Fibromyalgia, widespread general muscle pain and fatigue, has no known causes or effective treatments but small-fiber polyneuropathy (SFPN) has a clear pathology and is known to be caused by specific medical conditions, some of which can be treated and sometimes cured.
In a small group of patients with fibromyalgia, about half were found to have damage to nerve fibers in their skin and other evidence of small-fiber polyneuropathy.
Though fibromyalgia is recognized by the National Institutes of Health, the symptoms vary widely and it is used more as a blanket term. It is believed to affect at least 1 percent of individuals, primarily in Western countries and primarily women. That fibromyalgia shares symptoms with SFPN, a recognized cause of chronic widespread pain for which there are accepted, objective tests, may lead to more consistent treatments.
Designed to investigate possible connections between the two conditions, the current study enrolled 27 adult patients with fibromyalgia diagnoses and 30 healthy volunteers. Participants went through a battery of tests used to diagnose SFPN, including assessments of neuropathy based on a physical examination and responses to a questionnaire, skin biopsies to evaluate the number of nerve fibers in their lower legs, and tests of autonomic functions such as heart rate, blood pressure and sweating.
The questionnaires, exam assessments, and skin biopsies all found significant levels of neuropathy in the fibromyalgia patients but not in the control group. Of the 27 fibromyalgia patients, 13 had a marked reduction in nerve fiber density, abnormal autonomic function tests or both, indicating the presence of SFPN. Participants who met criteria for SFPN also underwent blood tests for known causes of the disorder, and while none of them had results suggestive of diabetes, a common cause of SFPN, two were found to have hepatitis C virus infection, which can be successfully treated, and more than half had evidence of some type of immune system dysfunction.
"This provides some of the first objective evidence of a mechanism behind some cases of fibromyalgia, and identifying an underlying cause is the first step towards finding better treatments," says Anne Louise Oaklander, MD, PhD, director of the Nerve Injury Unit in the MGH Department of Neurology and corresponding author of the Pain paper. "Until now, there has been no good idea about what causes fibromyalgia, but now we have evidence for some but not all patients. Fibromyalgia is too complex for a 'one size fits all' explanation."
"The next step of independent confirmation of our findings from other laboratories is already happening, and we also need to follow those patients who didn't meet SFPN criteria to see if we can find other causes. Helping any of these people receive definitive diagnoses and better treatment would be a great accomplishment."