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Surgeons Advised to Avoid Presence-or-Absence Method for Intraoperative Monitoring

Posted on: 03/13/2008
Damage to the spinal cord or spinal nerves is always a risk with spine surgery. To avoid neurologic damage, surgeons use intraoperative monitoring (IOM). With IOM, the surgical team uses equipment to test the function of the spinal cord and nerves during spine surgery. To do this, they monitor the contraction of target muscles. The role of IOM is to give the surgeon a warning before permanent nerve injury occurs.

There are many different ways to monitor motor function of the muscles during surgery. One commonly used method is EMG-based motor-evoked potentials (MEPs). MEPs monitor the contraction of muscles using electrical stimulation.

During surgery, while the patient is asleep, needle electrodes are placed in the muscle groups. Needles are placed in muscles that correspond to the area where the surgeon will be working. A machine can monitor electrical activity from the muscle.

In this study, MEPS from 903 patients are reviewed. Repeated transcranial electrical stimulation of the brain (TES) was the specific MEP method used. This is the favored choice of MEPs for many neurosurgeons. The goal was to find the most accurate way to interpret the MEPs.

In some surgical centers, the Presence-or-Absence method of reading MEPs is used. If the muscle contracts when stimulated, then everything is okay. The surgeon considers it a warning if the target muscle(s) don't contract. Measures are then taken to prevent neurologic injury.

Another way to interpret responses is the Threshold-level approach. This method tracks the intensity (in volts) of TES needed to cause a minimal muscle contraction. If the amount of voltage needed to cause a muscle contraction (the threshold) starts to go up, the surgeon is warned of the change.

The results of this study showed that using the Threshold-level alarm is an earlier warning system than the Presence-or-Absence approach. Postoperative weakness was avoided in patients for whom the Threshold method was used. Compared with the Presence-or-Absence approach, the surgeon was warned minutes-to-hours ahead by the Threshold method.

The authors conclude that the Presence-or-Absence method to interpret MEPs should not be used. The Presence-or-Absence method is an accurate and reliable method of triggering a warning. But the warning is delayed and may come too late to protect the patient's motor function.

References:
Blair Calancie, PhD, and Maria R. Molano, MD. Alarm Criteria for Motor-Evoked Potentials. In Spine. February 15, 2008. Vol. 33. No. 4. Pp. 406-414.

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