The Effects From Lumbar Nerve Root Transection in Rats on Spinal Somatosensory and Motor-Evoked Potentials

STUDY DESIGN: Spinal somatosensory-evoked potentials (SSEPs), elicited by mixed nerve and dermatomal stimulation, and compound evoked muscle potentials (CMAPs), elicited by lower thoracic interspinous space stimulation, were recorded in rats that underwent single nerve root transection. OBJECTIVES: To investigate and compare the effects of single nerve root transection using various electrophysiological models and to define a monitoring strategy that can easily and accurately predict compromise caused by a single lumbosacral nerve root injury. SUMMARY OF BACKGROUND DATA: Neuromonitoring is an important and effective preventive measure against neurologic complications during spinal surgery, but monitoring the lumbosacral nerve roots is not well established. METHODS: Rats received (Group I) a sham operation or had the (Group II) left L4, (Group III) L5, or (Group IV) L6 nerve root transected. SSEP were recorded at the thoracolumbar junction following stimulation of the sciatic nerve (M-SSEP) and the L5 dermatome (D-SSEP). CMAP was recorded at the intrinsic muscles of the foot by electrical stimulation in the lower thoracic spinal cord. Potentials recorded before and after a single nerve root transection were compared and correlated with clinical status by walking-track analysis 1 week later. RESULTS: Relative amplitudes in Group II were 54.1% (M-SSEP), 84.6% (D-SSEP), and 85.5% (CMAP); 25.2% (M-SSEP), 66.5% (D-SSEP), and 85.8% (CMAP) in Group III; and 66.5% (M-SSEP), 95.5% (D-SSEP), and 23.2% (CMAP) in Group IV. M-SSEP is sensitive but not specific to single nerve root injury. D-SSEP and CMAP are less sensitive but more specific. CONCLUSIONS: Injury to a single lumbosacral nerve root is diagnosed more easily with M-SSEP. With D-SSEP and CMAP, it was possible to differentiate the lesioned nerve root by stimulating the primary dermatome or recording from the innervated muscle. M-SSEP is an easy-sampling and appropriate tool for screening nerve root injury; its poor specificity may be overcome by using D-SSEP and CMAP in addition. Optimal monitoring of the lumbosacral nerve roots during lumbar spinal surgery requires the administration of M-SSEP, D-SSEP, and CMAP to provide independent verification of lumbosacral nerve root integrity and to allow detection of the occasional injuries that selectively affect either the sensory or motor system.