Time to first interictal epileptiform discharge in extended recording EEGs

The presence of interictal epileptiform discharges (IEDs) in the outpatient interictal EEG can provide evidence for a diagnosis of epilepsy and support for a specific epilepsy syndrome. However, there is variation in the duration of outpatient EEGs, and prolonged EEGs may have a higher yield for IEDs. The authors retrospectively reviewed 172 routine, extended, outpatient EEGs of at least 60 minutes duration in adults to determine the time to the first IED. The mean duration was 187 minutes. Twenty-six percent captured IEDs. The initial IED occurred in the first 20 minutes in 53% and after 20 minutes in 47%. The mean time to the first IED was 32.8 minutes with a range of 1 to 216 minutes and a standard deviation of 48.2 minutes. Results were skewed to the right (skew 2.22) with a median time to the first IED of 10 minutes. There was a longer time to the first IED in temporal epileptiform discharges compared with generalized discharges.