In the appropriate clinical setting, the presence of interictal epileptiform abnormalities (IEAs) on EEG supports the diagnosis of epilepsy. However, the absence of epileptiform abnormalities on EEG cannot exclude a diagnosis of epilepsy. The goal of our study is to determine the prevalence of IEAs in patients with confirmed epilepsy, determined by having at least one epileptic seizure recorded during video-EEG monitoring. In addition, we aimed to analyze the time to recording IEAs and seizures in correlation with patient age, duration of epilepsy, and seizure focus localization. We retrospectively evaluate EEG data for all patients admitted to the epilepsy monitoring unit over a 2-year period. Of the 151 patients included, 129 (86%) patients had IEAs and 22 (14%) patients had no IEAs. Age and duration of epilepsy were not independent predictors of whether IEAs were present on EEG. The duration of EEG monitoring and time to first seizure did not influence IEA detection. In patients with IEAs, the mean time to the first IEA was 1.57 days. By day 5, IEAs were observed in 95% of the patients who had IEAs present on EEG (82% of total patients). The majority (75%) of patients also had their first seizure by day 5. We concluded that five days of EEG recording is optimal to detect IEAs and seizures, and that more prolonged recording has a low yield. Failure to detect IEAs should be interpreted with caution, and is not useful for diagnostic purposes.
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