Supervision of ambulatory cassette EEG screening: a strategy based on the temporal distribution of epileptiform...

The interposition of screening personnel in the analysis of extended cassette EEG recordings requires a method for supervision by the responsible electroencephalographer. To this end, analysis was undertaken of the temporal distribution of epileptiform abnormalities and seizures on cassette EEG in 40 epilepsy unit inpatients for whom these abnormalities were confirmed by simultaneous cable telemetry. Epileptiform abnormalities were noted in the hour following sleep onset in 38 patients (92%). Seizures occurred primarily in wakefulness. One hundred cassette EEG tapes obtained for clinical purposes and previously found to contain evidence of epilepsy were then subjected to an abbreviated review. Eighty-seven contained epileptiform abnormalities in the first hour of sleep, and 7 additional tapes contained only EEG seizure activity coincident with reported clinical events. Considering that fewer than 20% of cassette recordings obtained in unrestricted clinical use are likely to harbor epileptiform abnormalities or seizures, correct identification of about 99% of tapes as normal or abnormal can be anticipated with an abbreviated review consisting of analysis of the first hour of sleep; and analysis of reported clinical events if no abnormalities are noted in the first hour of sleep. Such a review is sufficiently accurate to serve as a supervisory analysis of cassette EEG tapes that have been previously screened and reported to lack evidence of epilepsy.