What is the role of somatosensory evoked potentials (SEPs) in intraoperative monitoring?

Updated: Aug 20, 2019
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: Selim R Benbadis, MD  more...
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Answer

Intraoperative SEPs are recorded routinely using electrode strips or grids or electrodes applied directly to the exposed cortex during neurosurgical procedures in and around the somatosensory cortex. The N20 response is used to identify the primary somatosensory cortex in the postcentral gyrus. By inference, the motor cortex in the precentral gyrus can be localized as well, and this information is used to guide the surgical procedure. Scalp-recorded SEPs have also been used to monitor cerebral ischemia during vascular surgery, such as carotid endarterectomy; however, EEG monitoring is used more widely for this application. The value of regional cerebral oximetry adds nothing to the information already provided by EEG and SEP in determining when to place a shunt during CEA.

Somatosensory response can be measured by magnetoencephalography (see images below). The generators of the N20m and later waveforms can be modeled as equivalent current dipoles. Usually, individual digits and the lip are stimulated, and the single dipoles of the cortical generators are overlaid on co-registered MRIs to create a map of the somatosensory cortex. SEPs are usually collected in outpatients several days prior to the surgery so that the information can be used by neurosurgeons to help in planning the approach to the anticipated surgery. The information also can be used to provide patients with an idea of the relative risk of neurologic deficits that could result from the surgery.

Plot of 148-channel magnetoencephalographic record Plot of 148-channel magnetoencephalographic recording of somatosensory evoked response to tapping on the tip of the left fifth digit. As magnetic fields enter and exit the skull at different locations, the deflections are upgoing at some magnetometer channels and downgoing at other magnetometer positions. The N20m latency is highlighted.
First cortical somatosensory response (N20m) to le First cortical somatosensory response (N20m) to left fifth digit stimulation recorded by magnetoencephalography. Localization of the response is co-registered on the patient's MRI. Note that the response localizes to the postcentral gyrus.
The P34m response to tapping on the tip of the lef The P34m response to tapping on the tip of the left fifth digit as seen on magnetoencephalography.
Localization of the P34m response co-registered on Localization of the P34m response co-registered onto the patient's MRI.

Use of motor evoked potentials (MEP) and SEP monitoring during thoracic and thoracoabdominal aortic surgery is controversial. Motor and somatosensory evoked potentials during thoracic and thoracoabdominal aortic aneurysm repair were analyzed. Irreversible changes were significantly associated with immediate neurologic deficit, and the findings were identical for SEP and MEP in this variable, indicating that the more complex MEP measures do not add further information to that obtained from SEP. Normal SEP and MEP findings had a strong negative predictive value, indicating that patients without signal loss are unlikely to awake with neurologic deficit.

Combined neurophysiological intraoperative monitoring with EMG and SEP recording and the selective use of MEPs is helpful for predicting and possibly preventing neurologic injury during cervical spine surgery.

Intraoperative SEP monitoring was reliable in ruling out spinal injury in descending thoracic and thoracoabdominal aortic repair, but had a low sensitivity. It did not predict delayed neurologic deficit. Spinal SEP change was an independent predictor of mortality and correlated with low preoperative glomerular filtration rate. Effects of various anesthetics used during surgical monitoring need to be kept in mind. Sloan et al have shown drug synergy when isoflurane is mixed with nitrous oxide. If these agents are used for anesthesia, the combination of these agents may produce more amplitude and latency changes than expected from individual agents. [15]

Eager et al emphasized the importance of the use of multimodality methods, including the use of SEPs in spine cases based upon their study of 2069 cases. [16]


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