Abstracts

Rationale: High frequency oscillations are now routinely measured at the Montreal Neurological Institute in epileptic patients studied with intracerebral depth electrodes EEG (iEEG). These analyses need to be done during slow wave sleep. To maintain sterile conditions, scalp electrodes cannot be applied using collodion and, hence, sleep staging was so far obtained by looking at the presence of delta activity in depth electrode contacts combined to EMG and EOG patterns. This method does not allow the detection of vertex sharp waves, K-complexes and spindles, and sleep staging may therefore be difficult or inaccurate. In March 2010, we started using Subdermal Wire Electrodes (SWE, Ives et al.) for sleep staging in patients who undergo iEEG. Methods: From March 2010 to May 2011, SWE were placed in 18 patients for a total time of recording of 235 days (mean, 13 days; range, 2-22). Nine SWE were placed per patient according to the 10-20 international system (F3-C3-P3-Fz-Cz-Pz- and F4-C4-P4). SWE were inserted subdermally by the surgeon after the insertion of intracerebral electrodes under general anesthesia, and fixed at the skin with a suture. We evaluated the ability of SWE to detect sleep patterns, the quality of the EEG recordings, the quality of the imaging studies done during iEEG and complications due to SWE. For each patient, we analyzed at least one night of recording looking for vertex sharp waves, spindles, K-complexes, alpha activity and delta waves.Results: Eighteen patients were studied (8 males), and none reported pain explained by the SWE and no traumatic or infectious complications were observed. Troublesome artifacts were found in one electrode in one patient only. In six patients, occasional transient artifacts were observed, which did not prevent EEG interpretation. Impedances were measured the first day of recording in 12 patients and repeated during the last 2 days in three, and remained low and stable: they were at 1 to 9 kilo-ohms initially and at 1 to 8 kilo-ohms at the end. The use of SWE did not require any additional technical time, and caused no significant artefact to MR images obtained during iEEG evaluation (n=9 pts). One patient was recorded during wakefulness only. In all others alpha activity and sleep patterns (delta, spindles, vertex sharp waves and K-complexes) were easily visualized.Conclusions: SWE are easily inserted, they are well tolerated by patients, and they remain in place and are stable without further manipulations. They provide very good quality scalp EEG recordings for several days and without complications. Their usefulness was confirmed by their ability to easily detect sleep patterns and alpha activity. This study demonstrated that SWE is a safe and reliable method to measure scalp EEG over long periods of time. EEG recordings with SWE should be considered in subjects who undergo invasive EEG evaluations and in ICU patients.