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Cranial motor nerves are at risk of being injured during many neurosurgical operations of the skull base, such as operations to remove different kinds of tumor. Cranial motor nerves can also be at risk during operations on the vascular system of the brain. The risk of loss of function of cranial motor nerves during surgical procedures can be reduced by appropriate use of intraoperative neurophysiological monitoring, thus decreasing the risk of postoperative deficits that have more or less severe consequences. Methods are available that can monitor the motor function of cranial nerves CN III, CN IV, CN V, CN VI, CN IX, CN X, CN XI, and CN XII.

This chapter describes the normal anatomy and function of somatic peripheral nerves and different forms of injuries that can occur from trauma and other forms of insults. Because intraoperative monitoring of nerves of the autonomic system has not found practical use, this topic is not covered in detail. Chapter 13 provides a description of the practical aspects of intraoperative monitoring and diagnosis of pathologies of peripheral nerves.

Intraoperative Measurement of Nerve Conduction Monitoring of neural conduction is important for detecting surgically induced injuries to nerves and it is a prerequisite for reducing the risks of postoperative deficits. Several different techniques can be used for such monitoring. One method utilizes stimulation of a nerve and recording of the compound action potential (CAP) from another location on the nerve. Other methods use recording of somatosensory evoked potential or the F response1 and H response. These methods can be used for detecting partial or complete failure of neural conduction and for measurements of changes in neural conduction velocity. Such measures are important for detecting injuries caused by surgical manipulations. Similar electrophysiological methods can be used for finding the anatomical location of injuries to nerves ( Chap. 14). Intraoperative measurement of conduction of peripheral nerves plays an important role in guiding the surgeon in repair of injured nerves (discussed in Chap. 15).

The most direct way that intraoperative neurophysiological recordings can guide the surgeon in an operation is in identifying a specific nerve. This is of great importance when trying to identify cranial nerves in cases where the anatomy is distorted by a pathological process. Previous operations might have changed the anatomy, making it difficult to identify specific nerves solely on the basis of visual observation in a surgical field. Tumors and malformations of various kinds can have distorted the anatomy so that it becomes difficult to identify specific neural tissue. These problems could occur in connection with cranial nerves and peripheral nerves. Neurophysiologic methods can identify nerves in such situations, and in other situations, neurophysiological methods can confirm the anatomy.

Intraoperative neurophysiological recordings are not only beneficial for reducing the risk of postoperative deficits, but similar techniques can be used for diagnosis of peripheral nerve disorders and for guiding the surgeon in certain operations. Intraoperative measurements of neural conduction and neural conduction velocity can help to determine the nature of a specific pathology and to identify the anatomical location of the pathology of nerves. Such recordings can guide the surgeon to the proper anatomical location for surgical intervention and, indeed, might also help the surgeon choose the appropriate surgical intervention.

Because anesthesia could affect the results of intraoperative monitoring, it is important that the person who is performing the intraoperative neurophysiological monitoring understand the basic principles of anesthesia. The person who is responsible for monitoring should communicate with the anesthesiologist to obtain information regarding the type of anesthesia that is to be used, if there are changes made in the anesthesia during the operation, and, if so, what other drugs might be administered during the operation.

Intraoperative neurophysiological monitoring is a technique used to assist in the prevention of accidents in surgical operations, or rather reduce the risks of accidents. In order that intraoperative neurophysiological monitoring can serve that purpose adequately, it is important to reduce the risk of human mistakes, equipment failure, or electrode failure, which can jeopardize proper execution of intraoperative neurophysiological monitoring.

In the early days of intraoperative monitoring, either custom-made equipment or equipment taken from the clinical testing laboratory or the neurophysiological animal laboratories was used in the operating room. Now, there is specialized equipment commercially available for nearly all needs of intraoperative monitoring. This means that the persons who do monitoring do not need to know as much about recording and stimulating equipment as they did earlier. However, knowledge about the basic function of the equipment that is used for intraoperative monitoring is an advantage for optimal use of the equipment and for troubleshooting. The equipment now used for intraoperative monitoring is capable of appropriate signal processing and it has several possibilities for filtering the recorded responses. The user must have sufficient knowledge about the basis for filtering and signal averaging to use these methods in optimal ways. Modern equipment also have many options for display of recorded potentials.

Guidelines for intraoperative monitoring have been issued by various bodies. The Therapeutics and Technology Subcommittee of the American Academy of Neurology has concluded that the following are useful and noninvestigational: (1) EEG, compressed spectral array, and somatosensory evoked potential (SSEP) in CEA and brain surgeries that potentially compromise cerebral blood flow, (2) auditory brainstem response (ABR) and cranial nerve monitoring in surgeries performed in the region of the brainstem or inner ear, and (3) SSEP monitoring performed for surgical procedures potentially involving ischemia or mechanical trauma of the spinal cord (44). Earlier, the National Institutes of Health Consensus Development Conference (held December 11–13, 1991) stated in a “Consensus Statement” that