Surgical Neurophysiology: Part of the Team
 
Introduction
 
Surgical neurophysiology is a rapidly expanding and evolving field. When surgeons realized nerves were at risk of being inadvertently cut, stretched, or heated during surgery, they sought experts in electrophysiology to guide them intraoperatively with electrical stimulation and monitoring. As more surgeons were trained at institutions where monitoring was used, the demand increased steadily, and so did the sophistication of the surgical neurophysiologists' techniques.
 
Cranial nerve monitoring
 
Twelve cranial nerves connect the brainstem to muscles and senses of the head and neck. Tumors growing near these nerves are difficult to remove safely. In some cases the nerves can be clearly seen, but often the surgeon must search for a nerve by applying brief electrical pulses while the surgical neurophysiologist (SN) watches for electrical evidence of slight muscle contraction. Even when the nerves are visible, warning the surgeon of responses to manipulation of the nerves can ward off injury. In some cases a nerve must be sacrificed to allow complete removal of the tumor. The SN can help here by guarding nearby nerves and ensuring that only the correct nerve is cut. Not all nerves can be monitored. There are no reliable techniques to monitor vision and smell. Monitoring some muscles controlling the eye and throat poses a greater risk than the expected benefit. The facial nerve passes near the small bones of the middle ear, and can be monitored during surgeries to restore hearing or remove diseased tissue. Both children and adults may require such surgery.
 
Peripheral nerve monitoring
 
Tumors can grow near nerves in other parts of the body. Hip replacements pose a risk to the sciatic nerve because of surgical manipulations. The techniques of monitoring peripheral nerves are similar to those of cranial nerves: probing with electrical pulses and watching for electrical evidence of slight muscle contraction; and continuously watching for muscles' responses to surgeons' manipulations. Nerves can be crushed or cut in accidents. Repairing such injured nerves involves first finding the nerve remnants with the help of electrical stimulation.
 
Spinal cord monitoring
 
Tumors that grow in and near the spinal cord are dangerous to remove without monitoring. Back injuries, such as disk herniations and vertebral fractures, also pose a risk to the spinal cord, as do other deformities and diseases of the spine requiring major orthopaedic surgery. Three types of monitoring are usually used in spine surgeries: (1) to protect the nerves carrying sensory information from the limbs to the brain, electrical pulses are applied at the wrists and ankles, and the brain processing of the pulses are recorded from scalp electrodes; (2) to protect the nerves transmitting commands from the brain to control muscles, electrical pulses are delivered through the scalp, and electrical evidence of slight muscle contraction is recorded from the limbs; and (3) to protect and identify nerves emerging from the spinal cord to control limb muscles, continuous monitoring and electrical stimulation are used, as described above for cranial and peripheral nerves.
 
Brain monitoring
 
Repair of ruptured brain arteries usually requires temporarily clamping nearby arteries. Monitoring the sensory pathways ensures that enough oxygenated blood reaches the brain during the clamping time. Arteries in spasm and low blood pressure can be detected by watching for intermittent loss of signals, indicating the need for further intervention by the surgeon or anesthesiologist. To spare the brain regions controlling movement, the surgeon can stimulate with electrical pulses while the SN watches for electrical evidence of slight muscle contraction.
 
Roger S. Noss PhD, CNIM, D.ABNM
 
Paul Jasiukaitis PhD
 
Dr. Nicholas Barbaro, Medical Director of the Neuromonitoring Service
 
 
Links
 
Intraoperative Neurophysiological Monitoring
This article provides an overview of the various neurophysiological monitoring techniques used intraoperatively.