The analgesic effect of intravenous ketamine and lidocaine on pain after spinal cord injury

Background: Pain following spinal cord injury (SCI) is a therapeutic challenge. Only a few treatments have been assessed in randomized, controlled trials. The primary objective of the present study was to examine the analgesic effect of ketamine and lidocaine in a group of patients with neuropathic pain below the level of spinal cord injury. We also wanted to assess sensory abnormalities to see if this could help us to identify responders and if treatments resulted in changes of sensibility. Methods: Ten patients with spinal cord injury and neuropathic pain below the level of injury were included. The analgesic effect of ketamine 0.4 mg kg(-1) and lidocaine 2.5 mg kg(-1) was investigated. Saline was used as placebo. The drugs were infused over 40 min. A randomized, double-blind, three-period, three-treatment, cross-over design was used. Systemic plasma concentrations of ketamine and lidocaine were assessed. Pain rating was performed using a visual analogue scale (VAS). Sensory function was assessed with a combination of traditional sensory tests and quantitative measurement of temperature thresholds. Results: Response to treatment, defined as 50% reduction in VAS-score during infusion, was recorded in 5/10 in the ketamine, 1/10 in the lidocaine and 0/10 in the placebo groups. Neither ketamine nor lidocaine changed temperature thresholds or assessments of mechanical; dynamic and static sensibility. Nor could these sensory assessments predict response to treatment in this setting. Lidocaine and particularly ketamine were associated with frequent side-effects. Conclusion: Ketamine but not lidocaine showed a significant analgesic effect in patients with neuropathic pain after spinal cord injury. The pain relief was not associated with altered temperature thresholds or other changes of sensory function.