DISTINGUISHING FORMS OF GENERALIZED EPILEPSY USING MAGNETIC BRAIN-STIMULATION

In this study, we have used paired transcranial stimulation of the mot or cortex to test the hypothesis that cortical inhibition is decreased in juvenile myoclonic epilepsy (JME). The double shock technique was adopted here because it offers a means for highlighting abnormal inhib itory mechanisms. From previous experiments performed on healthy subje cts, it is known that a magnetic conditioning stimulus, of subthreshol d intensity, suppresses the MEP in response to a subsequent suprathres hold stimulus delivered after 1-4 msec. JME patients were selected as a potential contrast with other forms of idiopathic generalized epilep sy, because they complain of myoclonic jerks without loss of conscious ness, indicating with certainty a dysfunction of the motor cortex. Two patients with sporadic grand mal and one non-epileptic patient were a lso investigated. Paired stimulation was produced by a Bi-stim (Magsti m) stimulator, with a figure-of-8 coil placed over the hand area of th e motor cortex, and a set of interstimulus intervals (ISIs) ranging fr om 1 to 6 msec was analyzed. In JME patients there were two indication s of abnormality with respect to normal subjects and to the other epil eptic patients: (1) the absence of MEP suppression to paired stimulati on; (2) a progressive amplitude increase of MEPs to the test stimulus alone. In the two patients with the other form of epilepsy the pattern of inhibition was broadly preserved, even though there was some diffe rence from the normal profile. The results suggest that the loss of ME P inhibition can be regarded as a marker of JME.