Recruitment of motor cortex inhibition differentiates between generalized and focal epilepsy

Intracortical disinhibitory mechanisms play a crucial role in epilepsy. Therefore, the recruitment of motor cortical excitability was evaluated to distinct between focal and generalized epileptic syndromes. Twenty-five untreated patients with epilepsy and 20 controls were enrolled. Classification into focal (FE, n=10) or idiopathic generalized (IGE, n=15) epilepsy was based on seizure semiology, EEG and MRI. The recruitment of motor cortical inhibition and facilitation was measured by varying the stimulus intensity (SI) of the first conditioning stimulus in a paired-pulse transcranial magnetic stimulation (TMS) paradigm producing stimulus-response (S-R) curves of intracortical excitability. S-R curves were then compared with other commonly used TMS measures of cortical excitability [cortical silent period (CSP) and motor threshold (MT)]. In patients with IGE, inhibition occurred only at higher conditioning SIs compared to patients with focal epilepsy and controls. Recruitment of inhibition was unchanged in patients with focal epilepsy compared to controls. Recruitment of facilitation (ICF), CSP duration and MT, were not different between patients with FE or IGE or between patients and controls. These results suggest that the recruitment for motor cortical inhibition in patients with IGE is less effective. This may reflect a disturbed access to or an increased threshold of inhibitory neurons within the motor cortex. Impaired recruitment of inhibition might be a helpful parameter to access cortical excitability in newly diagnosed patients with generalized or focal epilepsy.