A Long-term Analysis of the Spinal Somatosensory Evoked Potential of Rats with Sciatic Constriction Injury

Background : This study was undertaken to evaluate the long-term effect of a peripheral neuropathic pain model, chronic constriction injury (CCI) of the sciatic nerve, through analyses of electrophysiologic change, sciatic nerve function, and pain behavior. Methods : CCI of the sciatic nerve was induced in twelve rats as described byBennett and Xie. Three parameters were monitored: spinal somatosensory evoked potential (SSEP) elicited by supramaximal stimulation of the posterior tibial nerve and recorded from the thoracolumbar junctional space; thermal hyperalgesia assessed by measuring paw withdrawal latency (PWL); and sciatic function index (SFI). All the values of these parameters were obtained before the CCI procedure (day 1 as a preoperative baseline, and again on days 5, 12, 19, 26, 33, 47, 61, 75, and 89). SSEP was alsomeasured 3 hr after the CCI operation. Datawere also obtained from contralateral limbs. Results : All rats with CCI developed thermal hyperalgesia on day 5, as indicated by a significant reduction in PWLin the CCI limbs and a deterioration of the SFI compared with baseline values. These effects persisted to day 89. In the electrophysiologic study, 3 hrs after the CCI operation, the amplitude significantly decreased and latency significantly increased in all SSEP recordings. The changes persisted and showed no further statistically significant deterioration or recovery. The data demonstrated that the major electrophysiologic change after a constriction injury was the loss of conduction ability across the injury site in the fast-conducting fibers, but in the slow-conducting fibers the conduction ability was still preserved, which occurred immediately after the operation and persisted consistently for 89 days in the rats with behavior manifestations of neuropathic pain. Conclusions : Our findings suggest that SSEP is simple to obtain and sensitive in the acute phase of electrophysiological changes, but is limited for long-term evaluation after CCI.