Intraoperative neurophysiological monitoring
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Intraoperative neurophysiological monitoring (IONM) or intraoperative neuromonitoring is the use of electrophysiological methods such as
electroencephalography Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
(EEG),
electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyog ...
(EMG), and
evoked potential An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a Stimulus (physiology), stimu ...
s to monitor the functional integrity of certain neural structures (e.g.,
nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
s,
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
and parts of the
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
) during surgery. The purpose of IONM is to reduce the risk to the patient of
iatrogenic Iatrogenesis is the causation of a disease, a harmful complication, or other ill effect by any medical activity, including diagnosis, intervention, error, or negligence. "Iatrogenic", ''Merriam-Webster.com'', Merriam-Webster, Inc., accessed 2 ...
damage to the
nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
, and/or to provide functional guidance to the
surgeon In modern medicine, a surgeon is a medical professional who performs surgery. Although there are different traditions in different times and places, a modern surgeon usually is also a licensed physician or received the same medical training as ...
and
anesthesiologist Anesthesiology, anaesthesiology, or anaesthesia is the medical specialty concerned with the total perioperative care of patients before, during and after surgery. It encompasses anesthesia, intensive care medicine, critical emergency medicine, ...
.


Methods

Neuromonitoring employs various
electrophysiologic Electrophysiology (from Greek , ''ēlektron'', "amber" etymology of "electron"">Electron#Etymology">etymology of "electron" , ''physis'', "nature, origin"; and , ''-logia'') is the branch of physiology that studies the electrical properties of bi ...
modalities, such as
extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
single unit and local field recordings, SSEP, transcranial electrical motor evoked potentials (TCeMEP), EEG, EMG, and
auditory brainstem response The auditory brainstem response (ABR), also called brainstem evoked response audiometry (BERA), is an auditory evoked potential extracted from ongoing electrical activity in the brain and recorded via electrodes placed on the scalp. The measured r ...
(ABR). For a given surgery, the set of modalities used depends in part on which neural structures are at risk. Transcranial Doppler imaging (TCDI) is also becoming more widely used to detect vascular
emboli An embolism is the lodging of an embolus, a blockage-causing piece of material, inside a blood vessel. The embolus may be a blood clot (thrombus), a fat globule ( fat embolism), a bubble of air or other gas (gas embolism), amniotic fluid (amniot ...
. TCDI can be used in tandem with EEG during
vascular surgery Vascular surgery is a surgical subspecialty in which diseases of the vascular system, or arteries, veins and lymphatic circulation, are managed by medical therapy, minimally-invasive catheter procedures and surgical reconstruction. The specialty ...
. IONM techniques have significantly reduced the rates of
morbidity A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
and
mortality Mortality is the state of being mortal, or susceptible to death; the opposite of immortality. Mortality may also refer to: * Fish mortality, a parameter used in fisheries population dynamics to account for the loss of fish in a fish stock throug ...
without introducing additional risks. By doing so, IONM techniques reduce health care costs. To accomplish these objectives, a member of the surgical team with special training in
neurophysiology Neurophysiology is a branch of physiology and neuroscience that studies nervous system function rather than nervous system architecture. This area aids in the diagnosis and monitoring of neurological diseases. Historically, it has been dominated b ...
obtains and co-interprets triggered and spontaneous electrophysiologic signals from the patient periodically or continuously throughout the course of the operation. Patients who benefit from neuromonitoring are those undergoing operations involving the
nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
or which pose risk to its anatomic or physiologic integrity. In general, a trained neurophysiologist attaches a computer system to the patient using stimulating and recording
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
s. Interactive software running on the system carries out two tasks: # selective activation of stimulating electrodes with appropriate timing, and # processing and displaying of the electrophysiologic signals as they are picked up by the recording electrodes. The neurophysiologist can thus observe and document the electrophysiologic signals in realtime in the operating area during the surgery. The signals change according to various factors, including anesthesia, tissue temperature, surgical stage, and tissue stresses. Various factors exert their influence on the signals with various tissue-dependent timecourses. Differentiating the signal changes along these lineswith particular attention paid to stressesis the joint task of the surgical triad: surgeon, anesthesiologist, and neurophysiologist.


