The Use of Muscle Relaxants in the Treatment of Non-Specific Low Back Pain

The Use of Muscle Relaxants in the Treatment of Non-Specific Low Back Pain
Reid Larson, PharmD, Welia Health

Background: How many people do you know with low back pain? Too many to count? If that is the case it is no surprise low back pain has been the leading cause of disability worldwide for the past 30 years according to the Global Burden of Disease study in 2017. First-line therapies include a range of non-pharmacological approaches, often accompanied with the use of non-steroidal anti-inflammatory drugs (NSAIDs), acetaminophen, muscle relaxants (MRs), and corticosteroids. Prescription MRs are the third most commonly prescribed class of medications for the treatment of low back pain and can be classified as non-benzodiazepine antispasmodics (carisoprodol, cyclobenzaprine, methocarbamol, tizanidine, etc.), antispastics (baclofen, dantrolene), and benzodiazepines (diazepam, clonazepam, etc.). Due to variable results in clinical trials, conflicting recommendations for the use of these medications exist between international clinical practice guidelines. For example, the American College of Physicians (ACP) guideline recommends non-benzodiazepine antispasmodics as the drug of choice for acute low back pain, the National Institute for Health and Care Excellence guideline does not make a recommendation on the use of MRs, and the Belgian Health Care Knowledge Centre discourages use of MRs entirely. A recent review by Cashin et al. summarized the available evidence on safety and efficacy of MRs to help guide healthcare providers in the treatment of low back pain with evidence-based pharmacotherapy.

Evidence: In an effort to quell this controversy, Cashin et al. recently published a systematic review consisting of 49 randomised controlled trials that evaluated the efficacy and safety of MRs versus placebo, usual care, or no treatment in adults (≥18 years) for the treatment of non-specific low back pain. The trials included were assessed for the risk of bias and certainty of the evidence, and data was extracted to produce outcomes of pain intensity, disability, acceptability, and safety. Of the 41 clinical trials, 31 were selected to be further analyzed via a meta-analysis. A total of 6,505 participants comprised the sample population included in the meta-analysis.

Acute, sub-acute, and chronic pain were defined as non-specific low back pain lasting 0 to 2 weeks, 2 to 6 weeks, and ≥12 weeks, respectively. The authors found, in the case of acute low back pain, that non-benzodiazepine antispasmodics were associated with a reduction in pain intensity compared with control (mean difference −7.7 [95% CI −12.1 - −3.3]). However, they did not find a reduction in disability (mean difference −3.3 [95% CI −7.3 - 0.7]) and the evidence suggested an increase in the risk of an adverse event (RR 1.6 [95% CI 1.2 - 2.0]). Interestingly, benzodiazepines were found to have a statistically significant reduction in disability in patients with sub-acute pain (mean difference −6.9 [95% CI −12.1 - −1.7]), but showed no reduction in disability in patients with chronic low back pain. Antispastic drugs were also associated with an increase in the risk of an adverse event versus control (mean difference 2.0 [CI 95% 1.1 - 3.8]) which were more likely to cause discontinuation of treatment (mean difference 34.6 [95% CI 2.1 - 568.0]).

Discussion & Clinical Impact: In the treatment of low back pain, this study has aided in summarizing the variable efficacy and safety data previously reported. Overall, non-benzodiazepine antispasmodic drugs appear to be more effective compared to benzodiazepines or antispasmodics in reducing acute low back pain. However, there are a number of limitations that may push clinicians to avoid their use in this setting. First, the mean differences reported were based on a 0 - 100 point scale, meaning even those effects that were statistically significant were relatively insignificant (i.e. less than 8 points on a 0 - 100 point scale) and may not be clinically meaningful. Second, the meta-analysis included three studies that evaluated the use of thiocolchicoside, a non-benzodiazepine antispasmodic not approved by the Food & Drug Administration due to the possibility of severe adverse effects. The addition of these studies may have skewed the overall results of the meta-analysis towards showing benefit of MRs. Furthermore, the risks associated with MRs may outweigh their benefits, especially in older adult populations. These risks include side effects of drowsiness, dizziness, agitation, irritability, headache, increased fall risk, and possible dependence or misuse.

Non-specific low back pain typically improves over time regardless of treatment modality. Therefore, per the ACP clinical guidelines on low back pain, first-line treatments should focus on non-pharmacological options such as physical therapy, superficial heat, massage, or acupuncture. If pharmacologic treatment is warranted beyond NSAIDs or acetaminophen, MRs may be an option in the acute setting only. Deprescribing practices should be considered in those initiated on MR therapy for chronic low back pain. Cashin et al. wrote the following conclusion to help guide clinical practice: “Although non-benzodiazepine antispasmodics might reduce pain intensity at two weeks or less for acute low back pain, the effect is unlikely to be considered clinically important.”



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