Focal epilepsy without interictal spikes on scalp EEG: A common finding of uncertain significance

Abstract Objective Interictal epileptiform discharges (IEDs) are important to identify the epileptogenic zone and to define epileptic syndromes. However, not all patients show IEDs on scalp EEG. We evaluate the likelihood of not findings spikes on prolonged Video-EEG Monitoring (VEM) in patients with focal epilepsy, and explore clinical correlates. Methods We retrospectively reviewed the VEM reports for all the patients admitted to the seizure monitoring unit in the Calgary Epilepsy Program between July’10 and August’17. Adult focal epilepsy patients, using the diagnostic criteria of the International League Against Epilepsy, who had at least three consecutive VEM days and one recorded seizure were included. Patients were categorized as spikers or non-spikers if any or no spikes were seen on VEM. We compared demographic, neuroimaging, epilepsy risk factor and seizure data. Results Of 933 patients, 345 fulfilled our eligibility criteria, 17% [55% males] non-spikers and 83% [53% males] spikers. There were no statistically significant differences between non-spikers and spikers in the studied clinical variables at our epilepsy centre. Average age and average duration of epilepsy were 39 and 13 years for non-spikers and 38 and 16 years for spikers. The average duration of VEM was 8–9 days in both groups. The most frequent seizure focus was in the temporal lobe in both groups (53% in non-spikers vs. 64% in spikers, p = 0.06). An epileptogenic lesion on MRI was identified in 26 (46%) of non-spikers and 158 (57%) of spikers (p = 0.16). Significance Approximately one out of six patients with focal epilepsy showed no IEDs despite prolonged VEM. There was no significant difference among the investigated clinical variables between these two groups of patients in our epilepsy centre. We hypothesise that patients without IEDs on scalp EEG may have smaller, deeper generators with lower levels of neuronal synchrony, which precludes the expression of high amplitude spikes detectable on scalp EEG.