A systematic review and meta-analysis found little efficacy and safety evidence supporting the use of muscle relaxants for low back pain. These findings were published in BMJ.
Investigators from Neuroscience Research Australia searched publication databases (Medline, Embase, CINAHL, CENTRAL, ClinicalTrials.gov, clinicaltrialsregister.eu, and WHO ICTRP) through February 2021 for randomized clinical trials of muscle relaxants for adults reporting nonspecific low back pain. A total of 49 trials were included in their review, and 31 of these trials — comprising 6505 study participants — were quantitatively analyzed.
The studies were published in peer review journals (n=35), in trial registry records (n=12), and conference abstract books (n=2). Muscle relaxants were administered orally (n=35), intramuscularly (n=10), intravenously (n=1), or in as a mixed modality (n=3). Most studies (n=32) used a placebo as the comparator.
Patients were recruited for acute low back pain (n=35), chronic low back pain (n=8), subacute low back pain (n=2), a mixture of back pain duration (n=2), or unspecified duration of low back pain (n=2).
For acute low back pain, nonbenzodiazepine antispasmodics reduced pain at ≤2 weeks compared with control (mean difference, -7.7; 95% CI, -12.1 to -3.3) but not at 3-13 weeks (mean difference, 0.6; 95% CI, -4.5 to 5.7). There was no evidence for improved disability at ≤2 weeks (mean difference, -3.3; 95% CI, -7.3 to 0.7) or 3-13 weeks (mean difference, 4.3; 95% CI, -1.4 to 10.1). No difference from controls was observed for acceptability (relative risk [RR], 0.8; 95% CI, 0.6-1.1).
At 3-13 weeks, benzodiazepine was associated with reduced disability among patients with acute low back pain (mean difference, -6.9; 95% CI, -12.1 to -1.7). This data was sourced from a single study.
Among patients with chronic low back pain, eszoplicone was associated with reduced pain intensity at 3-13 weeks (mean difference, -19.9; 95% CI, -31.5 to -8.3) according to a single study.
Regarding subacute low back pain, a trial of botulinum toxin found no reduction in pain intensity compared with control (mean difference, -19.0; 95% CI, -41.9 to 3.9; very low certainty evidence).
The 2 trials of mixed back pain found evidence of reduced pain intensity during the first 2 weeks of nonbenzodiazepine antispasmodic use (mean difference, -4.4; 95% CI, -6.9 to -1.9) but this difference waned during weeks 3-13 (mean difference, -5.8; 95% CI, -13.8 to 2.2). Nonbenzodiazepine antispasmodics were associated with reduced disability at ≤2 weeks compared with control (mean difference, -19.2; 95% CI, -27.7 to -10.7).
Nonbenzodiazepine antispasmodics increased the risk of an adverse event, compared with control (RR, 1.6; 95% CI, 1.2-2.0), as did antispastics (RR, 2.0; 95% CI, 1.1-3.8). Antispastics were also associated with increased risk for discontinuation due to an adverse event (RR, 34.6; 95% CI, 2.1-568.0).
Most trials (n=28) were at high risk for bias. Common reasons for bias included inadequate reporting of allocation concealment (n=33), random sequence generation (n=25), inadequate blinding (n=19), and failure to report intention-to-treat effects (n=8).
The study authors concluded that there was a paucity of clear evidence supporting the use of muscle relaxants for low back pain.
“We would encourage clinicians to discuss this uncertainty in the efficacy and safety of muscle relaxants with patients, sharing information about the possibility for a worthwhile benefit in pain reduction but increased risk of experiencing a non-serious adverse event, to allow them to make informed treatment decisions,” they advised.
Cashin AG, Folly T, Bagg MK, et al. Efficacy, acceptability, and safety of muscle relaxants for adults with non-specific low back pain: systematic review and meta-analysis. BMJ. Published online July 8, 2021. doi:10.1136/bmj.n1446