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What is Intraoperative Monitoring of Nerves?

Intraoperative monitoring of the nerves is used during surgery to help prevent damage to the nervous system and guide the surgeon to have a safe and optimal outcome.

Intraoperative monitoring, or IOM, is a technique used during surgery to monitor the condition of a patient’s nervous system throughout the surgical procedure. Monitoring the condition of the nervous system while the patient is asleep under general anesthesia helps prevent damage to the spinal cord, brain, or nerves. Therefore, the use of intraoperative monitoring reduces the risk of surgery-related nerve damage. IOM is often used during procedures such as spine or brain surgery, where surgical complications may cause a loss of neurological function. Intraoperative monitoring of nerves is also referred to as intraoperative neurophysiological monitoring or intraoperative neuromonitoring.  Evolving research in area of neuromonitoring has allowed the advancement of cutting edge neurosurgical techniques and helps us ensure the best possible outcomes.

Types of Intraoperative Monitoring

There are several types of intraoperative monitoring that can be utilized during surgery. Your surgeon may choose to use one or more of the below monitoring modalities based on the procedure being done. The main types include:

  • Motor Evoked Potentials, or MEPS, allows signals sent from the brain to certain muscle groups to be monitored.
  • Somatosensory Evoked Potentials/Dermatome Evoked Potentials, or SSEP/DEPS, allows signals sent to the brain from specific sensory areas to be monitored.
  • Electromyography, or EMG, allows signals within certain muscle groups to be monitored during spine surgery. The EMG technique monitors nerves controlling muscle groups during surgery on the part of the spine that controls those muscles.
  • Brainstem Auditory Evoked Responses (BAERS), allows signals from the nerves controlling hearing to be monitored continuously.
  • For example:
  • During neck, or cervical, surgery: The nerves in the arm and leg muscles are monitored.
  • During mid-back, or thoracic, surgery: The nerves in the abdominal and leg muscles are monitored.
  • During low back, or lumbar, surgery: The nerves in leg muscles are monitored. Sometimes nerves going to the bladder and bowel are monitored as well.
  • During brain surgery: the fiber tracts in the brain are tested and continuously monitored to prevent damage to important structures.  Sometimes awake or asleep motor or awake language mapping is used to identify critical pathways that help guide successful outcome.
  • During brainstem surgery around cranial nerves:  The cranial nerves that come out of the low part of the brain can be monitored continuously to help preserve hearing, facial movements, and swallowing functions.
  • During a surgery to remove a lesion or tumor directly on a nerve: The nerve of interested is tested and monitored during the procedure.


First, electrodes are placed on and/or under the skin at various locations along a nerve pathway. The electrodes are connected to a computer that analyzes information about any nerves being monitored.


Next, an electrode will stimulate the nerve with an electrical impulse. The impulse is brief and contains a very low amount electrical current.


Following the stimulation, the nerve’s activity is assessed. The other electrodes along the nerve pathway record the time it takes for electrical impulses to travel between them. The speed of the signal is calculated using the distance between electrodes and the time it takes the impulses to travel between them.

Using the Information

The baseline information is compared to that obtain throughout the surgical procedure. If the signal becomes slower and weaker, this reveals a problem with the nerve, like compression. Therefore, changes in signal responses allow a surgeon to quickly identify a problem and take corrective action before nerve damage becomes permanent.

This material is intended to give the patient an overview of surgical procedures and treatments and is not intended to replace the advice and guidance of a physician. Always consult with your doctor about the particular risks and benefits of your treatment.

Our Physicians

All physicians in this practice may use intraoperative nerve monitoring.