that could mask or prohibit data acquisition, such as operating lights or other emitters of radiated electrical activity, should be minimized pre-operatively.

This chapter attempts to provide a brief description and discussion of intraoperative monitoring techniques that are currently used by neurosurgeons, some of which have been utilized with great success over the years, and others which remain in an experimental phase. Intraoperative imaging and image-guidance will be covered in Chapter 7.


Electroencephalography (EEG) monitors and records spontaneously generated electrical potentials originating from the various surface cortical regions of the brain. EEG was first utilized as an intraoperative monitoring technique in 1965. It has subsequently grown in popularity owing to readily available equipment, familiar and consistent technique, relative simplicity of pre-operative set-up, and well- characterized patterns of response to various states of neurological function.

Traditional EEG relies on the application of a standard grid of scalp electrodes that are posi tioned using the International 10-20 Scalp Electrode Placement System (Fig. 1.1). Data are typically gathered from both cerebral hemispheres. Appropriate personnel, including an individual trained in interpretation of the ongoing recordings, are required to apply the electrodes and maintain the system throughout the surgical procedure. Conventional EEG recording generates a great deal of data and requires continuous monitoring by a trained individual; therefore, there has been some interest in developing computer-based methods of real-time EEG analysis. Several methods of digitally processing EEG signals with subsequent computer analysis have been described, which utilize Fourier transforms to provide spectral power representations that are easier to interpret than the raw EEG data (known as compressed spectral array, or CSA). However, there has been concern over the failure of CSA to detect mild changes that could be detected with analog EEG monitoring, and the simplified data provided by CSA are more likely to be complicated by artifacts introduced by the operative environment. This may be alleviated by comparison of selected segments of raw data with the histograms generated by the computer.

The primary utility of intraoperative EEG is in monitoring for the presence of prolonged and

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