Intraoperative Neurophysiological Monitoring

Anatomy and Physiology

Each technique of intraoperative neurophysiological monitoring monitors a specific neural pathway.

Somatosensory Evoked Potential (SSEP): SSEP monitors the dorsal column–medial lemniscus pathway, which mediates tactile discrimination, vibration, and proprioception.[2] Stimulation of sensory receptors in the skin initiates peripheral sensory nerves, which extend through the nerve root to the ipsilateral dorsal root ganglia at spinal levels. The projections from these first-order neurons form fasciculi gracilis and cuneatus, which carry impulse from the lower and upper extremities, respectively. The first synapse occurs in the lower medulla, then the impulses cross over at the level of the brainstem and form medial lemniscus. The impulse then ascends to the contralateral thalamus and relay information to the primary sensory cortex in the parietal lobe. In the upper extremities, the median and ulnar nerve are monitored, whereas, in the lower extremities, the posterior tibial and peroneal nerve are monitored.

Motor Evoked Potential (MEP): MEP monitor motor pathways, transcranial electrical stimulation elicits excitation of corticospinal projections at multiple levels. Depending on the intensity of stimulation and the placement of electrode, motor evoked potentials are generated at different levels of the brain, including superficial white mater just underneath the motor cortex, the deep white matter of the internal capsule, and pyramidal decussation. The electrical potential is recorded at the spinal cord or muscles. MEP is generated and transported via the pyramidal tract.

Visual Evoked Potential (VEP): VEP measures the functional integrity of the optic pathways from the retina to the brain's visual cortex in response to light stimulus. Visual stimulus is converted into nerve signals in the retina. These signals are transmitted via the optic pathway to the brain, from the retina to the optic nerve, optic chiasma, optic tract, lateral geniculate body, optic radiation, and visual cortex occipital lobe.[3]

Brainstem Auditory Evoked Potential (BAEP): BAEP monitors the function of the auditory nerve and auditory pathways in the brainstem. The auditory signal travels from the cochlear hair cell to the primary auditory cortex via the vestibulocochlear nerve, superior olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate body.[4]

Electromyography (EMG): EMG monitors somatic efferent nerve activity and evaluates the functional integrity of individual nerves. EMG monitors intracranial, spinal, and peripheral nerves during surgeries. Depolarization of a motor nerve produces electrical potential within the muscles innervated by that specific nerve, and the generated electrical activity is monitored using subdermal or intramuscular electrodes.[5]

Electroencephalography (EEG): The electrical activity measured by EEG is generated by groups of pyramidal neurons, which has cell bodies in the 3rd and 5th layer of the cerebral cortex.[6]

Indications

Intraoperative neurophysiologic monitoring (IONM) is recommended for individuals at increased risk of neurological injury during surgical procedures. The following are the indications for IONM.

Somatosensory Evoked Potential (SSEP) or Motor Evoked Potential (MEP)

• Spine and spinal cord surgery including scoliosis and Kyphosis correction with instrumentation, spinal cord decompression/stabilization, anterior and posterior spinal fusions (cervical, thoracic, and thoracolumbar), the release of tethered cord, correction of spina bifida, resection of the tumor, cyst, aneurysm or arteriovenous malformation of the spinal cord
• Brain and brain stem surgeries including craniotomy for tumor removal, craniotomy for aneurysm repair, arteriovenous malformation repair, localization of cortex during craniotomy, thalamotomy 
• Cerebrovascular surgery, including clipping of intracranial aneurysms, interventional neuroradiology
• Stereotactic surgery on the brain stem, thalamus, and cerebral cortex
• Pelvic fracture surgery
• Thoracoabdominal aortic aneurysm repair
• Repair of coarctation of the aorta
• Brachial plexus and lumbosacral plexus surgery
• Peripheral nerve repair
• Carotid endarterectomy 
• Thyroid surgery

Brainstem Auditory Evoked Potential (BAEP)

