Intraoperative neurophysiological monitoring

Intraoperative neurophysiological monitoring (IOM) describes a variety of procedures that are used to monitor the integrity of neural pathways during high-risk neurosurgical, orthopedic, vascular and other surgeries that may place the nervous system at risk for injury (e.g., parotidectomy; parathyroidectomy; thyroidectomy). IOM can assess central sensory and motor pathways, peripheral and central auditory pathways, cranial nerves, motor and sensory spinal nerves and roots, brachial plexus, and peripheral motor and sensory nerves. The principal goal of intraoperative monitoring is the identification of nervous system impairment in the hope that prompt intervention will prevent permanent deficits. One is using techniques to identify impending damage to the nervous system. The surgeon is alerted to the possible damage, and corrective action is taken to prevent the damage. Secondly the mapping techniques used to identify critical structures in the nervous system are identified electrophysiologically; the surgeon avoids these structures to prevent neurological damage from occurring. Correctable factors that can occur during surgery include circulatory disturbance, excess compression from retraction, bony structures or hematomas, or mechanical stretching.
Various types of intraoperative monitoring are described below:
Sensory-evoked Potentials
Sensory-evoked potential describes the responses of the sensory pathways to sensory or electrical stimuli. Intra-operative monitoring of sensory-evoked potentials is used to assess the functional integrity of central nervous system (CNS) pathways during operations that put the spinal cord or brain at risk for significant ischemia or traumatic injury. The basic principles of sensory-evoked potential monitoring involves identification of a neurological region at risk, selection and stimulation of a nerve that carries a signal through the at-risk region, and recording and interpretation of the signal at certain standardized points along the pathway. While monitoring the neural pathway at risk, another sensory pathway is monitored as a control. The control recording provides a method of differentiating the normal changes in the nerve pathway from the changes that occur as a result of the high-risk surgery that would place the neural pathway at risk. Sensory-evoked potentials can be further broken down into the following categories according to the type of simulation used:
  • Somatosensory-evoked potentials (SSEPs) are electrical waves that are generated by the response of sensory neurons to stimulation. Peripheral nerves, such as the median ulnar or tibial nerve, are typically stimulated, but in some situations the spinal cord may be stimulated directly. Recording is done over peripheral nerve, spinal cord, and brainstem and at the cortical level. SSEPs are used as a mapping technique to record directly from the cerebral cortex to identify the sensory areas of the cortex. Intra-operative monitoring of SSEPs is most commonly used during orthopedic or neurologic surgery in order to provide prompt intervention and reduce surgically induced morbidity and/or to monitor the level of anesthesia. Several different techniques are commonly used, including stimulation of a relevant peripheral nerve with monitoring from the scalp, from interspinous ligament needle electrodes, or from catheter electrodes in the epidural space.
  • Brainstem auditory evoked potentials (BAEPs) are generated in response to auditory clicks and can define the functional status of the cochlea, the auditory nerve, the brainstem and the primary auditory cortex.
  • Visual-evoked potentials (VEPs) are used to track visual signals from the retina to the occipital cortex using light flashes as a stimulating signal. VEPs are helpful as a mapping technique to identify visual system structures during tumor removal.
EMG (Electromyogram) Monitoring and Nerve Conduction Velocity Measurements
This type of monitoring can be performed in the operating room and may be used to assess the status of the peripheral nerves, (e.g., to identify the extent of nerve damage prior to nerve grafting or during resection of tumors). Additionally, these techniques may be used during procedures around the nerve roots and around peripheral nerves to assess the presence of excessive traction or other impairment. Surgery in the region of cranial nerves can be monitored by electrically stimulating the proximal (brain) end of the nerve and recording via EMG in the facial or neck muscles. Thus the monitoring is done in the direction opposite to that of sensory-evoked potentials, but the purpose is similar - to verify that the neural pathway is intact.
Motor - evoked Potential Monitoring
Motor-evoked potentials are recorded from muscles following direct or transcranial electrical stimulation of motor cortex or by pulsed magnetic stimulation provided by a coil placed over the head. Stimulation induces an electrical current in the brain or spinal cord which in turn can stimulate the motor neurons. Muscle activity is recorded by electrodes placed on the skin at prescribed points along the motor pathways. Motor evoked potentials, especially when induced by magnetic stimulation, can be affected by anesthesia.
EEG (Electroencephalogram) Monitoring
Spontaneous EEG monitoring can also be recorded during surgery and can be subdivided as follows:
  • EEG monitoring has been widely used to monitor cerebral ischemia secondary to carotid cross clamping during a carotid endarterectomy. EEG monitoring may identify those individuals who would benefit from the use of a vascular shunt during the procedure in order to restore adequate cerebral perfusion. Conversely, shunts, which have an associated risk of iatrogenic complications, may be avoided in those individuals in whom the EEG is normal. Carotid endarterectomy may be done under local anesthesia so that monitoring of cortical function can be directly assessed.
  • Electrocorticography (ECoG) is the recording of the EEG directly from a surgically exposed cerebral cortex. ECoG is typically used to define the sensory cortex and to map the critical limits of a surgical resection. ECoG recordings have been most frequently used to identify epileptogenic regions for resection. In these applications, electrocorticography does not constitute intraoperative monitoring.
