A brain symmetry index (BSI) for online EEG monitoring in carotid endarterectomy

Introduction: Carotid endarterectomy is a common procedure as a secondary prevention of stroke, and one of the early controversies in carotid surgery is centered around whether a shunt should be used during this-procedure. Although various EEG parameters have been proposed to determine if the brain is at risk during carotid artery clamping, the common procedure is still the visual assessment of the EEG. We propose a brain symmetry index (BSI), that has been implemented as an on-line quantitative EEG parameter, as an additional criterion for shunt need in carotid endarterectomy. Methods: The BSI captures a particular asymmetry in spectral power between the two cerebral hemispheres, and is normalized between 0 (perfect symmetry) and I (maximal asymmetry). The index was evaluated retrospectively in a group of 57 operations in which the EEG and the transcranial Doppler were used as criteria for shunt insertion. In addition, after online implementation of the algorithm, several patients have been evaluated prospectively. Results: If no visual EEG changes were detected, it was found that the change in BSI from baseline, DeltaBSI less than or equal to 0.03 in all patients. In none of these patients shunting was performed, except for I I in whom shunting was advised based on changes in the transcranial Doppler signal. None of these patients suffered from neurological complications. In those operations with visual EEG changes during test-clamping and selective shunting, we found that DeltaBSI greater than or equal to 0.06. In this group, one patient suffered from intraoperative stroke and one patient died, most likely from a hyperperfusion syndrome. Conclusions: The BSI may assist in the visual EEG analysis during carotid endarterectomy and provides a quantitative measure for electroencephalographic asymmetry due to cerebral hypo-perfusion. In patients with a change in the BSI (DeltaBSI) smaller than 0.03 during test clamping, visual EEG analysis showed no changes, whereas if visual EEG analysis did warrant shunting, it was found that DeltaBSI greater than or equal to 0.06. (C) 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.