• Acoustic neuroma resection
• Vestibular nerve section
• Vascular loop decompression
• Vestibular schwannomas
• Facial nerve decompression
• Brainstem tumor resection
• Auditory brainstem implant
• Posterior fossa procedures
• Functional localization of the cortex with direct cortical stimulation
• Assess auditory pathways within the brainstem
• Assess ischemia at the cochlea and eighth nerve

Visual Evoked Potentials or Response (VEP): 

• Monitoring the visual system during optic nerve surgery
• Orbital surgery
• Pituitary gland surgery

Electroencephalogram (EEG):

• Carotid endarterectomy
• Cerebral aneurysm clipping
• Epilepsy surgery
• Monitoring depth of anesthesia

Electromyography (EMG) 

• To monitor cranial nerve function during procedures including acoustic neuroma resection, microvascular decompression of the facial nerve, parotid tumor resection, vestibular neurectomy for Meniere disease, neurotologic/otologic procedures.
• Nerve root or spinal cord monitoring during spinal surgeries including spinal instrumentation (e.g., pedicle screw placement), a mechanical spinal distraction
• Resection of skull base tumors, spinal tumors
• Surgical excision of cranial nerve neuromas of motor cranial nerve
• Brachial or lumbosacral plexus surgery
• Neck surgery including thyroid surgery, neck dissections

Contraindications

There is no absolute contraindication for any of the techniques of intraoperative neurophysiological monitoring (IONM). Relative contraindications for motor evoked potentials are the presence of vascular clips, intracranial electrodes, pacemakers, other implanted bio-mechanical equipment, cortical lesions, skull defects, increased intracranial pressure, and history of epilepsy.[7][8] According to the guideline of the American Clinical Neurophysiology Society (ACNS), transcranial motor evoked potential (MEP) can induce seizures. The incidence is very low, so the history of epilepsy is not considered a contraindication to MEP monitoring.[9]

Equipment

The intraoperative neurophysiological monitoring (IONM) systems are equipped with all the channels to monitor different modalities of IONM needed for the specific type of surgery. The monitoring system displays continuous trends and raw signals, as well as it can also trigger the stimulation of specific muscle and nerve (peripheral, cranial) and records the signal. The system also records waveforms' time/date, detail waveform data, and the technologist's notes. The system usually has a provision for rejection of artifacts related to electrocautery.

Personnel

Intraoperative neurophysiological monitoring (IONM) is performed by dedicated neurophysiologists or IONM technicians who work under the direct supervision of professionals with training and experience in IONM. IONM team, along with anesthesia personnel, surgeon, and operating room staff, forms a multidisciplinary intraoperative team.

Preparation

An appropriately skilled intraoperative neurophysiological monitoring (IONM) team should be assigned for each patient and procedure. After a detailed preop evaluation (history, physical exam, and review of the medical record) and discussion with the surgeon to review the imaging, relevant neural anatomy, physiology, and planned procedure, the IONM team determines the appropriate modalities IONM required for the scheduled procedure. The team discusses IONM procedures and their risks with the patient and documents patient's data. IONM personnel discusses planned monitoring with nursing staff and determines a suitable location for monitoring equipment in the operating room. IONM personnel set up and check all the equipment before the patient arrives in the operating room. A suitable anesthesia plan is discussed with the anesthesia team as required for the surgical procedure and specific neuromonitoring modality. The technique of IONM involves the placement of electrodes or other monitoring devices under all aseptic skin preparation, acquisition, recording, and interpretation of high-quality data.[10] 

IONM personnel acquires baseline responses for needed monitoring modalities, informs and discusses alert criteria and testing strategies with the surgical and anesthesia team to coordinate monitoring as required during the procedure. IONM personnel also reviews the anesthesia regimen with the anesthesia team to optimize anesthesia maintenance during the procedure. The IONM record contains surgical event times, communication between teams, alert issued to surgical and anesthesia team, anesthesia drugs, and dosages used. Significant changes in the dose of anesthesia medications and physiological parameters, including heart rate, blood pressure, and temperature, are also recorded.

