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Muscle Relaxants 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.

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.