Intraoperative monitoring for major changes in somatosensory and transcranial electrical motor-evoked potentials during spinal surgery can predict the risk of partial postoperative paralysis, according to an updated evidence-based guideline developed by the Therapeutics and Technology Subcommittee of the AAN and the American Clinical Neurophysiology Society.
According to the new report, published in the Feb. 21 issue of Neurology, major changes in somatosensory-evoked potentials (SEPs) and transcranial electrical motor-evoked potentials (tceMEPs) — known collectively as EPs — can flag potential problems during surgery likely to result in permanent spinal damage and partial paralysis of the lower limbs (paraparesis), paraplegia, or even quadriplegia. The report includes a table of specific EP parameters to use during intraoperative monitoring.
Monitoring allows surgeons and anesthesiologists to take preventive measures to reduce the risk of such postoperative problems, according to lead author Marc R. Nuwer, MD, PhD, professor of neurology at the David Geffen School of Medicine at the University of California, Los Angeles (UCLA) and head of the clinical neurophysiology department at Ronald Reagan UCLA Medical Center. Dr. Nuwer highlighted for Neurology Today the main findings of the new evidence-based review.
WHAT WERE THE MAJOR FINDINGS?
We concluded that intraoperative monitoring with sensorimotor-evoked potentials, or EPs, can predict a patient's risk of paraparesis, paraplegia, and quadriplegia following spinal surgery, based on eight Class II studies and four Class I studies, those with the highest rating for their design integrity and analysis.
In the Class I studies, between 16 percent and 40 percent of monitored patients with EP changes developed postoperative onset paraparesis, paraplegia, or quadriplegia. In one Class I study, no events of paraparesis, paraplegia, or quadriplegia were reported among 17 intraoperative monitoring ofpatients without EP changes, whereas there were five postoperative motor adverse outcomes among the 16 patients with EP changes, or a 31 percent rate of adverse outcomes when EPs did change.
HOW ARE THE INTRAOPERATIVE MONITORING EPs USED DURING SPINAL SURGERY?
SEP involves ankle-wrist stimulation with neck and scalp recording, while tceMEP involves muscle recording after transcranial brain stimulation.
During SEP, electrodes are attached to the ankle, neck, and scalp. Once stimulation is applied, electrical signals travel upward along the sensory pathways. Any significant change in these signals as they reach the neck, and then the brain, alert the surgical team that there is a problem.
MEP stimulation sends signals in the descending direction.This technique evaluates the reaction of muscles to brain stimulation applied via electrodes placed on the scalp. These signals travel downward along the spine's motor pathways to the arms and legs, and muscle response is recorded.
By using the two processes, each moving in the opposite direction, it is possible to assess whether or not the spinal cord is correctly doing its work. If at any time during a surgical procedure there are major signaling interruptions, we know something is going wrong and the surgeon and anesthesiologist can take steps to correct the problem before it becomes permanent.
In most patients there is about a 15-minute window of opportunity for repairing a problem detected by intraoperative monitoring. If we intervene within this time frame there is a good chance of a positive outcome. Not always, but most of the time.
CAN YOU PROVIDE AN EXAMPLE?
Say a patient's blood pressure suddenly falls 20 or 30 points during a procedure. It can be the straw that breaks the camel's back. Many surgeons prefer blood pressure to be lower during surgery to reduce blood loss, so there is an increased risk of a critical lack of blood supply in an injured spinal area. If blood pressure falls too far and stays that way for several hours, it causes the equivalent of a spinal cord stroke with permanent, irreversible damage. But monitoring can detect when an area of the spine reacts to critical deficits in blood delivery and blood pressure can then be raised to alleviate the problem.
WHAT ELSE CAN BE DONE IF THERE WERE MAJOR EP CHANGES?
Surgeons and anesthesiologists can intervene in a variety of ways. They can modify the surgery by reducing the degree of distraction, adjust retractors, remove or adjust grafts or hardware, re-implant or unclamp arteries, place vascular bypass grafts, minimize the remaining portion of the surgery, or by taking other actions. Surgeons also have the opportunity to check a wake-up test in some patients.
Monitoring also benefits the operating staff. It can make surgeons feel more confident performing spinal procedures. That allows them to take on more high-risk cases than they do now, and perform a case more thoroughly than they would have without monitoring.
WHAT ABOUT PREDICTING THE RISK OF LESSER DEGREES OF MOTOR IMPAIRMENT?
There was not enough data to draw conclusions about any benefit in reducing less severe forms of motor impairment such as spinal root injuries.
WHICH MONITORING WORKS BEST, MEP OR SEP?
None of the studies compared the two, but when used together they have a high rate of accuracy in catching problems during spinal procedures in time for the surgical team to intervene.
Conducting both intraoperative mon-i–toring techniques together was found to be a reasonable choice for many patients; however, neither technique can predict the onset of paraplegia that is delayed until hours or days after surgery. This monitoring requires a trained neurophysiologist to supervise the technologists and interpret monitoring changes.
HOW WIDELY AVAILABLE ARE INTRAOPERATIVE MONITORING DEVICES, AND DO MOST INSURERS COVER THEIR USE IN SPINAL SURGERY?
The technology is readily available, and its use has been recommended for some time. It is already in the mainstream. The equipment costs around $40,000. A number of “mobile” companies provide equipment and trained technicians as needed, serving as third-party vendors to hospitals. They might come into a hospital once or twice a month, or as needed, whenever spinal surgeries are scheduled.
Insurers cover the extra cost of the monitoring during spinal surgery. It is considered as accepted as EEG monitoring of epilepsy patients in intensive care units.
WHAT ADDITIONAL RESEARCH IS NEEDED OR MIGHT BE USEFUL?
We found several areas where more study would be welcome. Pooling of results from a large series of well-monitored patients would permit a better understanding of certain areas where questions remain unresolved. For example, there was a low rate of false-negatives in MEP intraoperative monitoring in the studies we reviewed but we do not know if this can be generalized to the technique as a whole. Also, a better understanding of anterior spinal artery syndrome may help to reduce further the rates of both paraplegia and paraparesis after surgery.
In addition, if we could more clearly identify limitations specific to the individual EP monitoring techniques, and methods could be found to correct them; studies directly comparing SEP with MEP might be worthwhile.
WHAT NEEDS TO BE DONE TO INCREASE THE USE OF INTRAOPERATIVE MONITORING?
Spine surgeons should already be using MEP and SEP, but many do not. The challenge is getting the word out to community hospitals, where monitoring is used less often than in larger hospitals.
Also, there is a shortage of neurologists with training and experience with MEP and SEP monitoring during spinal procedures. The AAN offers introductory training sessions during its annual meeting, and many medical schools offer training as one of the four sub-specialty clinical categories in clinical neurophysiology programs. I am hopeful as word gets out about the benefits of intraoperative monitoring, more neurologists will seek training, gain experience, and apply monitoring EPs during spinal procedures in their regional hospitals.