Technique or Treatment

The most commonly used intraoperative neurophysiological monitoring (IONM) techniques for surgical procedures include:

1. Somatosensory sensory evoked potential (SSEP)
2. Motor-evoked potential (MEP)
3. Spontaneous and triggered electromyography (EMG)[11]

The evoked potential technique involves applying a stimulus that generates a neuronal response, measured as a graph of time (mS) on the x-axis and voltage(mV) on the y-axis, two significant characteristics of the measured waveform is amplitude and latency. These evoked potentials are very low amplitude and require averaging and summation across multiple stimulations to enhance their quality and distinguish these potentials from background noise and patients electroencephalogram. The time interval between electrical stimulation of neural structure and measuring the evoked response at the cerebral cortex is defined as latency. Evoked potential are classified according to the nerve tract being monitored.

The somatosensory evoked potentials (SSEP) are electrical potentials generated within the neuroaxis in response to stimulation of a peripheral nerve (e.g., the median nerve at the wrist or the posterior tibial nerve at the ankle)[12] (peripheral SSEP), or cervical spine (subcortical SSEP) or somatosensory cortex (cortical SSEP). These potentials travel from the periphery to the brain and are recorded by electrodes placed over the scalp and along the transmission pathway. SSEP is monitored continuously throughout procedures that give close to real-time monitoring of the sensory pathway.

Motor evoked potential (MEP) is generated by transcranial electrical stimulation using surface or subdermal needle electrodes on the scalp or direct electrical stimulation on the brain. Evoked potential are measured over the spinal cord below the surgery level or in the muscle of interest. At the spinal level, the evoked response is measured in the epidural or intrathecal space. Compound muscle action potential (CMAP) is routinely measured due to their sensitivity, specificity, and minimal invasiveness. The electrodes are placed on the muscle innervated by specific brain regions, nerve root, or cranial nerve needed to monitor. Frequently used sites are thenar muscles, tibialis anterior, and abductor hallucis; muscle group above the surgery level is used as a control.[13]

Brain stem auditory evoked potential (BAEP), are recorded by delivering an auditory stimulus to one ear; the stimulus is loud, repetitive ticks produced by a device placed over or in the auditory canal. The response is measured from electrodes placed on the scalp or external ear to record ipsilateral and contralateral signals.

Visual evoked potential (VEP) is the electrical potentials initiated by visual stimuli recorded from the electrode on the scalp over the visual cortex. VEP recording requires three electrodes; the mid-occipital electrode is placed right above the external occipital protuberance (inion); the lateral occipital electrodes are placed 4 cm lateral on each side of the mid-occipital electrode.[3] VEP waveforms are extracted by averaging electroencephalogram (EEG) signals.

Electromyography (EMG) records myoelectric signals from peripheral musculature to monitor selective nerve root function. One muscle group per nerve is monitored by using spontaneous EMG or triggered EMG technique. Spontaneous EMG, needle electrode is directly inserted into a muscle to record the electrical activity of that muscle; no stimulation is performed, surgical manipulation such as compression, stretching, or pulling of nerves produces myoelectric signals in the corresponding innervated muscles which are recorded. Triggered EMG, a monopolar electrode, stimulates the top of the pedicle screw with incremental current intensities. Needle electrodes are placed in the appropriate muscle groups to record electrical activity in response to the stimulation. Direct stimulation of nerve root using less than two mA can ensure an electrical activity in the appropriate muscle group. Triggered EMG is used to check proper pedicular screw placement; if the screw breaches the medial or inferior pedicle wall, it significantly reduces the stimulation threshold and increases the risk of damage to the exiting nerve root.[2]

An electroencephalogram (EEG) measures the activity of the cerebral cortical neuron via the scalp electrode. Each scalp electrode collects around 6 cm2 synchronous cortical activity. This electrical activity is the summation of the postsynaptic potentials (inhibitory and excitatory) from pyramidal cells near each recording electrode.[6] Invasive EEG (iEEG) is recorded with subdural grid electrodes, depth electrodes, or epidural peg electrodes. Subdural electrodes are implanted subdurally over the brain surface. Depth electrodes are placed in deep structures like the hippocampus, amygdala, insula, or lesions under neuronavigation guidance. Epidural peg electrodes are placed by creating a trephination of the skull and inserting the electrode with the tip touching the dura.[14]

