Hyperexcitable cortical responses in progressive myoclonic epilepsy - A TMS study

Objective: Transcranial magnetic stimulation (TMS) has allowed investigators to study intracortical inhibition and facilitation and sensorimotor integration in motor disorders and epilepsy. The authors used TMS to elucidate the pathophysiology of reflex myoclonus with giant somatosensory evoked potentials (SEP). Methods: The authors studied four patients with progressive myoclonic epilepsy. All patients had giant SEP elicited by mixed and digital nerve stimulation. They studied the response to paired-pulse TMS at interstimulus intervals (ISI) ranging from 1 to 15 ms and the conditioning effect of digital electrical stimulation at ISI ranging from 10 to 100 ms on the motor evoked potential amplitude to TMS. Results: Digital stimulation markedly facilitated conditioned motor evoked potentials at ISI ranging from 25 to 40 ms in all patients. This pattern was significantly different from the inhibition observed in controls (n = 12) at the same ISL In the patients, paired-pulse TMS showed a decrease in intracortical inhibition in the motor cortex in comparison with controls. Conclusions: These findings suggest cortical and subcortical components of abnormal sensorimotor integration in addition to hyperexcitability of the sensory and motor cortex in our myoclonic patients.