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UMSOD Researcher Studies Women's Responses to Pain

Written by Adam Zewe

Hundreds of studies over the past few decades have found that women show more sensitivity to acute pain than men, yet scientists still do not fully understand the basis of this difference. Professor Joel Greenspan, PhD, chairman of the Department of Neural and Pain Sciences, conducted a study of pain responsiveness as it relates to women's menstrual cycles and sex hormones to try and unlock part of the puzzle.

The fact that many research studies report women to have increased sensitivity to acute pain stimuli is not just a laboratory artifact, Dr. Greenspan says. Even chronic pain conditions, like temporomandibular joint disorders and fibromylagia, are much more common in women than in men. However, women aren't always more pain sensitive than men, and other chronic pain conditions show no sex difference in prevalence. "The reality is very complicated, so there are probably a number of factors that go into it. An increased response to pain could be biological, psychological or even psychosocial," states Dr. Greenspan.

Previous animal studies have shown that manipulating estrogen levels alters female responses to pain stimuli. However, human studies do support a role for estrogen in influencing pain sensitivity, as well. Building off this research, Dr. Greenspan and colleagues tested women's responses to pain at four different times throughout their menstrual cycles. He used pressure to cause acute muscle pain. While the women were feeling pain, Dr. Greenspan performed functional neuroimaging scans to study how their brains were responding to the painful pressure. "As a person experiences acute pain, one can measure an increased signal across multiple brain areas," Dr. Greenspan explains.

He saw the most dramatic changes in brain activity in the women's frontal lobes. The frontal lobe enables a person to identify an appropriate response and reaction to pain. For example, the frontal lobe might send signals that lead a person to avoid a painful stimulus. Other parts of the frontal lobe activate deeper brain regions that are responsible for reducing pain signals.

Even though the frontal lobe signals became stronger and weaker at different points in the menstrual cycle, the pattern varied for different brain regions. "We didn't get a smoking gun out of this study. The variation in brain activity among healthy women is subtle and complex," he states. The study also revealed that sex hormone variation across the menstrual cycle is only partly responsible for differences in brain response.

Looking forward, Dr. Greenspan would like to direct his research to women who suffer from chronic pain conditions. "The brain acts differently when a person suffers from chronic pain, and it is likely that hormone levels could have a more profound effect on brain activity under these circumstances," says Dr. Greenspan. He is hopeful that this research could lead to a deeper understanding of pain neurobiology and, ultimately, yield better treatments for both acute and chronic pain conditions.

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