Radicular derivation of sensory action potentials in lower extremity.

A patient with grade II spondylolisthesis and back pain underwent Harrington rod instrumentation. Postoperatively, he developed pain and numbness of the lateral aspect of the left foot and loss of the left AChilles reflex. Preoperative electrodiagnostic findings of the left lower extremity were normal. Two postoperative studies revealed that, concomitant with evidence of denervation if the left S1-supplied muscles, his sural sensory nerve action potential (SNAP) had been abolished but the superficial peroneal SNAP, recorded from the intermediate branch, was normal and equal to that of the contralateral foot. Though various dermatomal maps show this branch to be in the border zone between S1 and L5 dermatomes, the findings in this patient are evidence that the SNAP of the superficial peroneal nerve, recorded as described, is subserved by fibers in the L5 root. In lesions producing axonal loss in L5 root distribution, the amplitude of the superficial peroneal SNAP will be of value in distinguishing lesions at or distal to the dorsal root ganglion from those proximal to it.