Ipsilateral cortico-cortical inhibition in patients with cryptogenic partial epilepsy

An altered balance between excitation and inhibition has been postulated as the neural basis of epileptic phenomena. Transcranial magnetic stimulation (IMS) can give information on such balance in the human motor cortex. We studied with IMS a group of eight patients affected by cryptogenic partial epilepsy (CPE) (mean age 30, S.D. ± 11.8), whose epileptic focus was located in the right hemisphere, and nine normal controls (mean age 28.7 S.D ± 5.7). TMS was delivered through a round coil centered at the vertex; motor evoked potentials (MEPs) were recorded from the right and left first dorsal interosseous muscle (FDI). We determined the following variables: (i) the threshold for evoking MEPs in the resting and active FDI ; (ii) the cortical silent period; (iii) the ipsilatci-al cortico-cortical inhibition and facilitation curve (conditioning shock = 0.8× and test shock= 1.2× resting threshold). Either hemisphere was stimulated separately, and the results were subject to inter-side comparison. There was also a comparison between homologous hemispheres of patients and controls. In the patient group, the variables measured showed no difference between the right hemisphere, containing the epileptic focus, and the contralateral one. Comparing CPE patients with controls, we found that: (i) the resting motor threshold was increased in both the patient hemispheres; (ii) the active motor threshold was increased only in the hemisphere containing the focus; (iii) the cortical silent period was prolonged in the hemisphere contralateral to the focus; (iv) the ipsilateral cortico-cortical inhibition (for intervals 1 and 3 msec) was reduced in the patients, more evidently so for the hemisphere site of the focus.