Bed break-induced attenuation of downside leg sciatic nerve sensory and motor evoked potentials during lateral lumbar interbody (LLIF) fusion surgery

Background Multimodality intraoperative neuromonitoring (IONM) is used to map and monitor the femoral plexus during transpsoas lateral lumbar interbody fusion (LLIF) surgery. Approach side LLIF IONM alerts that portend a femoral plexopathy have been well-studied, but downside sciatic nerve distribution alerts have not been reported. The breaking of the bed to obtain better access to the disc space can cause attenuation of downside leg sciatic nerve distribution somatosensory, transcranial, and transabdominal motor evoked potentials (SSEPs, TcMEPs, and TaMEPs) and appears most common in females with a BMI ≥ 30. We evaluated the propensity and cause of downside leg sciatic nerve distribution SSEP, TcMEP, and TaMEP alerts during LLIF surgery. Methods A single-center 30-month retrospective examined the frequency of downside leg sciatic nerve distribution IONM alerts for 692 patients, 380 female (55%) and 312 male (45%), having LLIF spine surgery utilizing IONM consisting of SSEPs, TcMEPs, TaMEPs, electromyography (EMG) and electroencephalography (EEG). Downside leg IONM alerts were analyzed for correlation to patient BMI, sex, age, as well as the recovery of IONM signals to the length of time the bed remained broken. IONM data was correlated to immediate postoperative neurological evaluations and chart reviews. Results Of 692 LLIF surgeries, 18 (2.6%) downside leg sciatic nerve distribution SSEP, TcMEP, and TaMEP alerts were observed. No cases detected concomitant IONM changes involving downside-leg femoral nerve distribution. The attenuation and recovery of the downside leg sciatic nerve distribution IONM responses during these alerts appeared to correlate to the breaking and unbreaking of the bed. Of the 18 alerts, 17 occurred in females, 12 in females with BMI ≥ 30, and 16 in patients aged 60 or older. Chi-square analysis points to sex (female) and obesity (BMI ≥ 30) as characteristics that correlate to this phenomenon. Interestingly, none of the 18 patients, including the 2 where IONM responses did not return to baseline, suffered postoperative downside leg sciatic distribution motor or sensory deficits. Conclusions The possible mechanism of these downside leg alerts is compression or stretch of the sciatic nerve at the level of the pelvis, not leg ischemia. While none of the 18 alert case patients suffered postoperative deficits, best practice would be to partially or completely unbreak the bed as soon as feasible when the downside sciatic nerve distribution IONM signals are attenuated.