  • Intraoperative neurophysiologic monitoring, including, somatosensory-evoked potentials, brainstem auditory-evoked potentials, visual-evoked potential, EMG, motor-evoked potentials, and EEG, when performed during spinal, intracranial, vascular, or other surgery that may place the nervous system at risk for injury (e.g.thyroidectomy, parathyroidectomy, parotidectomy) is considered medically necessary if the medical appropriateness criteria are met. (See Medical Appropriateness below.)
  • Intraoperative neurophysiologic monitoring for other indications, including, but not limited to, the following: motor-evoked potentials using transcranial magnetic stimulation is considered investigational.
  • Any device utilized for this procedure must have FDA approval specific to the indication, otherwise it will be considered investigational.
See also: Visual and Auditory Evoked Potentials
  • Intraoperative neurophysiologic monitoring is considered medically appropriate if ANY ONE of the following criteria are met:
    • Somatosensory-evoked potentials (SSEPs) if ANY ONE of the following:
      • Neurosurgical, orthopedic, vascular or other surgical procedure placing the nervous system at risk for injury at spinal levels C1 - L2 and ANY ONE of the following:
        • Documentation of involvement of monitoring instrumentation that can effectively monitor tissue at risk, and documentation of exposure of multiple nerve roots at one or more levels of the spinal cord
        • Documentation of the existence of a need to monitor the connections between nerve roots and monitoring multiple muscles at multiple levels to detect possible injury to those connection
      • A cerebral vascular procedure used to restore normal anatomy and blood flow to cerebral artery complex including carotids, vertebral arteries, and cerebral arteries (e.g. carotid endarterectomy, cerebral aneurysm)
    • Electromyogram during a surgical procedure involving C1 - S1 and ANY ONE of the following:
      • Documentation of involvement of monitoring instrumentation that can effectively monitor tissue at risk, and documentation of exposure of multiple nerve roots at one or more levels of the spinal cord
      • Documentation of the existence of a need to monitor the connections between nerve roots and monitoring multiple muscles at multiple levels to detect possible injury to those connection
    • Electroencephalogram (EEG) for a cerebral vascular procedure used to restore normal anatomy and blood flow to cerebral artery complex including carotids, vertebral arteries, and cerebral arteries (e.g. carotid endarterectomy, cerebral aneurysm)
    • Brainstem auditory - evoked potentials (BAEPs) for any surgical procedure on or near the acoustic nerve, inner ear or brainstem
    • Visual-evoked potentials (VEPs) for any procedure on or near the optic nerve, cortex or chiasm
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Evoked potential studies are to be performed by the physician alone or by a technologist under direct supervision of the electrodiagnostic (EDX) consultant. The consultant must be trained or certified in the application, performance and interpretation of electrodiagnostic testing and licensed by the state in which the studies are performed. The technician performing the studies must be trained in electrodiagnostic testing and licensed or certified by the state (or by one of the state's health related boards if licensure or certification as a technician does not exist in a given state) in which the studies are performed.
Electrodiagnostic testing is not performed in a standard fashion, but must be specifically designed for each individual patient. It is often necessary to modify or add to the procedure during the examination, depending on the findings as they unfold. For this reason, collection of the clinical and electrophysiologic data must be entirely under the supervision of an EDX consultant who is physically present in the facility in which the studies are performed and actively involved in the examination of the findings as they unfold. (Refer to BCBST's
Staff Supervision Requirements for Delegated Services policy.)
According to the American Association of Neuromuscular & Electrodiagnostic Medicine, individuals performing electrodiagnostic studies should be performed by physicians properly trained in electrodiagnostic medicine. Eligibility requirements for the annual American Board of Electrodiagnostic Medicine (ABEM) examination include the completion of an accredited residency in neurology or physical medicine and rehabilitation with a minimum of 6 months of full-time equivalent formal clinical neurophysiology training and Board certification by the American Board of Psychiatry and Neurology (neurology or psychiatry) or the American Board of Physical Medicine and Rehabilitation (physiatry). One year of additional post-residency experience is also required. The ABEM is an independent credentialing body established by the AANEM. Certification in electrodiagnostic medicine by an examining board, such as the ABEM, is also recommended.
Resnick et al summarized that, “based on the medical evidence provided by the literature reviewed, there does not appear to be support for the hypothesis that any form of intraoperative monitoring improves patient outcomes following lumbar decompression or fusion procedures for degenerative spinal disease. Evidence does indicate that a normal evoked EMG response is predictive for intrapedicular screw placement (high NPV for breakout). The presence of an abnormal EMG response does not, however, exclude intrapedicular screw placement (low PPV). The majority of clinically apparent postoperative nerve injuries are associated with intraoperative changes in SSEP and/or DSEP monitoring. For this reason, changes in DSEP/SSEP monitoring appear to be sensitive to nerve root injury. There is a high false-positive rate, however, and changes in DSEP and SSEP recordings are frequently not related to nerve injury. A normal study has been shown to correlate with the lack of a significant postoperative nerve injury. There is no substantial evidence to indicate that the use of intraoperative monitoring of any kind proves useful information to the surgeon in terms of assessing the adequacy of nerve root decompression at the time of surgery”.
There is a lack of published studies to validate the use of intraoperative neurophysiologic monitoring for other indications.
There is a lack of well-designed randomized controlled trials and published studies that provide evidence of the safety and efficacy of the motor-evoked potentials using transcranial magnetic stimulation technology.
As of April 8, 2011, a U. S. Food and Drug Administration (FDA) web site search did not find any 510(k) premarket notification approvals for the motor-evoked potentials using transcranial magnetic stimulation devices.

Vertebral Anatomy