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.