Latency to first interictal epileptiform discharge in epilepsy with outpatient ambulatory EEG

The diagnosis and classification of epilepsy often relies upon the demonstration of interictal epileptiform discharges (IEDs). Routine 20-min EEG recording has low sensitivity, with multiple EEGs increasing sensitivity to a maximum of 77% (Doppelbauer et al., 1993). An alternate strategy is the use of prolonged continuous EEG; however, there are no data on the average latency to first IED with ambulatory monitoring.

In this retrospective study we reviewed 180 consecutive patients with epilepsy referred to a Specialist Epilepsy Unit who had undergone 96 h outpatient ambulatory EEGs, without medication withdrawal, where IEDs were recorded. Latency to, and factors affecting first IED were analysed.

Median latency to first IED was 316 min, (interquartile range 70-772 min, n=180). IEDs were recorded in 44% of patients within 4h, 58% within 8h, 85% within 24h and 95% within 48 h. Recording for the full 96 h period revealed only 5% further IEDs. Multivariate analysis showed the latencies to IEDs with generalised epilepsies were shorter than with focal epilepsies (p<0.0001).

In 95% of patients showing scalp IEDs a 48 h recording was sufficient for electro-clinical classification in this study.

Our data are the first to show the latency to recording interictal epileptiform discharges with prolonged outpatient EEG monitoring. These data are important in guiding diagnostic practice in Specialist Epilepsy Services.