spike discharges were seen at the pre-SMA. Each seizure initially had a tonic phase associated with an electrodecremental EEG pattern. It was followed by clonic convulsion as shown by clonic EMG discharges of the left tibialis anterior (TA) muscle. It had a duration of 300 to 500 msec, and on each occasion a positive cortical activity at the pre-SMA preceded the EMG onset by 110 msec, and a negative spike at the SMA-proper preceded the EMG onset by 50 to 60 msec. Epileptic discharges originating from the pre-SMA spread to the SMA-proper and possibly also to the primary motor cortex (MI) in this patient. Since both SMA-proper and MI could elicit EMG discharges through the independent corticospinal tracts having different conduction velocities, even a single spike arising from the SMA could give rise to a long EMG burst, which may play some role in the tonic convulsion which characterizes SMA seizures.