Intraoperative neurophysiological monitoring is used to decrease neurosurgical morbidity and detect neurologic compromise before the damage becomes irreversible. There are numerous modalities for intraoperative monitoring, including somatosensory-evoked potentials (SSEPs), motorevoked potentials (MEPs), brainstem auditory-evoked potentials (BAEPs), and visual-evoked potentials (VEPs). Their selection is based on the area at risk for injury. Monitoring evoked potentials (EPs) can be challenging for the anesthesia provider because many of the anesthetic agents currently used affect signal quality. The anesthesiologist or anesthetist must communicate with the surgeon and neurophysiologist prior to surgery regarding what EPs will be monitored in order to decide on an anesthetic plan that will optimize this monitoring.

EPs are measurements of the electrical potentials produced when the nervous system is stimulated in contrast to the electroencephalogram [EEG], which records spontaneous electrical activity generated by the central nervous system. EPs may be generated by sensory, magnetic, electrical, or cognitive stimulation. They are characterized by both latency and amplitude. The latency of an EP is measured in milliseconds and is the time between a stimulus and the occurrence of the EP. The amplitude is measured in millivolts and is the magnitude of the EP. EPs can be monitored noninvasively from electrodes on the skin or invasively by monitors placed within the surgery field.

SSEPs are in the form of intraoperative neurophysiological monitoring with the widest clinical application. This technique includes stimulating a peripheral sensory nerve repetitively (e.g., median nerve or posterior tibial nerve) and recording the response with electrodes placed over the primary sensory cortex. SSEPs use signal averaging (e.g., repetitive stimulation and response recording) to cancel out background noise (e.g., EEG). In certain surgeries, the recording electrodes can be placed directly in the epidural space immediately proximal to the site of interest. SSEPs monitor the well-being of the dorsal column functions (position, vibratory sense, and light touch) as well as portions of the brainstem and cerebral cortex. Specifically, the
pathway includes the peripheral sensory nerves (cell bodies in the dorsal root ganglia) → which ascend via the ipsilateral dorsal column to synapse in the medulla → secondary fibers decussate and ascend to the contralateral thalamus → tertiary fibers ascend from the thalamus to the primary sensory cortex (postcentral gyrus). Note that SSEPs provide little to no information on the anterior columns (read: Motor function), and there are reports of patients with normal intraoperative SSEPs who awaken with new motor deficits. Fortunately, this is rare despite the dorsal columns having a different blood supply (posterior spinal arteries) than the anterior columns (anterior spinal artery). However, it is theoretically possible to have hypoperfusion to the anterior columns (e.g, during a thoracic aortic aneurysm repair) that SSEPs may fail to recognize. SSEPs are monitored during aneurysm surgery (median nerve during middle cerebral artery aneurysms, posterior tibial nerve during anterior circulation aneurysms—remember your homunculus!), brain tumors including the posterior fossa, and some spine surgeries.

As spine surgery becomes more complex, the need to monitor the anterior columns is becoming increasingly important. Fortunately, our technical ability to monitor MEPs has improved rapidly since the 1990s. MEPs entail applying a current directly or transcranially to the primary motor cortex (precentral gyrus) or spinal cord to initiate an action potential. The action potential then descends from the motor cortex through the pyramidal decussation to the contralateral lateral corticospinal tract. These neurons then synapse on the ventral horn with an alpha motor neuron that travels to the muscle. MEPs can be measured at numerous points along this pathway. Neurogenic MEPs are responses recorded in the periphery following stimulation of the spinal cord. Myogenic MEPs are EPs recorded over the muscle belly as compound muscle action potentials (CMAPs).