Research: Intraoperative Neuromonitoring (IONM) .... or Intraoperative Monitoring (IOM)

Intraoperative neuromonitoring is used during surgical operations that place nerve cells in the brain, spinal cord, or arms/legs at risk of injury.  Surgical injury to nerves can occur from mechanical forces, such as cutting, stretching, or rapidly pressing on nerve tissue.  Nerve tissue is also extremely sensitive to blood flow and oxygen delivery; permanent injury to nerves can occur in as little as 3 minutes if they lose their oxygen supply.  Loss of blood flow might occur for a variety of reasons, including: 1) placing a retractor on a blood vessel; 2) placing a bone graft up against the spinal cord; 3) correcting a bend or curve in the spine; or 4) dislodging a blood clot during vascular procedures (e.g. carotid endarterectomy), where that clot travels to the brain and blocks smaller arteries.

Neuromonitoring tests are done while the patient having surgery is deeply anesthetized.  The specific tests vary depending on what part of the nervous system is at risk.  In general the idea is to activate nerve cells on one ‘side’ of the surgery (‘upstream’), causing these nerve signals to conduct through and beyond the region at risk from surgery, and record the response to this activation from a different population of nerve fibers (or from muscle fibers connected to the nerve fibers) that are ‘downstream’ from the surgery site.

If the signals downstream from where surgery is taking place are unchanged, this indicates that the nerves conducting that signal are OK, and have probably not been damaged by the surgery.  On the other hand, if the nerve signals downstream from surgery change beyond a certain amount from their initial properties (called their ‘baseline’ response), this indicates that the surgical procedure might be interfering with function in the nerves responsible for these waveforms.  When this happens, the surgical team is warned that the signals have changed or gotten weaker, giving the surgeons time to try and get the signals back again.

Intraoperative neuromonitoring is extremely sensitive to the effects of general anesthesia medications.  Successful IONM is impossible without the cooperation and support of the anesthesiologist (or nurse-anesthetist).  This person is responsible for keeping the patient deeply anesthetized (so that the patient neither remembers what happened during surgery nor moves), but not so deeply anesthetized so that all the signals used in the different types of IOM tests disappear, making IOM impossible.

My lab has published a number of papers in the field of IONM that have helped influence the field.  I showed that even very low levels of the anesthetic agent isoflurane make MEP monitoring difficult, if not impossible.  The same holds true for newer agents in this class, such as Sevoflurane and Desflurane.

My lab pioneered the use of stimulus-evoked EMG for testing the position of lumbosacral pedicle screws, and is currently funded by NIH to translate this approach to improve the safety of pedicle screw placement in the thoracic spine.

We’ve been using MEPs in the lab since 1985, and in the operating room since 1988.  Acting as Sponsor/Investigator, research from my lab led to marketing approval from the FDA for the Digitimer D185 multi-pulse electrical stimulator in 2002.

I’ve published several papers describing the advantages of using stimulus intensity threshold as the primary outcome measure for MEP monitoring; these can be found here and here.  I've also pointed out the risks of using the 'Presence or Absence' alarm criteria for interpreting MEPs.