Dextromethorphan, an over-the-counter medication that silences coughs, may help fibromyalgia patients quiet over-reacting nerves that amplify ordinary touches into agony.
A University of Florida study documents, for the first time, that dextromethorphan temporarily reduces the intensity of fibromyalgia "wind-up," a snowballing pain response to minor, repetitive physical contact. The discovery, described in the May issue of The Journal of Pain, also enables researchers to rule out one suspected cause of the phenomenon.
Fibromyalgia is an incurable illness that causes widespread muscle aches, stiffness, fatigue and sleep disturbances, according to the National Fibromyalgia Association. An estimated 10 million Americans suffer from the condition, most of them women. Current treatment strategies include pain medication, exercise, stretching, sleep management and psychological support.
Though the UF study did not establish guidelines for using dextromethorphan clinically, it suggests the drug may eventually be an option for treating fibromyalgia and other conditions involving heightened pain sensitivity, said rheumatology expert Roland Staud, M.D., a UF associate professor of medicine and the study's principal author.
"I think it's one piece of the mosaic," Staud said. "We currently have no single therapy in chronic pain that has a big effect. So what this really means for chronic pain patients is that they need to use a whole host of different interventions to decrease the pain they have. And in this, dextromethorphan may have a role in the future."
Dextromethorphan is popular in cold remedies because it elevates the threshold for the coughing reflex but does not cause physical addiction, according to the U.S. Drug Enforcement Administration.
But fibromyalgia patients should not resort to self-medicating by taking cough syrups for pain, Staud cautioned.
"Like every medication, dextromethorphan has side effects," he said. "At high doses, patients can have problems related to memory and confusion."
The underlying cause of fibromyalgia remains unknown, but in the past 25 years substantial progress has been made toward understanding the mechanisms behind specific features of fibromyalgia, Staud said. One is central sensitization, a feature of many chronic pain conditions in which the central nervous system - the brain and spinal cord - somehow magnifies pain signals to abnormally high levels, said Staud, who is affiliated with UF's McKnight Brain Institute.
Central sensitization is associated with wind-up, a phenomenon in which repeated touches - even handshakes or pats on the back - generate lingering pain that increases with each new contact, he said. A normal form of achy, lingering pain known as secondary pain affects anyone who suffers an injury.
The UF researchers - Staud, neuroscientist Charles Vierck, Ph.D., psychologist Michael Robinson, Ph.D., and Donald Price, Ph.D. - were surprised to learn that dextromethorphan eased fibromyalgia patients' wind-up pain to the same degree it soothed secondary pain induced in healthy volunteers, Staud said. The results indicate a long-suspected cause of wind-up may not exist.
Previous studies at other institutions had shown that dextromethorphan blocks the action of a chemical messenger called N-methyl-D-aspartate, or NMDA, which relays pain impulses in the spinal cord. Many fibromyalgia researchers have theorized that wind-up is caused by abnormalities in the spinal-cord structures that process NMDA.
The UF results suggest those structures function normally but that pain impulses are more amplified in fibromyalgia than in healthy participants, Staud said.
"This has refocused much of our research now," he said. Future UF studies will attempt to pinpoint where the pain impulses are originating.
In the current study, researchers worked with 14 women with fibromyalgia and 10 women who did not have the disease, using mechanical devices that tapped the participants' hands repeatedly. One part of the study involved contact with a heated probe, the other used a small rubber-tipped peg. The intensity of the heat or pressure of the stimulation was individually adjusted so that all participants reported feeling the same degree of pain.
Researchers then gave each participant a capsule containing 60 milligrams of dextromethorphan, 90 milligrams of dextromethorphan or a placebo containing none of the drug, and asked them to rate the amount of pain they experienced when the stimulation was repeated.
With the heat stimulus, 90 milligrams of dextromethorphan reduced wind-up pain, but 60 milligrams was no more effective than the placebo. With the pressure stimulus, 90 milligram and 60 milligram doses were equally effective, reducing wind-up pain.
The UF study indicates the need for further research on dextromethorphan, said fibromyalgia expert Laurence Bradley, Ph.D., a professor of medicine with the University of Alabama at Birmingham's division of clinical rheumatology and immunology.
"This is a topic that's actually received very little attention so far in the literature," Bradley said. "It would be a disservice to start to recommend that either patients or physicians begin experimenting right away with dextromethorphan, because I think there's some important questions about how to minimize the side effects with this agent."