Characterization of pain and pharmacologic responses in an animal model of lumbar adhesive arachnoiditis

Study Design. The study used a small animal laminectomy model with kaolin to investigate the relationship of cauda equina pathology to pain, and the effects of systemic and intrathecal drugs to reduce pain. Objective. To determine if higher amounts of cauda equina adhesions produce increased pain-related behavior following laminectomy and epidural kaolin application. Summary of Background Data. Clinically, the relationship of cauda equina pathology to pain is controversial. An established animal model produces cauda equina adhesions. The correlation of this anatomic pathology with pain-related behavior has not been examined. Methods. A laminectomy was performed in Sprague-Dawley rats at the L5-L6 vertebrae, and kaolin was applied extradurally. Sham control animals had vertebrae exposed but no laminectomy/kaolin. Animals were monitored for pain-related behavior using vocalization in response to straight leg raising. Histology was performed at 6 weeks postoperatively. Laminectomy animals were tested with systemic or intrathecal drugs to reduce straight leg raising vocalizations. Results. In laminectomy animals, the pain response developed over 6 weeks, with vocalizations increasing from 0 to 1.65, out of 5 leg lifts. Cauda equina clumping was related to pain severity. Systemic morphine 3 mg/kg, but not 1 mg/kg, decreased vocalization to straight leg raising. Intrathecal morphine at 30 nmol, but not 10 nmol, gabapentin at 60 nmol, but not 20 nmol, decreased vocalizations. Intrathecal clonidine up to 90 nmol did not reduce vocalization. Conclusions. An animal model of adhesive lumbar arachnoiditis yields a quantifiable pain-related response that can be used to evaluate the effects of various analgesic interventions.