Loss of Lower Extremity Somatosensory Evoked Potentials During Lumbar Laminectomy and Instrumented Fusion: A Case ReportGerald A. McNamee, R.EPT, CNIM, FACSNM

LOSS OF LOWER EXTREMITY SSEP DURING LUMBAR LAMINECTOMY

ABSTRACT

The utilization of lower extremity somatosensory evoked potentials (SSEP) for

monitoring of lumbar surgical procedures remains controversial in the surgical neurophysiology

community. The fact that multiple lumbar spinal nerve roots contribute to the SSEP prior to its

transition into the posterior columns of the spinal cord may lead to insensitivity in monitoring

these potentials. We present a case report of a patient undergoing a posterior approach, open,

lumbar laminectomy and instrumented fusion. This patient experienced an acute loss of both

cortical and subcortical posterior tibial nerve (PTN) SSEPs during lumbar laminectomy. Post-

decompression testing revealed a full amplitude recovery of the attenuated evoked potentials

forty minutes after loss of signal. An analysis of the case, literature review, and discussion on

the merits of SSEP monitoring in these below-the-conus medularis procedures, is presented.

Keywords: SSEP, Somatosensory Evoked Potential, Lumbar Spine Surgery, Surgical

Neurophysiology, IONM

LOSS OF LOWER EXTREMITY SSEP DURING LUMBAR LAMINECTOMY

CASE PRESENTATION

A 55 year old male with a past medical history of two level lumbar laminectomy and

lumbar stenosis presented for a L3/4 decompression and fusion with a revision lamino-

foraminotomies at the left L4/5 and bilateral L5/S1. The patient complained of unprovoked,

progressive, low back pain with radiation to bilateral lower extremities. The back pain is

described as aching and stabbing in nature. The patient has difficulty standing any significant

time or walking any significant distance. The patient presented conscious and fully oriented in

no acute distress. There were no gross trophic changes noted. Sensation was grossly intact to

bilateral lower extremity light touch. Strength testing revealed 5/5 (full strength) to bedside

examination from the (a) quadriceps, (b) tibialis anterior, (c) gastrocnemius, and (d) extensor

hallucis longus, bilaterally. There was no significant ankle clonus to acute dorsiflexion of either

foot.

Plain radiographs demonstrate a grade 1 spondylolisthesis at L3/4. Magnetic resonance

imaging (MRI) revealed L3/4 spinal stenosis, left L3/4 and bilateral L5/S1 intervertebral

foraminal narrowing. The conus medularis was noted at the L1 vertebral body level. The patient

had no other significant medical history or allergies to food/medicine. Preoperative screening

blood chemistry and lab results were unremarkable.

The anesthesia regimen for this case included volatile anesthetics, narcotics,

benzodiazepines, and neuromuscular blocking agents only to facilitate orotracheal intubation and

surgical exposure. The surgical neurophysiology montage included bilateral ulnar and posterior

tibial nerve SSEP and spontaneous electromyography (spEMG). EMG was collected from the

Vastus Lateralis, tibialis anterior and abductor hallucis which reflected innervation from the

L2-S1 spinal nerve roots, bilaterally. A Jackson frame was utilized for the procedure.

LOSS OF LOWER EXTREMITY SSEP DURING LUMBAR LAMINECTOMY

Post-prone positioning baselines showed symmetrical and monitorable waveform latencies and amplitudes with clearly defined and consistent cortical and subcortical depolarization waveform morphologies.

At 1015 a 3rd recording was run and all remained stable

LOSS OF LOWER EXTREMITY SSEP DURING LUMBAR LAMINECTOMY

At 1031 there is a slight shift in the p37 latencies of the PTSSEPs.

1031 LT Post Tibial SSEP Shift in the p37 with stable PF

LOSS OF LOWER EXTREMITY SSEP DURING LUMBAR LAMINECTOMY

At 1043 there is a loss of the Cortical PTSSEPs

1043 Loss of Left PTSSEP

LOSS OF LOWER EXTREMITY SSEP DURING LUMBAR LAMINECTOMY

At 1046 the loss was confirmed and surgeon notified.

At 1119 there continues to be a loss of the cortical PTSSEPs with preservation of the PF potential.

LOSS OF LOWER EXTREMITY SSEP DURING LUMBAR LAMINECTOMY

At 1123 surgeon tells tech decompression is done and PTSSEPs return bilaterally.

At 1202 Cortical PTSSEPs remained stable.

LOSS OF LOWER EXTREMITY SSEP DURING LUMBAR LAMINECTOMY

At 1221 they started closing and a last trace was run with everything stable.