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

•IED latency and frequency in EEG is not influenced by the presence of seizures.•IED latency correlates with epilepsy etiology, onset, and duration.•EEG recordings of 72 h should be sufficient to determine the presence of IEDs in epilepsy patients.•Even with long-term recordings, EEG may not show IEDs in 1 out of 5 epilepsy patients.

PurposeTo assess the latency of interictal epileptiform discharges (IED) and seizures in long-term EEG recordings of patients with epilepsy.MethodIED 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).ResultsIEDs 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 (CI: 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.ConclusionIf 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.