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KEY POINTS
Anesthetic strategies to enhance intraoperative monitoring of the nervous system include techniques that minimize interference with neurophysiologic monitoring as well as techniques that preserve neurocognitive function during structural and functional mapping in awake patients.
The cellular basis of normal electroencephalography (EEG) reveals a variety of pathways to produce alterations of electrical and neurocognitive function.
Synchronous EEG is seen with sleep, sedation and anesthesia, and cerebral ischemia.
Processed EEG algorithms can aid the objective assessment of EEG changes. As long as there is an understanding of the EEG features analyzed by these algorithms, pitfalls leading to inaccurate assessment can be avoided.
Achieving reliability with evoked potential monitoring depends on minimizing anesthetic effects, maintaining constant anesthetic levels, and ensuring adequate tissue perfusion.
Intraoperative wakefulness for cortical mapping has been achieved by a variety of techniques. For a successful procedure, all techniques must address maintenance of effective ventilation during craniotomy and a balance of clear sensorium and sufficient analgesia to enable effective patient participation.
Intraoperative computed tomography (iCT), intraoperative magnetic resonance imaging (iMRI), and biplanar angiography are opening a whole new realm of real-time anatomic and physiologic monitoring that will further enhance neurosurgical patient outcomes. It is essential that the anesthesiologist have a clear understanding of these technologies to facilitate the neurosurgical intervention while maximizing patient safety.
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The purpose of intraoperative neurologic monitoring is to detect changes in neurologic function that reflect injury to the central or peripheral nervous system. Neurologic function can be assessed by repeated neurologic examinations in awake patients or by inducing observable responses through electrical or magnetic stimulation of the nervous system. Real-time intraoperative imaging modalities complement classic intraoperative monitoring techniques by adding anatomic and physiologic data to guide tissue resection, implant placement, or vascular integrity.
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Because much of intraoperative neurophysiologic monitoring depends on some measurement of the electroencephalogram (EEG), this chapter starts with a brief overview of the cellular mechanisms responsible for the genesis of the EEG. Phenomena that modify electrical brain function are considered so readers can appreciate the possibilities and limitations of electrical monitoring in guiding anesthetic administration and safeguarding the integrity of the nervous system. The use of anesthetic techniques that minimize interference with monitored neurophysiologic function is presented, as are techniques that allow for rapid emergence enabling awake intraoperative neurologic testing and immediate postoperative assessment. Finally, we review intraoperative imaging modalities (intraoperative computed tomography [iCT], intraoperative magnetic resonance imaging [iMRI], and biplanar angiography) that further extend our ability to monitor the integrity of the nervous system during neurosurgical intervention.
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ELECTROENCEPHALOGRAPHY
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Electrical activity in the brain was first measured in 1875 by Richard Caton, who noted the electrical oscillations on the exposed cortical surface of animals. The physiologic mechanisms and cortical morphology responsible for generating this EEG originally presented by Martin1 are reviewed here.