Triggered EMG Potentials in Determining Neuroanatomical Safe Zone for Transpsoas Lumbar Approach: Are They Reliable?
Spine (Phila Pa 1976)
STUDY DESIGN: In vivo analysis in swine model OBJECTIVE.: The purpose of this study is to determine the accuracy of t-EMG and its reliability in LLIF surgery. We also aim to document changes in psoas muscle produced during the approach. SUMMARY OF BACKGROUND DATA: LLIF is preferred over direct anterior approach due to lower complications, blood loss, and shorter recovery time. Threshold-EMGs are utilized for real-time feedback about nerve location, however, neurological deficits are widely reported, and are unique to this approach. Multiple factors have been hypothesized including neuropraxia from retractor and compression from psoas hematoma/ edema. The variable reports of neurological complication even with t-EMGs indicate the need to study them further. METHODS: 8 swines underwent left-sided retroperitoneal approach. Nerve on the surface of psoas was identified and threshold-EMGs were obtained utilizing a ball-tip, and needle probe. 1 EMG and threshold responses required to elicit 20 muV responses were recorded for 2 mm incremental distances up to 10 mm. In the second part, a K-wire was inserted into the mid-lumbar disc space, and a tubular retractor docked and dilated adequately. Post-mortem CT scans were carried out to evaluate changes in psoas muscle. RESULTS: A triggered-EMG stimulus threshold of < 5 mA indicates a higher probability that the probe is close to or on the nerve, but this was not proportional to the distance suggesting limitations for nerve mapping. Negative predictive value of t-EMGs is 76.5% with the ball-tipped probe and 80% with the needle probe for t-EMG >/=10 mA and indicates that even with higher thresholds, the nerve may be much closer than anticipated. Postoperative hematoma was not seen on CT scans. CONCLUSION: Threshold measurements are unreliable in estimating distance from the nerve in an individual subject and higher values do not always correspond to a 'safe zone". LEVEL OF EVIDENCE: 5.