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Intraoperative Monitoring
Neurophysiological intraoperative monitoring (IOM) uses electroencephalography (EEG), electromyography (EMG), and evoked potentials (EPs) during surgery to improve outcome. These techniques warn the surgeon of impending complications in time to intervene and correct problems before they become worse. IOM can also identify motor or language cortex 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 without monitoring 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 are needed between surgeon, anesthesiologist, and neurophysiologist ( ). The monitoring team maintains open communication throughout surgery. 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 IOM techniques are adapted from common outpatient testing: for example, EEG, brainstem auditory evoked potential (BAEP), and somatosensory evoked potential (SEP) tests. Box 39.1 lists various techniques used in the operating room. EEG is used when surgery 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: for example, Fig. 39.1 . SEP is widely used for many procedures that risk impairment to the spinal cord, brainstem, or sensorimotor cortex.
BOX 39.1
Techniques Used for Intraoperative Monitoring and Testing
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Electroencephalography
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Electrocorticography
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Direct cortical stimulation
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Somatosensory evoked potentials
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Transcranial electrical motor evoked potentials
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Brainstem auditory evoked potentials
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Deep brain and brainstem electrical stimulation
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Electromyography
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Nerve conduction studies
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Direct spinal cord stimulation
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Reflex testing
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Pedicle screw stimulation testing
Some IOM techniques are 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 made from extremity muscles. MEP monitors corticospinal tracts during cerebral, brainstem, or spinal surgery. Electrocorticography (ECoG) measures EEG directly from the exposed cortex. ECoG guides the surgeon to resect physiologically dysfunctional or epileptogenic areas while sparing relatively normal cortex. Direct cortical stimulation applies very localized electrical pulses to cortex through a handheld wand. That electricity disrupts cortical function such as language, which can be tested in patients who are awake during a craniotomy. Direct cortical stimulation identifies language or motor regions so that they can be spared during resections. Similar direct nerve stimulation is used for cranial and peripheral nerves to locate them amid pathological tissue and to check whether a nerve is still intact. Electrical stimulation of the floor of the fourth ventricle during brainstem resection can identify tracts and nuclei of interest. The placement of spinal pedicle screws risks injury to nerve roots or spinal cord. To reduce that risk, EMG is monitored while electrical stimulation is delivered to the pedicle hole drilled in the spine or the screw. If the hole or screw has errantly broken through bone into the spinal canal or nerve foramen, the stimulation may elicit an EMG warning of misplacement. In-depth descriptions of each procedure are beyond the scope of this chapter. The reader is referred elsewhere for extensive coverage of intraoperative neurophysiological techniques ( ).
Spinal Cord Monitoring
SEP and MEP spinal cord monitoring is a good example of a common IOM technique. Electrical stimuli are delivered at a rate of several per second to the ulnar nerve at the wrist and the posterior tibial nerve at the ankle. Averaged SEP peaks are recorded at standardized surface locations over the spine and scalp. Small electrical potentials recorded during the 50 ms following stimulation indicate the transit and arrival of axonal volleys or synaptic events at peripheral, spinal, brainstem, and primary sensory cortical levels. SEP recordings are repeated every few minutes. MEP stimulating electrodes are located on the scalp over motor cortex. Electrical MEP pulses are strong enough to discharge the axon hillock of motor cortex pyramidal cells. The resulting action potentials travel down corticospinal tracts and discharge spinal anterior horn cells. MEP recordings are made from limb muscles at 25–45 ms after stimulation.
SEP and MEP peaks remain stable over time in uneventful spinal surgery. If values change beyond established limits, the monitoring team alerts the surgeon of increased risk of neurological impairment. Which peaks are preserved and which are changed can localize the side and level of impairment. In thoracolumbar surgery, SEP and MEP channels of the upper extremity serve as controls to separate systemic or anesthetic causes from thoracic or lumbar surgical reasons for change. The ulnar nerve is often used rather than the median nerve during cervical surgery for better coverage of the lower cervical cord. The peroneal nerve at the knee may substitute for the posterior tibial nerve at the ankle for elderly patients, those with diabetes, or others in whom a peripheral neuropathy may interfere with adequate distal peripheral conduction. Blockade of the neuromuscular junction is helpful in reducing muscle artifact in SEP but must be limited for use if MEP is monitored. Sometimes other incidental clinical problems are detected beyond the primary purpose of monitoring in the spinal cord, brainstem, or cortical regions. 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, IOM changes warn of a systemic problem such as hypoxia secondary to a ventilatory problem.
Interpretation
Interpretation of intraoperative neurophysiology includes two categories. One is monitoring , in which baseline findings are established and subsequent findings are compared with baseline. Alarm criteria are set in advance based on knowledge of how much change is acceptable without risk. The other category, testing , identifies structures and sets limits of resection. Testing is used in several ways. One is to identify a structure, such as finding the facial nerve buried within pathological tissue. Another is to identify motor or language cortex prior to a resection. A third example is identifying which cauda equina root is L5, or S1, or S2, identifying which is a sensory or a motor portion of a root, or identifying roots as opposed to filum terminale during tethered cord release.
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