Abstracts

Rationale: The traditional bandpass filter for clinical reading and interpretation of scalp EEG is 1 Hz -70 Hz. Like most if not all traditions, this one is mired in a deep ancient notion that slow activity in the EEG is of little clinical interest to the reader, and is to be relegated to researchers. Pioneering ideas of Ernst Rodin with respect to the unfortunately named "DC shift" were ignored by the epilepsy community until recently, when "surround inhibition", a borrowed term to describe inhibitory GABA-ergic neuron firing around seizing neuron populations, and the resultant slow wave activity around a seizure focus ?" was demonstrated. This pilot study addresses slow activity recorded from the scalp in patients with localization-related epilepsy. The study evaluates interictal and preictal ISA (infra-slow activity) in Phase I patients, with the hypothesis that scalp ISA is at least lateralizing, and possibly a localizing/lobarizing phenomenon, particularly useful in nonlesional neocortical epilepsy. Methods: 11 Phase I patients with intractable localization-related epilepsy were admitted to UAB EMU for presurgical workup. EEG records were analyzed visually with routine bandpass filter of 1-70 Hz, then with the low frequency cutoff dropped to 0.001 Hz. Presence or absence of regional or focal ISA was noted by an epileptologist reader for the entire available EEG record, interictally, periictally, and ictally. ISA was defined as 0.001-0.2 Hz activity that was clearly not an artifact. Semiology and ictal EEG findings were correlated with the presence of ISA. Results: In eight patients, ISA was present over the same hemisphere, but not always over the same area where interictal abnormalities were present, as well as ictal onsets. These 8 patients had frontal rather than temporal seizure semiology, frontal or indeterminate seizure onsets, and in one case, a frontal lesion. The remaining 3 patients had clearly temporal onset seizures, a lack of distinctly frontal clinical characteristics of their seizures, and, in two cases, temporal lesions. This 3-patient group did not exhibit ISA, preictally or interictally. Conclusions: In scalp EEG, it is hard to distinguish frontal onset and temporal onset seizures, especially in nonlesional cases. Partly this is due to the fact that frontal and temporal semiological features overlap, and partly because symptoms may well be due to seizure propagation from a symptomatically quiet ictal zone. This distinction, temporal vs frontal onset, however - is an important call to make for Phase II surgical planning. Focal or regional scalp-recorded ISA appears to be present in some partial epilepsies. Frontal rather than temporal onset and semiology are associated with interictal slow activity, and with periictal ISA. Seizure outcomes and intracranial EEG correlation of ISA remains to be explored, as the same cohort of patients proceeds to Phase II, involving either sEEG or other intracranial implantation. Funding: None.