Feasibility of Saphenous Nerve Somatosensory-Evoked Potential Intraoperative Monitoring during Lumbar Spine Surgery

Abstract

Study Design. Retrospective review. Objective. Assess the feasibility of saphenous nerve somatosensory evoked potentials (SN-SSEP) monitoring in lumbar spine surgeries. Background Context. SN-SSEPs have been proposed for detecting lumbar plexus and femoral nerve injury during lateral lumbar surgery where tibial nerve (TN)SSEPs alone are insufficient. SN-SSEPs may also be useful in other types of lumbar surgery, as stimulation of SN below the knee derives solely from the L4 root and provides a means of L4 monitoring, whereas TN-SSEPs often do not detect single nerve root injury. The feasibility of routine SN-SSEP monitoring has not been established. Methods. A total of 563 consecutive cases using both TN and SN-SSEP monitoring were included. Anesthesia was at the discretion of the anesthesiologist, using an inhalant in 97.7% of procedures. SN stimulation was performed using 13 mm needle electrodes placed below the knee using 200-400 μsec pulses at 15-100 mA. Adjustments to stimulation parameters were made by the neurophysiology technician while obtaining baselines. Data were graded retrospectively for monitorability and cortical response amplitudes were measured by two independent reviewers. Results. 98% of TN-SSEPs and 92.5% of SN-SSEPs were monitorable at baseline, with a mean response amplitude of 1.35 μV for TN-SSEPs and 0.71 μV for SN-SSEPs. A significant difference between the stimulation parameters used to obtain reproducible TN and SN-SSEPs at baseline was observed, with SN-SSEPs requiring greater stimulation intensities. Body mass index (BMI) is not associated with baseline monitorability. Out of 20 signal changes observed, 11 involved SN while TN-SSEPs were unaffected. Conclusion. With adjustments to stimulation parameters, SN-SSEP monitoring is feasible within a large clinical cohort without modifications to the anesthetic plan. Incorporating SN into standard intraoperative neurophysiological monitoring (IONM) protocols for lumbar spine procedures may expand the role of SSEP monitoring to include detecting injury to the lumbar plexus. Level of Evidence. 3