Atypical Interictal Epileptiform Discharges in Electroencephalography

A great deal of attention has been focused on "typical" interictal epileptiform discharges (IEDs) in the electroencephalography (EEG) literature. However, there is a paucity of data on "atypical" IEDs, namely, positive sharp waves (PSWs), focal triphasic sharp waves and spikes (FTSWs), sharp slow waves (SSWs), bifid spikes, and "notched" delta. In this present study, we sought to address the pathophysiology, characteristics, and diagnostic significance of "atypical" IEDs in clinical neuroscience.

We prospectively reviewed the EEGs of 1,250 patients from a heterogeneous population over a period of 2 years. We also documented demographic, clinical, and neuroimaging data.

Thirty-one patients had PSWs, 26 had FTSWs, 30 had SSWs, 24 had notched delta, and four had bifid spikes in their EEG data. Ninety-six percent of patients with PSWs had epilepsy whereas 100% of the FTSW and SSW groups had this diagnosis. In the ND group the rate of epilepsy was 95% and in the bifid spike group 75%. Accordingly, "atypical" IEDs are potentially epileptogenic patterns with localizing significance, occurring primarily in younger age groups. We also found that a significant number of these patients had congenital central nervous system anomalies.

We conclude that "atypical" IEDs are rare and under-reported EEG patterns that potentially signify focal epileptogenicity. Our data also stresses the significance of neuroimaging in investigating the possibility of an underlying congenital central nervous system anomaly in this population.