Surgical neurophysiology, also known as intraoperative neurophysiology monitoring (IONM), is a new and growing allied health field. The surgical neurophysiologist is an integral part of the surgical team, and works closely with the anesthesiologist or anesthetist, the surgeons, and other members of the team.
The neurophysiologist performs testing and monitoring of the nervous system during surgery to assist the surgeons in avoiding or reducing complications such as paralysis, hearing loss, or stroke (depending on the type of surgery), by detecting incipient injury in time to prevent or ameliorate it. Surgical neurophysiology also provides information to the surgeon for use in intraoperative decision-making.
What kinds of surgeries can be monitored? Surgical neurophysiology monitoring employs a wide variety of modalities, each with a very specific application. It is most applicable when there is a specific risk to some part of the nervous system. For some types of surgery, such as cerebellar tumors, there is no suitable monitoring technique. Some of the most commonly monitored surgeries include spinal surgery, certain types of brain surgery, some ENT procedures, peripheral nerve surgery, and vascular surgeries such as carotid endarterectomies and thoracic-abdominal aortic aneurysms (TAAA).
What testing modalities are performed in intraoperative monitoring? Many different modalities can be used in the OR. Frequently several modalities, such as SSEP, EMG and MEP (see below) are used together in the same surgery. Some of the most widely used modalities include:
- SSEP (Somatosensory Evoked Potentials)—the response recorded from the brain, nerve, or spinal cord to electrical stimulation of peripheral nerve. Used most often to monitor the integrity of the dorsal columns of the spinal cord during spine surgery; also used in some brain surgeries and peripheral nerve surgeries.
- TCeMEP ( Transcranial Electrical Motor Evoked Potentials): an electrical stimulus is applied to the motor cortex of the brain, and a response recorded from the spinal cord or from limb muscles. Works like SSEP (see above), but in the opposite direction, to monitor function of the motor tracts of the spinal cord.
- BSEP (Brainstem Auditory Evoked Potentials)—an electrical response, originating in the brainstem, to an auditory stimulus, usually a click delivered through small in-the-ear earphones. Used to monitor brainstem function and to help preserve hearing in acoustic neuroma and brainstem tumor cases.
- EMG (electromyography)—spontaneous EMG is used to detect incipient nerve damage in spine surgery (spinal nerve roots) and in skull base surgery (facial nerve and other cranial nerves). Evoked EMG, using an electrical stimulus delivered through a hand-held probe used by the surgeon, is also used to identify and test nervous structures.
- Pedicle Screw Stimulation: evoked EMG obtained by stimulating a screw placed in part of a vertebra called the pedicle. Since a nerve root lies immediately beneath each pedicle, a response obtained at too low a stimulus intensity level indicates a breach in the pedicle. Used to avoid nerve root damage caused by such a breach.
- EEG (Electroencephalogram)—spontaneous brain activity is recorded to monitor functional integrity of the cerebral cortex, specifically to avoid injuries caused by ischemia (reduced blood flow) during carotid endarterectomies and aneurysm clippings.
- ECOG (Electrocorticography)—EEG recorded directly from the exposed surface of the brain to help define the borders of resection (tissue removal) in epilepsy surgeries and craniotomies for brain tumors.
- Direct Cortical Stimulation: Also used in epilepsy and tumor surgeries, to identify and map eloquent areas of the brain (speech and motor areas)
- TCD (Transcranial Doppler)—blood flow velocity in the internal arteries of the brain is measured using an ultrasound beam, analogous to clocking a baseball pitch with a radar gun. Used to monitor cerebral blood flow in carotid endarterectomies.
Who are surgical neurophysiologists? Surgical neurophysiology, though rapidly evolving into an established profession, began as an interdisciplinary field. Neurophysiologists come from a variety of backgrounds, including medicine (especially neurology and physiotry); audiology; neuroscience; and neurodiagnostic technology.
What kinds of credentials do neurophysiologists obtain? In addition to relevant board certification within their respective fields such as neurology, audiology and neurodiagnostic technology, surgical neurophysiologists in the United States can obtain two kinds of certification:
- Certification in Neurophysiologic Intraoperative Monitoring (CNIM) from the American Board of Registered Electroneurodiagnostic Technologists (https://abret.org). This is technical certification intended for technologists who work under supervision by a neurophysiologist or neurologist.
- American Board of Neurophysiologic Monitoring (ABNM) certification (http://abnm.info) This is a professional level certification intended for neurophysiologists working independently.
What training and career opportunities are there? The American Society of Electroneurodiagnostic Technologists (ASET), and the American Society of Neurophysiological Monitoring (ASNM ), maintain job listings. ASNM’s listings are on their website, https://asnm.org; contact information for ASET is on their website, https://aset.org. Some employers offer on-the-job training programs; generally a background in neurodiagnostic technology, neuroscience, audiology medicine, or another health profession such as nursing or physician’s assistant is helpful.
Short training programs such as weekend seminars are offered by ASNM and by ASET; also by the Larry Head Institute (https://larryheadinstitute.com) and Knowledge Plus, Inc. (https://eknowledgeplus.com). The Barrow Neurological Institute (https://thebni.com) offers brief onsite training opportunities. Atlanta Institute for Clinical Neuroscience (http://aicn.org) conducts a clinical training program for surgical neurophysiologists, leading to a master’s degree.
University of Texas at Dallas (https://utdallas.edu/dept/bbs/grad/Masters/Mcog/Mcog_main.htm) offers a master’s degree program, within their Neuroscience graduate program, which includes courses by IONM pioneer Aage Moller. Some audiology graduate programs may include limited IONM training.
How did surgical neurophysiology develop? The earliest intraoperative neurophysiology was probably the famous work of Wilder Penfield and others in the 1920’s. Penfield mapped exposed motor and speech cortex by electrical stimulation. In the 1960’s and 1970’s, EEG recordings were made from exposed cerebral cortex in epilepsy and tumor surgeries.
In the 1970’s, following the development of commercial evoked potential equipment, SSEP was used to prevent paralysis in scoliosis surgeries; BSEP and facial nerve EMG began to be used in skull base surgeries at about this time to prevent facial paralysis and hearing loss, and EEG monitoring began to be used in carotid endarterectomies to prevent ischemic strokes during surgery. The use of SSEP monitoring has become generalized to a wide variety of spinal and other surgeries, and some form of intraoperative neurophysiology monitoring has become the standard of care in many types of surgeries.
With the widespread popularity of several modalities of IONM, the specialty began to emerge as neurophysiologists, audiologists, technologists and others began to develop the skills to perform multiple types of monitoring. The technology has steadily improved, the knowledge base has greatly expanded with research and clinical experience, and new applications have been developed. The most recent major advance in the field has been the development of transcranial electric motor evoked potential (TCeMEP) monitoring.
Where is surgical neurophysiology headed in the future? Surgical neurophysiology continues to advance, with the development of new applications such as brainstem mapping, spinal cord mapping, monitoring for position-related nerve injuries, and many others. The surgical neurophysiologist requires increasing knowledge, versatility and sophistication. The greatest challenge faced by this evolving field is the need for standardized education, training and credentialling. Many neurophysiologists envision a structure like that of audiology, with graduate degrees in the field and state licensure.
© Copyright, 2004. Jerry Larson, CNIM, D. ABNM, jerry@neuromon.com. For more information see www.asnm.org.
Last updated: March 2004