Monitoring Spinal Function During Spine Surgery

Any spine surgery is a very delicate operation. Care must be taken to prevent damage to the spinal cord, spinal nerves, and blood vessels supplying these neural components. Damage to the blood vessels and loss of blood supply to the spinal cord can have serious consequences.

Surgeons have an important tool available during spinal surgery to monitor patients called intraoperative neuromonitoring or IOM. IOM methods include the wake-up test, somatosensory-evoked potentials (SSEP), transcranial motor-evoked potentials (tcMEP), spinal cord MEPs, spontaneous electromyography (sEMG), and triggered electromyography (tEMG).

Each one of these tests has its own purposes and functions. But the basic idea behind this type of monitoring is to make sure moment-by-moment during the procedure that no injury has occurred. This is called real-time monitoring. Warning is given so that any damage can be prevented or reversed.

The tests must be accurate enough to avoid any false positives or false negatives. A false positive means the test says there’s a problem when there really isn’t one. A false negative is a test that doesn’t indicate a problem when there is one.

In this study, neurosurgeons from the University of Pennsylvania and University of Virginia reviewed studies published on intraoperative neuromonitoring (IOM). They wanted to know how sensitive are each of the tests. Surgeons need to know what test values require immediate action.

Having these tests makes it possible to perform more complex spinal surgeries. That’s important for patients with severe scoliosis undergoing spinal correction to get the best possible result. The same is true for cancer patients with spinal tumors that have to be removed. It allows the surgeon to be more aggressive when it’s needed and with less risk of complications.

For each of the IOM tests, the authors provide a description of the test, when it would be used, and what the research reports about reliability, validity, and effectiveness of each test. Surgeons are given ways to avoid problems and obstacles with each test. A summary of all the technical information is provided with key points from the article offered in the conclusion.

Here’s a sample of the type of information surgeons can obtain from this review. The wake-up test (gradually reducing the amount of anesthesia until the patient wakes up enough to move their arms and legs) has many more drawbacks than benefits compared to the other tests. It’s easy to do but only offers a one-time look at what’s going on when really ongoing monitoring is much better. It should only be used along with a more consistent test.

Somatosensory-evoked potentials (SSEPs) became popular in the late 1980s and early 1990s. They were thought to be reliable but it turned out there was a high rate of false negatives. SSEPs don’t monitor all aspects of spinal cord, spinal nerve, and vascular (blood supply) function. They are not reliable to test motor (muscle) function and should not be used with patients who already have a neurologic problem. Again, it’s best to use this test along with others and to gear it toward specific cases depending on what type of surgery is being done.

Motor-evoked potentials (MEPs) apply electrical or magnetic impulses to the part of the brain that controls motor function (movement). Electrodes are applies to the scalp. Specific muscles are monitored this way. The problem with this test is that when the muscles contract, the patient moves, and the surgeon has to stop operating until the test is completed. That lengthens the time the patient is under anesthesia and total time in the operating room. It’s a safe test but can’t be used with patients who have skull defects, pacemakers or other implanted devices, or epilepsy.

It’s clear that there’s not one-individual test that works for all patients or that monitors all functions of the spinal cord. If the surgeon wants to monitor both sensory and motor function, then more than one test will certainly be needed. That’s referred to as multimodality intraoperative monitoring or MIOM. MIOM is a great help when the surgeon is trying to remove a spinal tumor completely but without damaging the neural tissues or creating paralysis or other disability.

If one test (such as the SSEP) is too slow to provide the kind of information needed, then MEP can be used. Studies show that when MIOM is used, there are far fewer cases of false-negatives. When problem do develop, they are only temporary and the patients recover within hours to days. The problem is that most surgeons don’t have access to all the different testing procedures and often have to rely on the one or two that are available.

For now, the use of intraoperative monitoring (IOM) is still optional, not required in all spinal surgeries. Because there’s not enough evidence to support specific protocols, there isn’t a legal requirement yet for use of these tests. Whenever IOM or multimodal intraoperative monitoring (MIOM) is used, the surgeon is advised to keep very careful records of everything that is done, how it is done, the exact time each step is performed, and the results.

Patients must understand that even with the best of testing available, problems can develop — even permanent paralysis is still a possibility. Intraoperative monitoring (IOM) isn’t really needed for the more simple spinal procedures, so patients shouldn’t expect this to be a standard part of every spinal operation.

The surgeon who understands IOM will know when and how to use it best. The details in this article will help aid in providing technical information needed about these tests. None of this information and none of the tests can replace a clear understanding of neurologic and vascular anatomy. Likewise, final outcomes of complex spinal surgery still require a high level of technical skill on the part of the surgeon.