Intraoperative neurophysiologic monitoring during spinal procedures

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What is intraoperative neurophysiologic monitoring?

Intraoperative neurophysiologic monitoring refers to the various neurophysiologic techniques used to assess functional integrity of the nervous system during surgical procedures that place these structures at risk.

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What neurologic structures are at risk during spinal surgery?

• Spinal cord and/or nerve roots at the surgical site

• Spinal cord and/or peripheral nervous system structures remote from the surgical site, including spinal nerve roots, brachial plexus, lumbosacral plexus, and peripheral nerves (e.g., placed at risk of injury from positioning of extremities, head, or neck)

• Optic nerve

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What criteria need to be met if intraoperative neurophysiologic monitoring is used during spinal surgery?

Three criteria need to be met if monitoring is used during spinal surgery:

• Neurologic structures are at risk

• Those structures can be monitored reliably and efficiently by qualified personnel

• The surgeon is willing and able to alter surgical technique based on information provided

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List common types of spinal procedures during which intraoperative neurophysiologic monitoring is commonly utilized.

• Correction of spinal deformities (scoliosis, kyphosis, spondylolisthesis)

• Insertion of spinal implants (e.g., pedicle screw systems, intervertebral body fusion devices, especially if inserted via a direct lateral surgical approach)

• Decompression performed at the level of the spinal cord

• Surgical treatment of spinal cord tumors

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Which intraoperative personnel may perform intraoperative neurophysiologic monitoring? Why is it important for surgeons to be knowledgeable regarding their qualifications?

Intraoperative neurophysiologic monitoring involves a technical component and an interpretive component. Technical aspects of monitoring include equipment set-up and execution of specific monitoring modalities. Interpretative aspects of monitoring include development of an intraoperative spinal monitoring plan and assessment of the clinical significance of waveform changes during the spinal procedures. The surgeon should be knowledgeable regarding the credentials of spinal monitoring service providers since the medicolegal burden of interpretation becomes the responsibility of the surgeon if the monitoring provider is not certified to interpret neurophysiologic potentials. Personnel who may be involved in provision of spinal monitoring services include:

• Physician: Most often a neurologist who may work with or without technologists

• Neurophysiologist, D.ABNM (Diplomate of the American Board of Neurophysiologic Monitoring): Neurophysiologist with a minimum of a master’s and often a doctorate degree in audiology, neurophysiology, or neurosciences.

• Neurophysiologist: Noncertified neurophysiologist with a minimum of a master’s degree and often a doctorate in neurophysiology or neurosciences.

• Certified neurophysiologic intraoperative monitoring technologist (CNIM): Usually a registered electroencephalography (EEG) technologist who has completed a course of study, demonstrated competence in acquiring intraoperative data over a number of cases, and passed a nationally recognized technologist examination.

• Audiologist (CCC-A, Certified Clinical Competence-Audiology): Certified audiologist with a minimum of a master’s degree in audiology and related neurophysiology.

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What mechanisms may be responsible for neurologic injury during spine procedures?

• Direct injury due to surgical trauma (e.g., during spinal canal decompression or placement of spinal implants)

• Traction and/or compression affecting neural structures. Injury may occur during spinal realignment and deformity correction using spinal instrumentation or as a result of epidural hematoma following corpectomy procedures. Traction or compression injury to neural structures remote from the surgical site may occur due to patient positioning (e.g., brachial plexus traction injury, ulnar nerve compression injury).

• Ischemia resulting in decreased perfusion of the spinal cord and/or nerve roots, resulting in ischemic injury to neurologic structures (e.g., following ligation of critical segmental vessels supplying the spinal cord or after an episode of sustained hypotension). Ischemia is the most common mechanism responsible for neurologic injury during scoliosis surgery.

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What techniques are available for monitoring spinal cord function?

• Stagnara wake-up test

• Ankle clonus test

• Somatosensory-evoked potentials (SSEPs)

• Transcranial electric motor-evoked potentials (tceMEPs)

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What techniques are available for monitoring nerve root function during spinal surgery?

• Electromyographic (EMG) monitoring

• tceMEP monitoring from multiple myotomes

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What is the Stagnara wake-up test?

The Stagnara wake-up test is used to assess the gross integrity of spinal cord motor tract function during spinal surgery. Discussion of this test with the patient before surgery increases its success. During the procedure, anesthesia is temporarily reduced to a degree where the patient is able to follow simple commands (move both hands and then both feet). Most patients have no recollection of being awakened, and those who recall do not report the experience to be unpleasant. This test has significant limitations. It does not provide information about spinal cord sensory tract function or individual nerve root function. In addition, it cannot be administered in a continuous fashion during surgery. A spinal cord injury may not manifest immediately following a specific surgical maneuver and thus may not be detected with a wake-up test. In addition, impending spinal cord compromise due to ischemia cannot be detected using this test. Furthermore, during the wake-up test, patient movement may disrupt sterility of the operative field or displace the endotracheal tube. The limitations associated with clinically based tests, such as the wake-up and ankle clonus tests, stimulated the development of intraoperative neurophysiologic monitoring techniques.

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