Complications

The overall risk of intraoperative neurophysiological monitoring (IONM) is low. Electrical safety is paramount in the operating room. Patients under anesthesia cannot report discomfort or pain; so, it is vital to ensure that IONM equipment is checked before a safe operation. Monitoring equipment malfunctioning can lead to local skin burns and other serious complications. An additional reported risk is seizure activity, high frequency (50 to 60 Hz) electrical brain stimulation can lead to seizure activity due to abnormal neuronal discharges.[15] 

Masseter muscles' stimulation and forceful jaw movement during MEP monitoring can lead to a tongue laceration, tooth fracture, or mandible fracture. These risks can be avoided by using bite blocks.[7] In rare cases, patients may experience tingling, bruising, soreness, and swelling at the needle insertion sites.[16] Invasive electroencephalogram monitoring during epilepsy surgery may also lead to adverse events; the risk is low, but the most commonly reported incidents are intracranial hemorrhage, superficial infection, cerebral infections, and elevated intracranial pressure.[14]

Clinical Significance

The risk of development of neuro-deficit following spinal surgery with or without any recognizable adverse event is known. Intraoperative neurophysiological monitoring (IONM) use during spinal surgery, including motor-evoked potentials (MEPs), somatosensory-evoked potentials (SSEPs), and electromyography (EMG), leads to early recognition and management of any signal changes during the procedure, thus predicts a favorable surgical outcome. Loss of IONM signals or any variation from baseline IONM signal during surgery indicates a neural injury and predicts postoperative neuro deficits' development.

Multiple factors, including anesthetic agents, blood pressure, body temperature, oxygenation, hypocapnia, and any technical problem, can affect the IONM signal. Intravenous anesthetics are compatible with IONM; inhaled anesthetics leads to dose-dependent suppression of amplitude and increases in latency; muscle relaxants are only used for intubation as they block neurotransmission. Mechanical compression of neural tissue increases latency and decreases amplitude; a decrease in blood supply decreases amplitude.

Evoked potentials signal changes with the change in body temperature, so it has been recommended to maintain a temperature close to baseline within a range of +/- 2 degrees C to 2.5 degrees C; at core temperatures below 28 degrees C, no MEPs, and SSEPs are recorded. The partial pressure of carbon dioxide levels less than 20 mmHg causes cerebral vasoconstriction leading to neural tissue ischemia associated with a change in cortical MEP and SSEP readings.

Studies have shown that with the use of IONM, there are significant improvements in neurological outcomes.[17]

Enhancing Healthcare Team Outcomes

Intraoperative neurophysiological monitoring (IONM) is considered the standard of care during many procedures, including spinal, intracranial, and vascular surgeries, where there is a risk of neurological damage.[18] [Level 1] Effective communication and close cooperation between multidisciplinary teams, including intraoperative neuromonitoring (IONM) technician, a neurophysiologist, anesthesiology, and surgery team, is required for high-quality perioperative care to detect and prevent neurologic injuries.

Enhancing communication between multidisciplinary teams increases the safety, quality, efficacy, and efficiency of perioperative care by decreasing adverse events and improving perioperative outcomes.[19] [Level 1] Various modalities of neurophysiological monitoring are used to monitor neural structures during different types of surgery. IONM team collaborates with the surgical and anesthesiology team to optimize signal acquisition and provide real-time analysis, interpretation, and timely communication of signal changes, which allows the surgeon to operate safely and avoid neural tissue injury.

Continuous IONM is a helpful real-time adjunct to detect malpositioning related nerve injury and anesthesia-related signal changes. The anesthesiology team relies on the IONM team to identify physiological deterioration. It allows the anesthesiology team to adjust their anesthetic dosing to provide a condition that supports neurophysiological monitoring.