• R V Nielsen, J S Fomsgaard, H Siegel, R Martusevicius, O Mathiesen, and J B Dahl.
• Department of Neuroanaesthesiology, Rigshospitalet - Glostrup, Copenhagen University Hospital, Glostrup, Denmark.
• Acta Anaesthesiol Scand. 2016 Sep 1; 60 (8): 1152-60.

BackgroundChlorzoxazone is a muscle relaxant administered for musculoskeletal pain, and as an analgesic adjunct for post-operative pain. Chlorzoxazone for low back pain is currently not advised due to the lack of placebo-controlled trials. We explored the effect of chlorzoxazone on acute pain after spine surgery.MethodsOne hundred and ten patients were randomly assigned to 500 mg oral chlorzoxazone or placebo in this blinded study of patients having spine surgery under general anaesthesia. In the 4 h trial period analgesia consisted of IV patient-controlled analgesia (morphine bolus 2.5 mg). Primary outcome was pain during mobilization (visual analogue scale) 2 h after the intervention. Secondary outcomes were pain at rest, opioid consumption, nausea, vomiting, sedation and dizziness.ResultsFor pain during mobilization 2 h after intervention, there was no significant difference between groups: 51 (21) vs. 54 (25) mm in the chlorzoxazone and placebo groups, respectively, mean difference 3 mm (95% CI -8 to 10), P = 0.59. For pain during mobilization and at rest (wAUC 1-4 h), there were no significant differences between groups. There was no significant difference in total IV morphine use 0-4 h: median 10 (7-21) vs. 13 (5-19) mg in the chlorzoxazone and placebo groups, respectively, P = 0.82. We found no significant difference in adverse effects.ConclusionNo analgesic effect of single-dose chlorzoxazone was demonstrated in patients with acute pain after spine surgery. Based on these findings, chlorzoxazone cannot be recommended for immediate treatment of acute pain after such procedures.© 2016 The Acta Anaesthesiologica Scandinavica Foundation. Published by John Wiley & Sons Ltd.

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