Basic Science (33)

Abstract

Perfusion of the mechanically compressed lumbar ganglion with lidocaine reduces mechanical hyperalgesia and allodynia in the rat. (University of Arkansas for Medical Sciences, Little Rock, AR) J Neurophysiol 2000;84:798–805.This study used an animal model of lumbar radiculopathy to investigate the neurological mechanisms of cutaneous hyperalgesia and tactile allodynia. The rat L5 dorsal root ganglion (DRG) was chronically compressed by inserting a hollow perforated rod into the intervertebral foramen. The DRG was constantly perfused through the hollow rod with either lidocaine or normal saline delivered by a subcutaneous osmotic pump. Behavioral evidence for neuropathic pain after DRG compression involved measuring the incidence of hindlimb withdrawals to both punctate indentations of the hind paw with mechanical probes exerting different bending forces and to light stroking of the hind paw with a cotton wisp. Behavioral results showed that for saline‐treated control rats: the withdrawal thresholds for the ipsilateral and contralateral paws to mechanical stimuli decreased significantly after surgery and the incidence of foot withdrawal to light stroking significantly increased on both ipsilateral and contralateral hind paws. Local perfusion of the compressed DRG with 2% lidocaine for 7 days at a low flow‐rate (1 μl/h), or for 1 day at a high flow‐rate (8 μl/h) partially reduced the decrease in the withdrawal thresholds on the ipsilateral foot, but did not affect the contralateral foot. The incidence of foot withdrawal in response to light stroking with a cotton wisp decreased significantly on the ipsilateral foot and was completely abolished on the contralateral foot in the lidocaine treatment groups. Conclude that compression of the L5 DRG induced central pain syndrome that included bilateral mechanical hyperalgesia and tactile allodynia. Results also suggest that a lidocaine block, or a reduction in abnormal activity from the compressed ganglia to the spinal cord, could partially reduce mechanical hyperalgesia and tactile allodynia. Comments by Marshall Devor, PhD.Tonic compression of the dorsal root ganglion (DRG) in animal preparations, and presumably also in man, causes sensory cells to become electrically hyperexcitable. The resulting spontaneous discharge is a cause of ongoing paresthesias and pain. Many believe that in addition, this ectopic activity can trigger and maintain central sensitization in the spinal cord, resulting in tactile hypersensitivity (allodynia and hyperalgesia) in the body parts innervated by the ganglion. This second effect of ectopic DRG activity, however, is controversial. Zhang and collaborators now provide strong new support for this idea by showing that lidocaine infusion into the DRG, with consequent block of the ectopic DRG firing, considerably reduces allodynia and hyperalgesia for the duration of the infusion (weeks). Here is a novel therapeutic approach that deserves a try.