What are intraoperative somatosensory evoked potentials (SEPs)?

Updated: Oct 25, 2019
  • Author: Andrew B Evans, MD; Chief Editor: Selim R Benbadis, MD  more...
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The basis of intraoperative SEP as a representative index of motor function is based on the fact that the vascular compromise that may cause motor dysfunction or loss also affects the lateral corticospinal tract and the dorsal spinocerebellar tract. In general terms, the regions of both motor and sensory pathways (lateral corticospinal tract and dorsolateral spinal cord and the alpha motor neurons) are served by the same vascular supply.

Intraoperative SEP poses a special challenge and requires close cooperation with the operating room staff, as well as special equipment and careful attention to limit electrical noise. Careful checking of the ground is important, as is good shielding. The list of interfering factors is long and includes electrocautery equipment, nerve stimulators, and electric drills. Anesthetic agents and level of anesthesia also are interfering factors.

The following considerations should be kept in mind:

  • Access to the patient is important, but the recording should not interfere with the operative procedure; a baseline recording should be done before the operation to establish the patient’s own normal value under less stressful circumstances

  • The operative plan and what the surgeon expects from the monitoring must be discussed in advance; communication with the entire staff is important

  • A note should be made of any preexisting diseases (eg, diabetes) that might interfere with successful SEP testing

Stimulus pulse duration was evaluated by Luk et al, who found a duration of 0.3 msec to be the recommended choice for tibial SEPs during intraoperative monitoring. [57] Stimulus duration affected amplitude but not latency.

SEP monitoring during lumbosacral spinal stenosis is a well-known procedure. Weiss found that even in routine surgical procedures, monitoring the SEP along with EMG helps the surgical team to avoid neurogenic complications. [58]

Recording motor evoked potential (MEP) and SEP during thoracoabdominal aortic surgery to assess ischemia of the spinal cord has been valued by a number of authors as a means of lowering the risk of postoperative neurologic injury. [59] Polo et al found SEP and MEP to be useful in scoliosis surgery for assessing hypotension-related anoxic cord injury. [60] Weigang et al found SEP monitoring to be useful in thoracoabdominal aortic endovascular stent grafting for prevention of spinal cord anoxia. [61]

Arrington et al used similar methods in pelvic fractures and acetabular surgery and found that combined SEP and MEP recording prevented damage to the sciatic nerve. [62] Mills et al found that monitoring radial nerve SEP was helpful in humeral nailing procedures. Schwartz et al reported that monitoring ulnar SEP was predictive of brachial plexus injury during surgery for correction of scoliosis. [63] However, Deutsch et al found that the false-negative rate was 9% for SEP in anterior spinal surgical approaches. [64]

Hyun et al reviewed 85 cases of intraoperative monitoring using a combination of SEP and MEP and found that the combination of SEP and MEP had a higher sensitivity for detecting postoperative motor abnormalities than either modality alone did. [65]

In this study, no postoperative neurologic abnormalities were seen when SEP and MEP parameters remained stable. [65] In 20 of the 85 cases, the MEP was abnormal, with or without SEP changes. In 7 of these cases, the MEP recovered, and there were no resulting neurologic deficits. The remaining 13 patients did not have recovery of the MEP and had transient or permanent neurologic abnormalities. Four patients had SEP changes with no MEP abnormalities that did not correlate to postoperative motor dysfunction.

Baba et al reported the results of spinal cord EP monitoring for cervical and thoracic compressive myelopathy, finding that the preoperative EPs were not clearly helpful in predicting outcome but that early postoperative recovery of the EPs correlated with clinical improvement. [66]

In this study, epidural spinal cord EPs were recorded in 95 patients undergoing surgery for cervical or thoracic compressive myelopathy. [66] Abnormal spinal cord EPs correlated significantly with the severity of spinal cord compromise and symptoms (eg, myelopathy). All of the thoracic myelopathy cases and 91% of the cervical myelopathy cases exhibited abnormal EPs.

Davis et al reported the use of ulnar nerve SEPs to detect and prevent position-related neuropathy in the first pediatric patient in the world to undergo robotic-assisted thyroidectomy; they concluded that upper-extremity SEPs should be routinely performed during this procedure. [67] Patient positioning for this approach, as well as retraction during exposure, has the potential to result in postoperative brachial plexopathy similar to what is seen in other types of surgery.

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