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Intraoperative Neuromonitoring for Cervical Spine Surgery

Intraoperative monitoring (IOM) refers to the application of various tests incorporating electrical stimulation and detection with human neurophysiology to reduce the risks of intraoperative complications like nerve or spinal cord damage. These tests can detect changes in the patient’s nervous system in a real-time fashion (as they occur) while the patient is under general anesthesia, allowing the surgeon to act immediately to potentially reverse or minimize the resulting, potentially devastating neurological deficit.

I IOM depends on a team approach to produce its most effective results. Your anesthesiologist must be intimately involved, as various inhalational agents to maintain general anesthesia and low body temperature will prevent obtaining reliable results. Your surgeon must be able to interpret the events identified by IOM as a potential problem and act accordingly, such as remove an offending screw or check for compression of the spinal cord as a result of manipulation. Finally, a highly-trained neurophysiologist must be available to place electrodes (tiny needles or patches) over your body once general anesthesia has been induced to allow for monitoring of your nervous system throughout the surgery as well as to interpret the waveforms. Electrodes are typically placed in and on the hands, feet, head, and adjacent to specific muscles and nerves, depending on the procedure to be performed. The neurophysiologist will continuously run electrophysiologic tracings throughout the procedure, interpret the results, and notify the surgeon immediately if a significant change in amplitude or latency of the waveforms occurs, indicating potential changes in neurologic function. This information will then be relayed to your surgeon, who can initiate an appropriate intervention, as required.

IOM first began in the 1970s as an adjunct to scoliosis surgery. Over the past decade, IOM has become routine and in most cases considered the standard of care during most extensive spinal procedures. The risks of IOM are extremely few and for the most part minimal in severity. In general, these would include but not be limited to bleeding or hematoma formation at the site of electrode insertion; infection at the electrode site; or damage to the muscle from repetitive stimulation. It should be noted that IOM does not eliminate intraoperative complications; it is used as an adjunct to recognize and help prevent some of the complications. As the medical profession comes to increasingly appreciate the value of this technology, in most cases, the cost of IOM is provided by your insurance company.

IOM of cervical spine surgery can take various forms:

  1. Somatosensory Evoked Potentials (SSEP’s): Anatomically, SSEP’s measure the dorsal column pathways of the spinal cord and the peripheral nerves and receptors responsible for a light touch, vibration, and position sensation through the body below the base of the skull. The peripheral nerve most commonly stimulated by SSEP’s include the ulnar and median nerves in the arms and the posterior tibial and peroneal nerves in the legs. SSEP tracings are measured both regarding amplitude (height) and latency (spacing or timing) of waveforms. A significant decrease in amplitude (greater than 50%) or a prolongation of latency (greater than 10%) portends a possible problem. Possible causes of false positives include but are not limited to: use of certain inhalational anesthetic agents; lowered body temperature; compression of peripheral nerves; and low blood pressure. SSEP’s are highly specific (in the range of 95%) but unfortunately have a low sensitivity to detect new motor deficits during surgery as they monitor sensory pathways. Therefore, they are commonly combined with transcranial MEP monitoring.
  2. Electromyography (EMG’s): EMG recordings are based on determining skeletal muscle activity by placing paired electrodes into or near the muscles supplied by nerves at risk during the surgical procedure. Because EMG’s are more sensitive to nerve root injury, they are used in conjunction with SSEP’s and can detect up to 30% more irritation of cervical nerve roots compared to SSEP monitoring alone Bursts of muscle and/or nerve irritation can be the result of mechanical stimulation (e.g., surgical dissection), metabolic insults, nerve retraction or stretch, or temperature changes. For EMG monitoring to be effective, neuromuscular blocking agents during the monitored portion of the surgery are contraindicated.
  3. Transcranial Motor Evoke Potentials (MEP’s): MEPS’ are considered the gold standard for monitoring and detecting new motor deficits. By exciting cortical neurons with transcranial electrical stimulation, the integrity of the pathways including the corticospinal tracts, spinal cord motor neurons, and peripheral nerves can be effectively monitored. Muscles used in MEP monitoring ar most commonly the smaller muscles of the hands and feet as they are most densely supplied by the motor (corticospinal) tracts of the spinal cord. MEP’s can be obtained very quickly (less than 10 seconds of monitoring time). Very rare complications of MEP monitoring include movement-related injuries such as tongue lacerations from jaw muscle contractions; cardiac arrhythmias; seizures; headaches; or damage to motor neurons resulting in weakness.
  4. Intraoperative Screw Stimulation (Triggered EMG’s): Using a hand-held probe, screws placed at the time of surgery for stabilization of the spine can be directly stimulated to determine the threshold level at which adjacent nerve roots are stimulated by the current. This gives the surgeon an idea of the proximity of the screw to the nerve itself and whether the bone has been breached adjacent to the nerve (seen as a low threshold stimulus) Screw stimulation with triggered EMG has proven to be very effective in detecting malposition of instrumentation, allowing the surgeon to immediately reposition the affected screw. False positives can occur with decreased bone integrity (multiple surgeries with altered bony anatomy, invasion by tumor, osteoporosis, and osteopenia). In our practice, combined with intra-operative imaging, this has virtually eliminated the need to take patients back to the OR for a malpositioned screw.
  5. Recurrent Laryngeal Nerve Monitoring (RLNM): The RLN is a branch of the vagus nerve that runs through the soft tissues of the neck to innervate the intrinsic muscles of the larynx. The nerve is vulnerable to stretch injury during exposure of the lower cervical and upper thoracic spine during anterior approaches; damage to the nerve results in hoarseness of the voice. In about 90% of cases, the nerve will recover on its own within 3-4 months after surgery. Monitoring of the RLN function during anterior cervical approaches is beneficial to minimize RLN palsy.
  6. Electroencephalography (EEG): Using cranial leads to monitor brainwave activity, EEG can provide valuable information as to the depth of anesthesia experienced by the patient on a second-by-second basis.

What can the surgeon do if intraoperative changes of concern occur during IOM? First and foremost is to assess and restore normal physiologic functioning. This entails re-establishment of normal blood pressure, temperature, and oxygen levels, as well as checking arterial blood gases and hemoglobin levels. Physical manipulation should be discontinued, especially distraction of the bony elements of the spine. The positioning of the patient should be checked to make certain undue pressure on the peripheral nerves is not an issue. Inhalational anesthetic agents may be the culprit and should be discontinued. Finally, if all these measures do not return the waveforms to normal, hardware should be removed to eliminate possible neural compression. Overall, the most common causes of signal abnormalities include malpositioning of a screw, effects of low blood pressure or low temperature, patient positioning, deformity correction, or pressure due to pressure or traction caused by retractors.

In summary, IOM using SSEP’s, MEP’s, EMG’s, triggered EMG’s, RLN monitoring, and EEG’s, allows the spine surgeons to monitor the integrity of the brain, spinal cord, nerve roots and peripheral nervous system in a real-time fashion to continuously minimize, prevent, or reverse intra-operative injury leading to neurologic deficits during cervical surgery. This has been shown to limit post-operative neurologic deficits to 0.1 to 0.2% of patients overall undergoing spine surgery. In our hands, changes detected by IOM have most commonly resulted in an adjustment in patient positioning to prevent the development of intra-operative peripheral nerve palsies (median, ulnar, and posterior tibial) due to pressure compared to actual nerve root or spinal cord injuries. In combination, IOM has proven to be very safe and effective, especially for cervical procedures, with sensitivities in the range of 92% and specificity in the range of 98%.

If you have questions regarding any aspect of cervical spine surgery or IOM, call us today at 1-888-854-9274.