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Surgery can generally be regarded as a risk-filled method for treating diseases, and it has a potential for causing injury to the nervous system. Because such injuries might not be detected by visual inspection of the operative field by the surgeon, they could occur and progress without the surgeon’s knowledge. Intraoperative neurophysiological monitoring involves the use of neurophysiological recordings for detecting changes in the function of the nervous system that are caused by surgically induced insults.

Intraoperative neurophysiological monitoring is often associated with reducing the risk of postoperative neurological deficits in operations where the nervous system is at risk of being permanently injured. Although the main use of electrophysiological methods in the operating room might be for reducing the risk of postoperative neurological deficits, electrophysiological methods are now in increasing use for other purposes. For example, electrophysiological methods are now regarded as necessary for guiding the placement of electrodes for deep brain stimulation or for making lesions in specific structures for treating movement disorders and pain. Intraoperative electrophysiological recordings can also help the surgeon in carrying out other surgical procedures. Finding specific neural tissue such as cranial nerves or specific regions of the cerebral cortex are examples of tasks that are included in the subspecialty of intraoperative neurophysiological monitoring.

To understand why and how neuroelectrical potentials, such as evoked potentials, might change as a result of surgical manipulations, it is necessary to understand the basic principles underlying the generation of the neuroelectrical potentials that can be recorded from various parts of the nervous system. In this volume, we discuss electrical potentials that are generated in response to intentional stimulation and we describe how the waveform of such recorded potentials might change as a result of injury to nerves or nuclei. It is also important to understand the nature of the responses that might be elicited by surgical manipulations of neural tissue and from surgically induced injuries. Further, it is important to know where in the nervous system specific components of the recorded evoked potentials are generated, so that the exact anatomical location of an injury can be identified on the basis of changes in specific components of the electrical potentials that are being monitored.

Intraoperative neurophysiological monitoring employs methods and techniques similar to those currently used in the clinical neurophysiology laboratory, but there are several important differences between recording sensory evoked potentials and electromyographic (EMG) potentials for diagnostic purposes in the clinic and for doing so in order to detect changes in neural function during an operation. The operating room is usually regarded to be an electrically hostile environment, which differs from the clinical neurophysiological laboratory where recording of EMG responses and sensory evoked potentials such as auditory brainstem response (ABR), somatosensory evoked potentials (SSEP), and visual evoked potentials (VEP) are usually done in electrically and acoustically shielded rooms. In the operating room, many other kinds of electronic equipment are connected to the patient.

Knowledge about the anatomy and physiology of the auditory system is a prerequisite for understanding not only the normal function of the auditory system but also the changes in function that might result from surgical manipulations of the auditory nerve and other, more central structures.

The eighth cranial nerve (CN VIII) is at risk of being injured by surgical manipulations in microvascular decompression (MVD) operations to relieve trigeminal neuralgia (TGN), hemifacial spasm (HFS), glossopharyngeal neuralgia (GPN) (70,71), and in connection with MVD operations of the eighth nerve in patients with tinnitus and disabling positional vertigo (DPV) (72).

Intraoperative recordings of somatosensory evoked potentials (SSEPs) were among the earliest used electrophysiological methods for monitoring function of the spinal cord and, for that matter of any neurological system. Orthopedics was the first specialty of surgery in which this method was used beginning in the 1970s in operations for scoliosis (112–114). When SSEPs are monitored during operations involving the spinal cord, the responses are usually elicited by electrical stimulation of a peripheral nerve and recorded from electrodes placed on the scalp. The SSEPs obtained in that way are generated by successive excitation of neural structures of the ascending somatosensory pathway. These potentials thus consist of different components that appear with different latencies ( the description of the neural generators of the SSEP in Chap. 5).

Intraoperative monitoring of visual evoked potentials (VEPs) during neurosurgical operations has been described by several investigators (63 – 65, 68, 83) for the purpose of preserving vision in operations in which the optic nerve or optic tract is being manipulated or in operations that involved the occipital cerebral cortex (163). It has been found difficult, however, to obtain reliable recordings of VEPs in anesthetized patients who did not undergo intracranial procedures (164).

The anatomy and the physiology of motor systems have been studied extensively in animal experiments. However, the animals used in the 1970s were mainly cats, the motor systems of which have considerable differences from that of humans. Even when monkeys were used for such studies, it became evident that their motor systems were different from that of humans. The limited possibilities of studying especially the neurophysiology of the human motor systems has caused knowledge about the human motor system to be limited. Studies in the operating room have contributed valuable information about the human motor system. This chapter will provide a basic description of the anatomy and functional organization of the motor system. When the information stems from studies in animals, it will be pointed out that the description might have discrepancies regarding the situations in humans. We will describe the spinal motor system and cranial nerve motor system separately in this section.

This chapter concerns practical aspects on monitoring spinal motor systems. (Monitoring of cranial motor nerves is discussed in Chap. 11.) It discusses techniques for stimulation of the motor cortex and the spinal cord and for recording the responses.