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

Introduction - Saphenous nerve (SN)SSEPs have been proposed to detect lumbar plexus and femoral nerve injury during lateral transpsoas lumbar surgery, as 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. TNSSEPs, commonly monitored during lumbar surgeries, derive principally from L5 and S1 and often do not detect single nerve root injury. The feasibility of routine SNSSEP monitoring has not been established.
Aims/Objectives- To determine the feasibility of SNSSEP monitoring without special modification of the anesthetic technique.
Methods- 563 consecutive cases of lumbar surgery, including laminectomies, posterior fusions, and anterior, posterior and transforaminal interbody fusions, monitored using both TNSSEPs and SNSSEPs at single center. Monitorability at baseline and baseline amplitude of TNSSEPs and SNSSEPs, as well as stimulation parameters were recorded. Anesthesia was at the discretion of the attending anesthesiologist; in 99% cases a halogenated inhalations agent at concentration of < 0.5 MAC was used along with an infusion of propofol. SN stimulation was performed using 13mm needle electrodes placed below the knee, TN was stimulated at the ankle. Stimulation parameters for TNSSEPs and SNSSEP were determined in real-time by the technologist. Biphasic stimulation was used in all cases. If needed to obtain monitorable signals, stimulus trains of 4 pulses delivered at 1 ms intervals were used rather than single pulses. Data were graded retrospectively for monitorability and TNSSEP and SNSSEP cortical amplitudes were measured by two independent reviewers.

Results- In a total of 563 patients, 97.9% of TN and 92.2% of SN-SSEPs were monitorable. In general, SNSSEPs required greater stimulation intensities and longer pulse durations that TNSSEPs (Table 1). In addition, the stimulus repetition rate was lowered from the standard 4.79 Hz in 14% of TNSSEPs and 25% of SNSSEPs (p <.05). Pulse train stimulation was employed in 8% of TNSSEPs and 29% of SNSSEPs (p <.05). Mean SSEPs amplitude was 1.4 uV for TN-SSEPs and 0.8 uV for SN-SSEPs (p <.05). Reviewer classification of SSEP monitorability correlated an amplitude threshold of 0.27 uV with an area of the curve (AUC) of 0.993. BMI(obesity vs non obesity) does not affect the individual monitorability of TNSSEPs(p>0.05) or SNSSEPs(p>0.05). Out of 20 SSEP alerts unexplained anesthetic changes, 8 involved both nerves, 11 only SN, and 1 only TN.
Conclusions- It is feasible to monitor SNSSEP monitor during lumbar surgery conducted using balanced anesthesia, with a halogenated inhalational agent along with propofol. Although they are smaller than TNSSEPs and adjustment of stimulations parameters is often necessary, monitorable SNSSEPs can be obtained in almost all cases. Saphenous nerve SSEP monitor may expand the role of neuromonitoring to detect root injury during lumbar surgery.