Introduction

Because 90% of adults experience back pain at some point in their lives, and because some 50 billion dollars are spent dealing with it in the US economy alone, much medical research throughout the world has sought to ameliorate this condition. Non-specific treatment is usually effective, as 70-90% of acute low back pain (LBP) resolves within 5 weeks; in such cases, the etiology is either a mechanical one, or unknown. Currently, pharmacologic treatment of chronic LBP consists of nonsteroidal anti-inflammatory drugs, antispasmodics, antidepressants, muscle relaxants, and opioids.

The intramuscular injection of a solution of reconstituted freeze-dried botulinum toxin A (Botox) is a safe procedure, and relieves the pain of several movement disorders, chronic myofascial pain, and cervical pain syndromes.

In the paper summarized in this report, the authors investigated the efficacy of paravertebral muscle injections of Botox unilaterally, on the more painful side, in patients with chronic LBP.

Method

The study, which was randomized and double-blind, entered 31 patients, of whom 15 received Botox and 16 received normal saline as placebo.

When reconstituted, the botulinum toxin A had a concentration of 100 units/mL; it was colorless and caused no pain when injected intramuscularly, like saline. Each of the five unilaterally located lumbar muscle sites received 40 units of solution, given through a 27-gauge needle using a 1 mL-tuberculin syringe. Patients were injected only once on the side of pain or pain predominance.

While most patients had no clear cause for their chronic LBP, 6 had a history of disc disease and 4 had a history of remote low back trauma. MRI showed no acute pathology in any patient.

Patients with LBP of less than 6 month's duration, those under age 18, those with systemic inflammatory disease, acute pathology on MRI, known allergy to Botox, current or planned pregnancy, disorders of neuromuscular transmission, injections of anesthetics or steroids to the lumbosacral spine within the prior 12 months, and those involved in litigation or seeking disability were excluded from the study. If the patients were taking medications, they were advised to continue taking them.

The health professional rating the pain levels used two methods: 1) a visual analogue scale (VAS), where the patient marked his or her assessment of the pain along a horizontal 10 cm. line, from a stated rating of "no LBP" at the extreme left to "worst LBP" at the extreme right of the line; and 2) the Oswestry Low Back Pain Questionnaire (OLBPQ) that requested information about functional ability regarding tasks of daily living (pain, personal care, lifting, walking, sitting, standing, sleeping, sex, social life, and traveling).
Both methods have been found to be valid, internally consistent, and reliable measures of pain and disability in patients with LBP.

The VAS was used before injection, and at 3 and 8 weeks after. The OLBPQ was used before and 8 weeks after the injection. Patients could report side effects at anytime.

For the VAS, the clinical response was considered significant when the difference between pre-and post-treatment scores was at least 50%. For the OLBPQ, a significant functional improvement was considered when at least a two-grade improvement over the base-line value occurred in one or more functional subsets.

Results

The group had a mean age of 46 years (20-73) and a mean duration of pain of 6 years (6 months - 30 years), a mean pretreatment pain on the VAS of 7.5 in the Botox group and 7.0 in the saline group. The pretreatment OLBPQ showed functional impairments in 1 to 7 subsets with no significant difference between the two groups. Three patients who failed to report for follow-up were excluded from the analysis, leaving 14 subjects in each treatment group who completed the study.

At 3 weeks, 86% (13 of 15 patients) in the Botox group and 31% (5 of 15 patients) in the saline group reported some degree of pain relief. The changes exceeded 50% (VAS score) in 73% (11 of 15) in the Botox group compared with 25% (4 of 16) in the saline group (p=0.012).

At 8 weeks, 60% (9 of 15) in the Botox group and 12.5% (2 of 16) in the saline group reported pain relief exceeding 50% (VAS score) (p=0.009). The OLBPQ rating revealed improvement in 67% (10 of 15) in the Botox group and 19% (3 of 14) in the saline group (p=0.011).

At 6 months after the treatment, only 10 patients in the Botox group could be reached located. Six of them reported cessation of the analgesic effect occurring 3-4 months post-injection.

No patients had a worsening of pain or function after Botox injections, while two patients reported worsening after saline injections. The injections themselves caused no side effects and were well tolerated by all participants.

Comment

Although these results were very encouraging, the authors of the study warn that they should be interpreted cautiously because the number of patients studied was small. From a practical viewpoint, it would be important to find out whether positive effects would continue to be achieved with repeated injections, such as has been reported for the use of Botox in certain dystonias and spasticity.

The mechanism of action of Botox in providing benefit in these patients is intriguing. The effectiveness of some treatments for LBP (antispasmodics, muscle relaxants) suggests that increased paraspinal muscle activity is an important factor; persistent muscle tension may cause the build-up of painful metabolites. The increased muscle activity can be demonstrated by electromyographic studies, and it would be instructive to see the effect of Botox using this technique.

The authors of the study offer several explanations of how botulinum toxin A might work at the neuromuscular level. One of these is that Botox metabolites might have an analgesic effect per se. Clearly further studies are needed.