By Melissa Schorr

SAN FRANCISCO, Nov 15 (Reuters Health) - Brain scans have revealed that women with the chronic condition fibromyalgia differ from women with depression in their sensitivity to pain, researchers reported here Wednesday at the American College of Rheumatology's annual meeting.

Fibromyalgia, a condition that affects 2% of Americans, usually women, causes muscle pain, stiffness and fatigue. The cause is unknown.

Because nearly half of fibromyalgia patients have suffered clinical depression at some point in their lives, some doctors consider the condition to be a physical manifestation of an underlying mood disorder like depression, said Dr. Leanne R. Cianfrini, a psychology researcher at the University of Alabama at Birmingham.

"Because fibromyalgia doesn't have a clear-cut etiology (or cause), it leads many rheumatologists to interpret their pain as a simple physical manifestation of an underlying depression," she said. "This can be frustrating and counterproductive to patients."

To clarify whether there were physiological differences between patients with fibromyalgia and patients with depression, the investigators compared pain thresholds and brain activity among 21 women with fibromyalgia, 8 women with depression and 22 healthy women.

Cianfrini and colleagues administered pressure calculated to be a level above each woman's pain threshold to three points on the women's bodies. The women were asked to evaluate their pain levels. The researchers also used a brain-imaging scan to measure each woman's brain blood flow while she experienced pain.

The investigators found that the women with fibromyalgia had lower pain thresholds and reported more pain after pressure stimulation than the healthy women. The fibromyalgia patients also showed greater activation of brain structures that process pain after relatively low levels of pressure.

The pain threshold and experience of pain among the depressed women was similar to that of the healthy women, the study found.

"We can't deny depression is associated with fibromyalgia, and it may exacerbate it," Cianfrini said. "But depression does not seem to be a necessary factor."

She advised fibromyalgia patients whose doctors seem resistant to treat them to tell them about her findings. "The pain is not due to depression and if they treat depression, your pain may not necessarily go away," she said. "Patients should say, 'Let's treat my pain, because it's real.'"

In a similar study, Dr. Richard H. Gracely, a research psychologist at the National Institutes of Health, presented findings on how the brains of fibromyalgia patients react to pain.

Gracely and colleagues used a brain scan technique called fMRI to compare fibromyalgia patients with healthy patients experiencing pain. The research team found that patients with fibromyalgia who were given relatively low levels of pressure seemed to experience the same amount of pain and subsequent brain activity as healthy people experiencing high levels of induced pain.

"One of the big issues of pain patients is credibility--they don't have the luxury of physical signs, nobody believes they have what they say," Gracely said. "Fibromyalgia patients particularly had it that way because even rheumatologists didn't believe it was necessarily pain."

However, these findings provide physical evidence to confirm what patients report, he added. "Brain imaging is a way to show there is something physical that matches with what they're saying," Gracely said. "It's welcome news to people who have this."

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