Seizure Onset Determination

Digital electroencephalography has greatly expanded the opportunities for data analysis. Although commercial software packages are available they seem not to be used as widely in the preoperative work-up of epilepsy patients as might be warranted. This review will demonstrate that seizure onset can be better defined by judicious use of post hoc filter settings, expanded electrode coverage, and special electrode montages. In scalp recordings, ictal baseline shifts and infraslow activity (ISA) can be evaluated with conventional EEG systems by opening the high-pass filter to 0.01 Hz; in intracranial recordings high-frequency activity (>60 Hz; HFA) can be observed in addition. Inasmuch as ISA and HFA have considerably smaller electrical fields than the conventional frequencies they may better define seizure onset than might be possible otherwise. It is recommended that to determine the clinical value of ISA and HFA for epilepsy surgery, retrospective analyses of seizure data, which include assessment of ISA and HFA, be performed from patients who have undergone surgical resections of epileptogenic tissue. These may yield information as to whether or not the epileptogenic areas of ISA and HFA had been included in the resected tissue and their relationship to surgical outcome can then be determined.