Are muscle relaxants effective against low back pain?

The clinical efficacy and safety of muscle relaxants remain indefinite, with very low certainty evidence showing that nonbenzodiazepine antispasmodic agents for the treatment of acute low back pain may provide small but clinically nonsignificant improvement in pain intensity at 2 weeks or less, according to a recent study.

Additionally, the risk of adverse events (AEs) but not serious AEs appears to increase with the use of nonbenzodiazepine antispasmodics, but evidence for this ranges from low to very low certainty.

“Although the observed effect of nonbenzodiazepine antispasmodics in reducing pain compared with control at 2 weeks or less was statistically significant, the magnitude of the effect was too small to be considered clinically important,” the researchers said.

In this systematic review and meta-analysis, the databases of Medline, Embase, Cinahl, Central, ClinicalTrials.gov, clinicaltrialregister.eu, and WHO ICTRP were searched for randomized controlled trials comparing muscle relaxants with placebo, usual care, waiting list, or no treatment in adults with nonspecific low back pain from inception to 21 February 2021.

Two independent reviewers identified the studies, extracted data, and assessed the risk of bias and certainty of evidence using the Cochrane risk-of-bias tool and Grading of Recommendations, Assessment, Development and Evaluations, respectively. Random effects meta-analytical models were used to estimate pooled effects and corresponding 95 percent confidence intervals (CIs).

Forty-nine trials met the eligibility criteria, of which 31 (n=6,505 participants) were quantitatively analysed. [BMJ 2021;374:n1446]

Very low certainty evidence indicated that nonbenzodiazepine antispasmodics for acute low back pain at 2 weeks or less correlated with lower pain intensity vs control (mean difference [MD], –7.7, 95 percent CI, –12.1 to –3.3) but not with reduced disability (MD, –3.3, 95 percent CI, –7.3 to 0.7).

Furthermore, low and very low certainty evidence showed that nonbenzodiazepine antispasmodics could elevate the risk of an AE (relative risk [RR], 1.6, 95 percent CI, 1.2–2.0) and might have little to no effect on acceptability (RR, 0.8, 95 percent CI, 0.6–1.1) compared with control for acute low back pain, respectively.

“The number of trials investigating other muscle relaxants and different durations of low back pain were small and the certainty of evidence was reduced because most trials were at high risk of bias,” the researchers said.

International clinical practice guidelines are conflicted as regards recommending the use of muscle relaxants: of the 15 clinical practice guidelines, six recommended muscle relaxants to manage low back pain, five do not recommend them, and four do not offer any recommendation. [Expert Rev Clin Pharmacol 2019;12:145-157; Eur Spine J 2018;27:2791-2803]

“Large, definitive, placebo-controlled trials are urgently needed to evaluate the efficacy and safety of muscle relaxants,” the researchers said.

“New trials should follow the core outcome set for nonspecific low back pain and the recommendations of the Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials, to improve the pooling of results and comparability between trials,” they added. [Eur Spine J 2015;24:1127-1142; Pain 2005;113:9-19]