Trans-cranial motor evoked potential detection of femoral nerve injury in trans-psoas lateral lumbar interbody fusion

Trans-psoas lateral lumbar interbody fusion (LLIF) is frequently associated with neurological complications, limiting its value as a less invasive procedure. The routine use of EMG neuromonitoring has been inadequate to detect iatrogenic injuries; significant postoperative deficits have gone undetected by EMG. An effective way to monitor for these intraoperative neurological events is not yet well established. To our knowledge, detection of lumbar plexus injury during LLIF by trans-cranial motor evoked potentials (MEP) without corresponding change in EMG has not been reported in the literature. Three cases are presented to illustrate the potential utility of trans-cranial MEP monitoring during trans-psoas LLIF. We introduce a modified intraoperative neuro-monitoring (IONM) protocol for LLIF surgery, which includes MEP in addition to spontaneous and triggered EMG. Postoperative neurological outcome was correlated with the IONM findings. In each case, loss of quadriceps MEP signals occurred during LLIF at L4/L5, and after prolonged retraction (27, 25 and 61 min respectively). The EMG, however, did not show any abnormal activity. Two patients had post-operative quadriceps weakness, concordant with MEP data. The third patient, in whom the MEP signals returned to normal after expeditious removal of the retractor, did not exhibit quadriceps weakness, also concordant with MEP data. These cases contribute to the developing perception that stand-alone EMG nerve monitoring is not adequate for trans-psoas surgery. The addition of MEP may improve the sensitivity of IONM during trans-psoas surgery. Multimodality IONM may offer the opportunity to intervene on evolving iatrogenic nerve injuries, and may reduce the incidence of adverse postoperative findings.