Timing of congruent and discordant interictal epileptiform <scp>EEG</scp> discharges in focal epilepsy

OBJECTIVE: Interictal epileptiform discharges (IEDs) can provide additional information for localizing the epileptogenic zone (EZ). However, due to the brevity of routine electroencephalography, IEDs are often not recorded and there is a lack of general recommendation regarding the recording duration METHODS: In our study, we analyzed the mean latency until the first IED was detected in 589 patients with focal epilepsy. IEDs were distinguished as either congruent (occurring within the EZ) or discordant (occurring outside the EZ). Patients were assigned to one of four regional subgroups according to the localization of their EZ: frontal, temporal, central, and parieto-occipital. RESULTS: A total of 50 040 IEDs were analyzed across temporal (n = 252), frontal (n = 127), central (n = 5), and parieto-occipital epilepsy (n = 27). The first IED was significantly more likely to be congruent with the EZ in temporal lobe epilepsy (p < 0.01); otherwise, it was not statistically significant. The mean latency was 19.73 h for congruent IEDs and 23.27 h for discordant IEDs. Overall, 90% of the first IEDs were detected within 49.20 h, with the longest latency observed in frontal lobe epilepsy. SIGNIFICANCE: Our study demonstrates the latency to the first IEDs on EEG, differentiating between IEDs that are congruent and discordant with the EZ. This allows for a more precise estimation of the necessary length of an EEG recording to evaluate the EZ.