Chapter 32E Clinical Neurophysiology
Intraoperative Monitoring
Techniques
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).