Clinical Neurophysiology: Intraoperative Monitoring

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Chapter 32E Clinical Neurophysiology

Intraoperative Monitoring

Marc R. Nuwer

Chapter Outline

Techniques

Spinal Cord Monitoring Techniques  

Interpretation

Monitoring  
Testing  

Response to Change

Prediction of Deficits

Anesthesia

Clinical Settings

Summary

Neurophysiological intraoperative monitoring (IOM) uses electroencephalography (EEG), electromyography (EMG), and evoked potentials during surgical procedures to improve surgical outcome. When problems begin, these techniques warn the surgeon in time to intervene and correct the problem before it becomes worse or permanent. IOM methods also can identify neurological structures such as language cortex, so as to spare them from resection. A surgeon can rely on monitoring for reassurance about nervous system integrity, allowing the surgery to be more extensive than would have been safe without monitoring. Some patients are eligible for surgery with monitoring who may have been denied surgery in the past because of a high risk of nervous system complications. Patients and families can be reassured that certain feared complications are screened for during surgery. In these ways, monitoring extends the safety, range, and completeness of surgery.

Effective collaboration and communication is needed among surgeon, anesthesiologist, and neurophysiologist, who typically maintain communication throughout a specific procedure. An experienced electrodiagnostic technologist applies electrodes and ensures technically accurate studies. The interpreting neurophysiologist is either in the operating room or monitors continuously online in real time.

Techniques

Many intraoperative techniques are adapted from common outpatient testing: EEG, brainstem auditory evoked potential (BAEP), and somatosensory evoked potential (SEP) tests, for example. EEG is used for surgery that risks cortical ischemia, such as aneurysm clipping or carotid endarterectomy. BAEP is used for procedures around the eighth nerve or when the brainstem is at risk in posterior fossa procedures. SEP is widely used for many kinds of procedures in which the spinal cord, brainstem, or sensorimotor cortex is at risk.

Other techniques are more specific to the operating room. Transcranial electrical motor evoked potential (MEP) tests are evoked by several-hundred-volt electrical pulses delivered to motor cortex through the intact skull. Recordings are from extremity muscles. This monitors the corticospinal tracts during cerebral, brainstem, or spinal surgery. Electrocorticography (ECoG) measures EEG directly from the exposed cortex. This guides the resection to include physiologically dysfunctional or epileptogenic areas while sparing relatively normal cortex. Direct cortical stimulation applies very localized electrical pulses to cortex through a handheld wand. The electricity disrupts cortical function such as language, which can be tested in patients awake during portions of the craniotomy. Stimulation near motor cortex can produce movement. These techniques identify language or motor regions so they can be spared during resections. Similar direct nerve stimulation is used for cranial and peripheral nerves to locate them amid pathological tissue and check whether they still are intact. One version is stimulation at the floor of the fourth ventricle or during brainstem resection to identify tracts and nuclei of interest. For spinal procedures using pedicle screws, risk is incurred to the nerve roots or spinal cord during screw placement. To reduce that risk, EMG is monitored while electrical stimulation is delivered to the hole drilled in the spine or the screw as it is being placed. If the hole or screw errantly has broken through bone into the spinal or nerve root canal, stimulation will elicit an EMG warning of misplacement. In-depth descriptions of each procedure is beyond the scope of this chapter. The reader is referred elsewhere for extensive coverage of intraoperative neurophysiological techniques (Nuwer, 2008).

Spinal Cord Monitoring Techniques

SEP and MEP spinal cord monitoring is a good example of a common IOM technique. SEP electrical stimuli are delivered to the median nerve at the wrist or the posterior tibial nerve at the ankle. Stimuli are strong enough to evoke a muscle twitch at the thumb or foot muscles. Several hundred stimuli are delivered at about 5 per second. Averaged recordings are made at standardized surface locations over the spine and scalp. Small electrical potentials are recorded during the 25 to 45 msec after the stimulation, corresponding to transit of the axonal volley or synaptic events at the peripheral, spinal, brainstem, and primary sensory cortical levels. After establishing baseline values for typical peak latencies and amplitudes, this stimulation and recording is repeated every few minutes. MEP stimulating electrodes are placed on the scalp over motor cortex. Strong enough electrical pulses are delivered to discharge the axon hillock of motor cortex pyramidal cells. The resulting action potentials travel down corticospinal tracts and discharge spinal anterior horn cells. Recordings are made from limb muscles in the absence of neuromuscular blockage drugs; that EMG is seen at 25 to 45 msec after stimulation.

In uneventful spinal surgery, the measured peaks remain stable over time. When values change beyond established limits, the monitoring team warns the surgeon of an increased risk of neurological impairment. Which peaks are preserved and which are changed can localize the side level of impairment. Each of the four limbs is monitored. In thoracolumbar surgery, the median SEP nerve channels and upper extremity MEP channels serve as controls to separate systemic or anesthetic causes of change from thoracic or lumbar surgical reasons for change. Sometimes other nerves are stimulated for SEP. The ulnar nerve may be substituted for the median nerve during cervical surgery, so as to better cover the lower cervical cord. The peroneal nerve at the knee may substitute for the posterior tibial nerve at the ankle for elderly patients, diabetics, or others in whom a peripheral neuropathy may interfere with adequate peripheral conduction. Neuromuscular junction blockade is helpful to reduce muscle artifact in SEP but cannot be used if MEP also is monitored. Sometimes other incidental clinical problems are detected beyond the primary purpose of spinal cord, brainstem, or cortical region monitoring. For example, a developing plexopathy or peripheral nerve compression can be spotted by loss of the peripheral peak, which may be easily treated by repositioning an arm. Occasionally, changes warn of a systemic problem such as hypoxia secondary to a ventilation problem.

Interpretation

Interpretation of intraoperative neurophysiology includes two categories. One is monitoring

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