1 Introduction , history , and sta ffi ngfor intraoperative monitoring

Neurophysiologic intraoperative monitoring (IOM) has grown over three decades into a method widely used to prevent neurologic injury during surgery. Common IOM techniques include electroencephalography (EEG), electromyography (EMG), evoked potentials (EPs), and nerve conduction velocity (NCV). Intraoperative monitoring can warn the surgeon of changes in time to correct problems and prevent postoperative neurologic deficits. It may also identify some systemic problems. By using IOM to assess a patient’s neurologic safety, a surgeon may provide a more thorough procedure or operate on a high-risk patient who might otherwise be turned away. Finally, patients and families can take comfort that neurologic risks are being evaluated during the case. False alarms do occur, e.g., in 1% of scoliosis spinal-cord monitoring. False alarms are cases when the IOM warns of changes, but the patient awakens from surgery without a new deficit. Some may be false alarms caused by technical failures, difficulty obtaining good quality tracings, or anesthetic changes. In other cases, IOM sometimes raises an alarm, interventions are accomplished, and yet the patient awakens with a neurologic injury. Raising an alarm does not necessarily prevent deficits. False-negative cases are those in which a patient suffers a postoperative neurologic injury that was not predicted by IOM. In spinal-cord SEP (somatosensory evoked potential) monitoring, the false-negative rate is around 0.1% of SEP-monitored scoliosis procedures (Nuwer et al., 1995). Some false-negatives are due to deterioration soon after surgery. Some injuries are in pathways that were not monitored, e.g., some root lesions. Occasionally, false-negative cases are due to errors by the IOM team, who failed to recognize changes when they occurred. Finally, the existence of some otherwise unexplainable false-negative monitoring cases is a reminder that no technique is 100% accurate in predicting outcomes. History of monitoring Early uses of neurophysiologic monitoring during surgery date back to the first half of the twentieth century. Penfield (Penfield and Boldrey, 1937) used direct cortical stimulation in patients undergoing epilepsy surgery to define the locations of motor and sensory cortex. Direct recording of EEG from exposed cerebral cortex (electrocorticography, ECoG) guided the surgeon to resect regions of epileptic discharges, slowing, or lack of fast activity (Jasper, 1949; Marshall and Walker, 1949). Both techniques are still used. More than a decade later, routine scalp EEG was used during carotid endarterectomy (CEA) (Thompson, 1968; Wylie and Ehrenfeld, 1970; Sharbrough et al., 1973) to assess for cerebral ischemia during carotid clamping. Electroencephalography accurately measures degrees of cerebral ischemia during carotid endarterectomy (CEA) (Sundt et al., 1974). It has provided an excellent substitute for keeping the patient awake during CEA or using other ischemia testing techniques and has become commonly used as a safeguard for CEA patients. Spinal-cord IOM was first investigated in the early 1970s by Japanese investigators using epidural recordings of spinal potentials evoked by direct spinal stimulation (Shimoji et al., 1971; Imai, 1976). These techniques were validated as accurate methods for monitoring spinal cases (Tamaki et al., 1972, 1981). Spinal IOM using somatosensory evoked potentials (SEPs) was developed in the mid-1970s. This involved using middleand long-latency 50–200ms cortical potentials during orthopedic procedures (Nash et al., 1974, 1977; Nash and Brodkey, 1977), but the attempts were too prone to disruptive signal noise, variability, and sensitivity to anesthesia. Grundy (1982), an anesthesiologist working with Nash, described ways to reduce anesthetic effects and extended the techniques to neurosurgery. These early SEP cortical IOM techniques used filters at 1–100Hz,