Fibromyalgia may be more common than previously believed, especially among men, according to a new Mayo Clinic study. Researchers there used two different methods to analyze how common the condition is in one Minnesota county. The findings, published in Arthritis Care & Research, offer new insights into a disorder that is difficult to diagnose and treat.

Fibromyalgia causes widespread muscle and joint pain as well as fatigue, sleep problems and cognitive difficulties. Although considered a rheumatic disease, it doesn’t damage the joints or other tissues. Instead, symptoms result from changes in the way the brain and spinal cord process and transmit pain signals.

Until recently, fibromyalgia has been thought to affect women in more cases than men. But experts now say men may be underdiagnosed. Study co-author Daniel Clauw, MD, a professor of medicine in the division of rheumatology and director of the Chronic Pain and Fatigue Research Center at the University of Michigan in Ann Arbor, believes men might account for as many as one-third of undetected fibromyalgia cases. One of the main reasons it’s undetected is because of the way the disorder is diagnosed, says Dr. Clauw, a leading researcher of fibromyalgia pain.

Since 1990, fibromyalgia has been diagnosed using a checklist developed by the American College of Rheumatology (ACR). The checklist has been criticized for omitting key symptoms – such as fatigue, depression, sleep difficulties and cognitive problems – and requiring a difficult-to-perform “tender point” exam – a physical check for 18 predetermined painful areas on the body.  

In 2010, the ACR published a new checklist that includes important symptoms (such as fatigue) previously left out. The checklist was revised in 2011 so fibromyalgia could be diagnosed without a tender-point exam.

For the study, Mayo Clinic researchers used both the 1990 and the 2010 checklists to look for fibromyalgia in the population of Olmsted County, Minn. “It was like using two different lenses with two different powers,” says lead author Ann Vincent, MD, medical director of the Fibromyalgia and Chronic Fatigue Clinic at Mayo Clinic in Rochester, Minn.

First, researchers retrospectively reviewed the medical records of nearly 3,500 patients with possible fibromyalgia. Of these, roughly one-third had been officially diagnosed by a doctor using the 1990 criteria. After taking into account age and sex, researchers determined fibromyalgia affected 1.1 percent of the county’s population.