Estimating the importance of epileptiform abnormalities discovered on cassette electroencephalographic monitoring.

Assessment of the importance of interictal epileptiform abnormalities discovered with cassette electroencephalographic (EEG) monitoring requires some appreciation of the frequency with which such abnormalities may be encountered in individuals without epilepsy. From a clinical experience involving more than 2500 patients, we have defined a group of 184 patients referred because of headache, with no additional referral information to suggest seizures. Only one (0.5%) of these patients had epileptiform abnormalities on cassette EEG, yielding 95% and 99% confidence limits for the incidence of epileptiform abnormalities in the unselected nonepileptic headache population of 1.5% and 1.8%. Presuming that the incidence of cassette EEG epileptiform abnormalities in the healthy population would be no higher than in this patient group, we suggest that epileptiform abnormalities are no more likely to be incidental findings on cassette EEG monitoring than on routine EEG. Consequently, the detection of such abnormalities seems a worthwhile aspect of cassette EEG interpretation when the goal of monitoring is the detection of evidence to support a diagnosis of epilepsy.

Spike voltage topography identifies two types of frontotemporal epileptic foci.

We characterized voltage topography of frontotemporal EEG spikes in 24 patients with complex partial seizures and identified two distinct patterns. "Type 1" spikes possessed a "dipolar" field with a negative region over the inferolateral temporal scalp and a positive region over the contralateral, centroparietal scalp. "Type 2" spikes showed only a broad, frontotemporal negative field. One or the other spike type predominated in all but two patients. Correlations with clinical data and intracranial EEG suggest that type 2 spikes arise from temporal or frontal neocortex, while type 1 spikes involve mesial temporal structures as well as lateral cortex.

Spike voltage topography and equivalent dipole localization in complex partial epilepsy.

The EEG of 45 patients with complex partial epilepsy was recorded from standard and supplementary inferior temporal electrode sites for 2 or more days via cable telemetry onto video (VHS) tape (22-25 channels, common reference). Epochs with "temporal spikes" were read into a topographic EEG device where individual spikes were visually identified and averaged in sums of 8-32. Analysis of spike voltage topography revealed two distinct patterns - dipolar, Type 1 and non-dipolar, Type 2. One or the other spike type predominated in all but two patients. Application of source modeling techniques (3 shells, single dipole, 6 parameters) to the spike topography data revealed that both spike types had similar equivalent dipoles in terms of location and orientation, except for vector elevation. However, calculated dipoles for Type 1 spikes were more stable over the course of the spike peak. Correlations with clinical data and intracranial EEG suggest that Type 1 spikes originate in mesial temporal structures, while Type 2 spikes arise from temporal or frontal neocortex. Spike voltage topography and equivalent dipole localization appear to be useful in the presurgical evaluation of patients with focal epilepsy.