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 101.5h (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 9.3h (CI: 7.5-11.4) occurring in 7.3%, 9.7%, 74.6%, 87.9%, and 96.4% within 20 min, 30 min, 24h, 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 21.6h, 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.