High Frequency Oscillations in the Ripple Band (80-250 Hz) in Scalp EEG: Higher Density of Electrodes Allows for Better Localization of the Seizure Onset Zone.

High frequency oscillations (HFO) are known as markers of epileptic areas in intracranial EEG and possibly scalp EEG. We compared distributions of HFO in the ripple band (80-250 Hz) in intracranial and scalp EEG with either a conventional 10-20-montage (10-20-EEG) or a high density recording using 128 electrodes (HD-EEG). HFO were visually identified, in all intracranial EEG channels (80-500 Hz) and all channels of the 10-20-EEG (scalp EEG 80-250 Hz). For the HD-EEG, HFO were analyzed in regions of interest using areas with HFO as seen on the 10-20-EEG as well as areas in the clinically-defined seizure onset zone (SOZ). 13 patients were included in the study, of whom 12 showed HFO in the ripple band. In 8 patients HD-EEG revealed additional regions of ripples compared to the 10-20-EEG. With HD-EEG, areas of highest ripple rates were corresponding between scalp and intracranial EEG in 7 patients (58%) and 8 (67%) patients showed highest ripple rates over the SOZ. In contrast, with 10-20-EEG only 2 patients (17%) had corresponding areas of highest ripple rates and only 3 patients (23%) showed highest ripple rates over the SOZ. HD-EEG proved to be better to identify scalp HFO in the ripple band compared to standard 10-20-EEG. Moreover, ripples in 10-20-EEG seem to lead to false localization of epileptic areas. In contrast ripples detected with HD-EEG were located over the seizure onset zone and maybe a promising tool to localize epileptic tissue in the future.