The muscle relaxing properties of “muscle relaxants” arise not from direct activity at the muscular or neuromuscular junction level but rather from an inhibition of more central polysynaptic neuronal (nerve cells that end in synapses) events. These agents have also been shown in some studies to demonstrate superior analgesia to either acetaminophen or aspirin, and it remains uncertain if muscle spasm is a prerequisite to their effectiveness as analgesics.
Range of Motion
Muscle relaxants are often prescribed in the treatment of acute low back pain in an attempt to improve the initial limitations in range of motion from muscle spasm and to interrupt the pain-spasm-pain cycle. Limiting muscle spasm and improving range of motion will prepare the patient for therapeutic exercise.
Types of Muscle Relaxants
In an attempt to determine the mechanism of action of carisoprodol (Soma®) in the treatment of low back pain, a double blind study was carried out comparing its effectiveness to that of a sedative control, butabarbital (a sedative), and a placebo in the treatment of 48 laborers with acute lumbar pain. Carisoprodol was found to be significantly more effective in providing both subjective pain relief and objective improvements in range of motion when evaluated by finger to floor testing. The results of this study suggest that the effects of carisoprodol are not secondary to its sedative effects alone.
In 1989, Basmajian compared the effectiveness of cyclobenzaprine (Flexeril®) alone with diflunisal (Dolobid®), placebo, and a combination of cyclobenzaprine and diflunisal in the treatment of acute low back pain and spasm. During the ten-day study period, the combined treatment group demonstrated significantly superior improvements in global ratings on day four, but not on day two or seven. This study suggested some effectiveness of combined analgesic and muscle relaxant therapy when utilized early in the initial week of pain onset.
Borenstein compared the effects of combined cyclobenzaprine and naproxen (Naprosyn®) with naproxen alone and also found combination therapy to be superior in reducing tenderness, spasm, and range of motion in patients presenting with ten days or less of low back pain and spasm. Adverse effects, predominantly drowsiness, were noted in 12 of 20 in the combined group and only four of 20 treated with naproxen alone.
Cyclobenzaprine and carisoprodol were compared in the treatment of patients with acute thoracolumbar pain and spasm rated moderate to severe and of no longer than seven days duration. Both drugs were found to be effective, without significant differences between the treatment groups. Significant improvements were noted in physician rated mobility and in patients’ visual analogue scores on follow up days four and eight. While 60% of patients experienced adverse effects in the form of drowsiness or fatigue, these differences were not significantly different between groups, and only eight percent of patients from each group discontinued treatment.
Baratta found cyclobenzaprine, 10-mg t.i.d. (three times per day), superior to placebo in a randomized, double blind study of 120 patients with acute low back pain presenting within five days of symptom onset. Significant improvement was noted in range of motion, tenderness to palpation, and pain scores on follow up days two through nine. Sixty percent of treatment group patients reported drowsiness or dizziness compared with 25% of those in the placebo group.
In an earlier study, diazepam (Valium®) was found to offer no significant subjective or objective benefit, when compared to placebo, in patients treated for low back pain. Carisoprodol was found to be superior to diazepam in the treatment of patients with “at least moderately severe” low back pain and spasm of no longer than seven days duration. In this study, the overall incidence of adverse reactions was higher in the diazepam treated group but was not of statistical significance.
Origin of Muscle Spasm
Muscle spasm of local origin needs to be clinically differentiated from spasticity and sustained muscle contraction in the setting of the central nervous system (CNS) and upper motor neuron injury. Baclofen (Lioresal®) and dantrolene sodium (Dantrium®) are two agents whose use is indicated in the setting of spasticity of CNS etiology. Dantrolene sodium is of particular interest, as its mechanism of action is purely at the muscular level where it serves to inhibit the release of calcium form the sarcoplasmic reticulum.
Casale studied the effectiveness of dantrolene sodium, 25-mg daily, in the treatment of low back pain and found patients to demonstrate significant improvements in visual analogue scores, pain behavior, and electromyographic (EMG) evaluations of “antalgic reflex motor unit firing,” when compared with the placebo group. The findings of this study are interesting in that they demonstrate improvement secondary to a pure muscle relaxant, which does not possess other outside anti-nociceptive properties.
Baclofen is a derivative of gamma-aminobutryic acid (GABA) and is believed to inhibit mono and polysynaptic reflexes at the spinal level. Treatment with baclofen was compared to placebo in a double blind, randomized study of 200 patients with acute low back pain. Patients with initially severe discomfort were found to benefit from baclofen, 30- to 80-mg daily, on days four and ten of follow up. Forty-nine percent of treatment patients complained of sleepiness, 38% of nausea, and 17% discontinued treatment.
Sedation: Side Effect
Sedation is the most commonly reported adverse effect of muscle relaxant medications. These drugs should be used with caution in patients driving motor vehicles or operating heavy machinery. More absolute contraindications do exist to the use of carisoprodol, cyclobenzaprine, and diazepam. Rare idiosyncratic reactions have also been reported to carisoprodol and its metabolites such as meprobamate. Benzodiazepines have potential for abuse and their use should be avoided. By initially prescribing muscle relaxants at bedtime, the physician might take advantage of their sedative effects and minimize daytime drowsiness.
These agents have been found to be effective when used either alone or in combination with an analgesic/anti-inflammatory agent within seven days of symptom onset. The prescribing physician should monitor patients receiving these medications and tailor dosages in an attempt to minimize the drowsiness and sedation often associated with their use. The use of benzodiazepines does not appear to offer any significant benefit to patients experiencing acute low back pain. Further research is needed before the role of baclofen and dantrolene sodium in the treatment of muscle spasm of local origin can be more clearly defined.
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