Motor Evoked Potentials for Femoral Nerve Protection in Transpsoas Lateral Access Surgery of the Spine

Abstract

Detecting potential intraoperative injuries to the femoral nerve should be the main goal of neuromonitoring of lateral lumber interbody fusion (LLIF) procedures. We propose a theory and technique to utilize motor evoked potentials (MEPs) to protect the femoral nerve (a peripheral nerve), which is at risk in LLIF procedures. MEPs have been advocated and widely used for monitoring spinal cord function during surgical correction of spinal deformity and surgery of the cervical and thoracic spine, but have had limited acceptance for use in lumbar procedures. This is due to the theoretical possibility that MEP recordings may not be sensitive in detecting an injury to a single nerve root considering there is overlapping muscle innervation of adjacent root levels. However, in LLIF procedures, the surgeon is more likely to encounter lumbar plexus elements than nerve roots. Within the substance of the psoas muscle, the L2, L3, and L4 nerve roots combine in the lumbar plexus to form the trunk of the femoral nerve. At the point where the nerve roots become the trunk of the femoral nerve, there is no longer any alternative overlapping innervation to the quadriceps muscles. Insult to the fully formed femoral nerve, which completely blocks conduction in motor axons, should theoretically abolish all MEP responses to the quadriceps muscles. On multiple occasions over the past year, our neuro-monitoring groups have observed significantly degraded amplitudes of the femoral motor and/or sensory evoked potentials limited to only the surgical side. Most of these degraded response amplitudes rapidly returned to baseline values with a surgical intervention (i.e., prompt removal of surgical retraction).

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

“…We would not expect to see any changes in the motor evoked potential amplitudes in the sciatic innervated muscles; tibialis anterior, gastrocnemius and abductor hallucis. We believe this publication is important as the case studies confirm the utility of motor evoked potentials for femoral nerve protection in LLIF procedures that have been described by other authors [5][6][7]19]. …”

“…We have read with great interest and enthusiasm the published article by Chaudhary et al entitled ''Trans-cranial motor evoked potential detection of femoral nerve injury in transpsoas lateral lumbar interbody fusion'', as this has been our research focus for the past 5 years and we have subsequently published two articles and multiple abstracts since 2012 on neuromonitoring for femoral nerve protection in lateral lumbar interbody fusion procedures (LLIF) [1][2][3][4][5][6][7]. Chaudhary's publication is of great importance as femoral nerve injuries are the most feared neurological complication during LLIFs and the result can be devastating for the patient.…”

“…Literature has suggested that the complete loss of the motor evoked responses from the quadriceps is most likely correlated to a conduction block of the more distal lumbar plexus elements including the fully formed femoral nerve [5,6].…”

Neurological complications in adult spinal deformity surgery

“…Spontaneous and triggered EMG is sensitive for direct nerve injury, but is not reliable for indirect nerve injuries from prolonged compression of the nerves between the retractor blades and the transverse processes. MEPS, however, have recently shown promise [82,83] and can be used with spontaneous and triggered EMG. Triggered EMG is recommended for transpsoas dissection and placement of retractor blades whereas MEPs can detect indirect nerve injury.…”

“…Triggered EMG is recommended for transpsoas dissection and placement of retractor blades whereas MEPs can detect indirect nerve injury. However, the interpretation of MEPs is affected by radicular overlap (overlapping muscle innervation from adjacent roots) [82], limited sampling of motor neurons (MEPs measure about 4-5 % of motor axons in a muscle) [84], and variable excitability of neurons, anesthetics, and hypotension [85].…”