Chapter 33. Intraoperative Neurologic Monitoring

1. 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 the structure and function mapping in the awake patient.

2. The cellular basis of normal electroencephalography (EEG) reveals a variety of pathways to produce alterations of electrical and neurocognitive function.

3. Synchronous EEG is seen with sleep, sedation and anesthesia, and cerebral ischemia.

4. 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.

5. Achieving reliability with evoked potential monitoring depends on minimizing anesthetic effect, maintaining a constant anesthetic level, and ensuring adequate nervous tissue perfusion.

6. 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 during cortical mapping.

7. Subarachnoid block for placement of epidural stimulating electrodes allows for maintained patient perception of electrode stimulation while providing effective anesthesia for laminotomy.

Intraoperative neurologic monitoring is based on detecting changes in neurologic function that reflect injury to the nervous system. Neurologic function can be assessed intraoperatively either by repeated neurologic physical examinations or by inducing observable responses through electrical or magnetic stimulation of the nervous system. 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 and electrical activity 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 are presented as are techniques that allow for rapid emergence enabling intraoperative neurologic testing, immediate postoperative assessment, and continuous postoperative assessment in the intensive care unit.

Electroencephalography

Electrical activity in the brain, which is what an EEG records, 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 the EEG are presented by Martin,1 and some essential features are summarized here.

Neural Basis of Electroencephalography

The EEG is derived from postsynaptic potentials on pyramidal cells. There are 2 major classes of cortical neurons, pyramidal and nonpyramidal. Pyramidal cells derive their name from their distinctive shape and are notable as the only neuron that projects axons out of the cerebral cortex and locally as well. Their apical dendritic spines are oriented perpendicularly to the cortical surface and extend through the lamina of the cortex, enabling connection with all of the nonpyramidal cortical cells. These nonpyramidal cells serve to modulate ...