Surgical procedures

Patients benefit from neuromonitoring during certain surgical procedures, namely any surgery where there is risk to the
nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
. Most neuromonitoring is utilized by spine surgeons, but neurosurgeons, vascular, orthopedic, otolaryngologists, and urology surgeons have all utilized neuromonitoring as well. The most common applications are in spinal surgery; selected brain surgeries;
carotid endarterectomy Carotid endarterectomy is a surgery, surgical procedure used to reduce the risk of stroke from carotid artery stenosis (narrowing the internal carotid artery). In endarterectomy, the surgeon opens the artery and removes the plaque. The plaque for ...
;
ENT Ents are a species of beings in J. R. R. Tolkien's fantasy world Middle-earth who closely resemble trees; their leader is Treebeard of Fangorn forest. Their name is derived from an Old English word for giant. The Ents appear in ''The Lord of ...
procedures such as
acoustic neuroma A vestibular schwannoma (VS), also called acoustic neuroma, is a benign tumor that develops on the vestibulocochlear nerve that passes from the inner ear to the brain. The tumor originates when Schwann cells that form the insulating myelin sheath o ...
(vestibular schwannoma) resection, parotidectomy; and
nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
surgery. Motor evoked potentials have also been used in surgery for
thoracic aortic aneurysm A thoracic aortic aneurysm is an aortic aneurysm that presents primarily in the thorax. A thoracic aortic aneurysm is the "ballooning" of the upper aspect of the aorta, above the diaphragm. Untreated or unrecognized they can be fatal due to dissec ...
. Intraoperative monitoring is used to : * to localize neural structures, for example to locate
cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
during skull base surgery; * to test function of these structures; and * for early detection of intraoperative neural injury, allowing for immediate corrective measures. For example, during any surgery on the
thoracic The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
or
cervical In anatomy, cervical is an adjective that has two meanings: # of or pertaining to any neck. # of or pertaining to the female cervix: i.e., the ''neck'' of the uterus. *Commonly used medical phrases involving the neck are **cervical collar **cerv ...
spinal column, there is some risk to the spinal cord. Since the 1970s, SSEP (
somatosensory evoked potentials An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light fla ...
) have been used to monitor spinal cord function by stimulating a nerve
distal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
to the surgery, and recording from the cerebral cortex or other locations
rostral Rostral may refer to: Anatomy * Rostral (anatomical term), situated toward the oral or nasal region * Rostral bone, in ceratopsian dinosaurs * Rostral organ, of certain fish * Rostral scale, in snakes and scaled reptiles Other uses * Rostral colu ...
to the surgery. A baseline is obtained, and if there are no significant changes, the assumption is that the spinal cord has not been injured. If there is a significant change, corrective measures can be taken; for example, the hardware can be removed. More recently, transcranial electric motor evoked potentials (TCeMEP) have also been used for spinal cord monitoring. This is the reverse of SSEP; the motor cortex is stimulated transcranially, and recordings made from muscles in the limbs, or from spinal cord caudal to the surgery. This allows direct monitoring of motor tracts in the spinal cord. EEG
electroencephalography Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
is used for monitoring of cerebral function in neurovascular cases (cerebral aneurysms, carotid
endarterectomy Endarterectomy is a surgical procedure to remove the atheromatous ''plaque'' material, or blockage, in the lining of an artery constricted by the buildup of deposits. It is carried out by separating the plaque from the arterial wall. It was firs ...
) and for defining tumor margins in
epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...
surgery and some cerebral tumors. EEG measures taken during anesthesia exhibit stereotypic changes as
anesthetic An anesthetic (American English) or anaesthetic (British English; see spelling differences) is a drug used to induce anesthesia ⁠— ⁠in other words, to result in a temporary loss of sensation or awareness. They may be divided into two ...
depth increases. These changes include complex patterns of waves with
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
slowing accompanied by
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
increases which typically peak when
loss of consciousness Loss may refer to: Arts, entertainment, and media Music * ''Loss'' (Bass Communion album) (2006) * ''Loss'' (Mull Historical Society album) (2001) *"Loss", a song by God Is an Astronaut from their self-titled album (2008) * Losses "(Lil Tjay son ...
occurs (loss of responses to verbal commands; loss of righting reflex). As anesthetic depth increases from light surgical levels to deep anesthesia, the EEG exhibits disrupted rhythmic waveforms, high amplitude burst suppression activity, and finally, very low amplitude isoelectric or 'flat line' activity. Various
signal analysis Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing ''signals'', such as sound, images, and scientific measurements. Signal processing techniques are used to optimize transmissions, di ...
approaches have been used to quantify these pattern changes and can provide an indication of loss of recall, loss of consciousness and anesthetic depth. Monitors have been developed using various
algorithm In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algorithms are used as specificat ...
s for signal analysis and are commercially available, but none have as yet proven 100% accurate. This is a difficult problem and an active area of medical research. EMG is used for cranial nerve monitoring in skull base cases and for nerve root monitoring and testing in spinal surgery. ABR (a.k.a. BSEP, BSER, BAEP, etc.) is used for monitoring of the acoustic nerve during acoustic neuroma and brainstem tumor resections.


Licensure, certification, credentialing, and evidence

In the US, IONM licensure has not been legislated at the state or federal level. Issues of licensure are discussed in ASET's 68-page
white paper A white paper is a report or guide that informs readers concisely about a complex issue and presents the issuing body's philosophy on the matter. It is meant to help readers understand an issue, solve a problem, or make a decision. A white paper ...
on occupational regulation. Worldwide, there are at least two private certifications available: CNIM (Certified in Neurophysiological Intraoperative Monitoring) and D.ABNM (Diplomate of the American Board of Neurophysiological Monitoring). Though not governmentally regulated, certain health care facilities have internal regulations pertaining to neuromonitoring certifications (see below). The CNIM is a more widely known credential throughout the United States. The Certification for Neurophysiological Intraoperative Monitoring (CNIM) is awarded by the American Board of Electroencephalographic and Evoked Potential Technologists. As of 2010, minimum requirements include 1) a B.A., B.S. ath 22) R.EP.T or R.EEG.T Credential ath 13) A minimum of 150 surgeries. Path 1 is a 200 question exam costing $600. Path 2 is a 250-question exam. A 4-hour multiple-choice computer-based exam is offered twice a year. Currently, there are a little over 3500 board certified clinicians. Audiologists may received board certification in neurophysiological intraoperative monitoring via AABIOM. The exam has 200 multiple choice questions covering 6 areas: Anesthesia, Neuroscience, Instrumentation, Electro-physiology, Human physiology / anatomy, Surgical Applications. There are several organisations that certify MDs in the field including the American Clinical Neurophysiology Society (www.acns.org) and the American Board of Electrodiagnostic Medicine. The optimal practice model is under discussion at the present time (2013) as is the relevant qualifications for supervision. Outside the US there many different styles of IOM. The evidence-based support for IOM is growing. There is a debate over whether IOM required controlled studies such as randomized trials, or whether expert consensus suffices.


References

{{Anesthesia Diagnostic neurology Electrodiagnosis Electroencephalography Anesthesia Medical monitoring