Frequency of the electroencephalographic discharge in seizures of focal and widespread onset in intracerebral recordings.

The information provided by the EEG during an epileptic seizure is of critical importance in the localization of an epileptic focus. Localizing information is usually provided by the region of onset, by the predominance of the discharge during the seizure, and by postictal slow waves. In patients with intracerebral electrodes, we investigated another aspect of the discharge: its highest frequency. We divided seizures into those with a focal onset and those with a regional onset. Results indicated clearly that fast frequencies (15-30 Hz) were much more frequent during seizures of focal onset than during seizures of widespread onset. Highest frequencies were not necessarily observed at the onset of focal seizures but could be observed at any time during a seizure of focal onset. High frequencies appear to characterize small epileptogenic zones (EZ). Our results may be helpful in the interpretation of intracerebral EEGs when large brain areas remain unexplored and one is often uncertain of the extent of an EZ.