Latency of interictal epileptiform discharges in long-term EEG recordings in epilepsy patients

Purpose: To assess the latency of interictal epileptiform discharges (IED) and seizures in long-term EEG recordings of patients with epilepsy. Method: IED latency was measured in 210 consecutive patients (mean (SD) age 38.6 +/- 13.9 years) with active epilepsy and the relationship to clinical variables was analyzed retrospectively. Median duration of EEG recording was 101.5 h (95% confidence interval [CI] 92 to 117 h). Results: IEDs were absent in 45 (21.4%) and present in 165 (78.6%) patients who had a longer duration (p < 0.001) and early onset (p < 0.01) of epilepsy and more often had IEDs in prior standard EEGs (p < 0.01), a structural etiology (OR 2.4, CI: 2.1-2.7), or temporal lobe epilepsy (OR 9.6, CI: 9.0-10.2). IED latency did not correlate with other clinical variables. Median latency to the emergence of the first IED was 9.3 h (Cl: 7.5-11.4) occurring in 7.3%, 9.7%, 74.6%, 87.9%, and 96.4% within 20 min, 30 min, 24 h, 48 h, and 72 h, respectively. Seizure frequency was higher in patients with (n = 165) than without IEDs (n = 45) (72.1% vs. 46.6%, p < 0.01) and seizure latency (median 21.6 h, CI: 16.8-27.3) was influenced by the presence of IEDs, whereas the presence of seizures did not influence the latency and frequency of IEDs. Conclusion: If present, in the majority of epilepsy patients IEDs occurred during the first 72 h of long-term video-EEG recording. Repeated video-EEG or video recordings of habitual seizures are needed to minimize false negative studies. (C) 2015 British Epilepsy Association. Published by Elsevier Ltd. All rights reserved.