Muscle injections produce clinically significant analgesic effects in patients with fibromyalgia syndrome

Patients with musculoskeletal pain syndrome including fibromyalgia (FM) complain of chronic pain from deep tissues including muscles. Previous research supports the role of impulse input from deep tissues for FM clinical pain. We hypothesized that blocking abnormal impulse input with intramuscular lidocaine would reduce decrease primary and secondary hyperalgesia and FM patients’ clinical pain. We enrolled 62 female FM patients into a double-blind controlled trial of 3 groups who received either 100 mg or 200 mg lidocaine or saline placebo injections into both shoulders and buttocks. Study variables included clinical pain as well as pressure and heat hyperalgesia. A natural history condition was included to estimate the effect size of saline placebo injections. Clinical FM pain significantly declined by 34% after either lidocaine or saline injections and secondary heat hyperalgesia at the arms and legs was also unspecifically reduced after muscle injections. In contrast to these unspecific effects, primary and secondary mechanical hyperalgesia decreased significantly more after lidocaine than placebo injections. Patient expectations of pain relief accounted for 15 % of the analgesic effect of muscle injections. Thus muscle injections can provide clinically relevant reductions of overall clinical FM pain. These effects appear to be mediated by a combination of specific and unspecific factors including impulse blockade from muscles and patient expectations. In addition, needling of deep tissues appears to be analgesic which may be related to activation of endogenous pain modulatory mechanisms. Specific effects of lidocaine on mechanical hyperalgesia emphasize the role of peripheral impulse input for FM pain processing abnormalities.