Pain and Gender: Is There A Difference?
Fibromyalgia Network, June 1998
NIH conference on April 7-8 provided a variety of speakers who all attempted
to answer the question, "Is there a difference in how men and women
process pain?" "We know that men's and women's brains are different,"
joked one presenter, "because women have a large processing center
for chocolate cravings while men have one for sex!"
On a scientific level, genetics expert Jeffrey Mogil, Ph.D., from the University of Illinois at Urbana-Champaign commented: "One thing that should be kept in mind, in addition to sex differences that are a result of hormonal status, is that males and females have different brains from the start." Specific brain findings by Dr. Mogil and others are given below, followed by sections of other data presented at this two-day conference held in Bethesda, MD.
Many complicated gender-specific differences in the pain system of rodents have been documented, but what remains unknown is the genetic basis for these findings. Mogil thinks he has found a section on chromosome 4 in male mice that could explain why most studies have shown that women have lower pain thresholds than men. This particular section of the gene controls part of the opioid system in male mice, but it has no effect in female mice. This opioid receptor is present in the brain, spinal cord and other neurons, and Mogil suspects that it is responsible for regulating baseline pain sensitivity.
David Borsook, M.D., Ph.D., at Massachusetts General Hospital in Boston, is attempting to look at differences in brain function of humans when a painful stimulus is applied to a person's body. The procedure is called functional magnetic resonance imaging (fMRI). Borsook has divided his healthy female subjects according to where they are in their menstrual cycle: the mid-folicular phase which is around day 7 and the mid-luteal phase which is around day 22. He has found that women in the first phase of their cycle (folicular) and men look the same on fMRI measurements. However, there are distinct pain processing differences cropping up in women who are in the second half of their menstrual cycle.
"We think that these data provide the beginnings for implications of pain processing in the brain as a result of the estrogen and progesterone phases," says Borsook. He added that his fMRI technique could provide an objective way to measure responses to pain medications and, perhaps, pinpoint the brain processing centers that may be functioning differently between men and women.
Serotonin and Hormonal
So what are the differences between males and females? The serotonin concentration in the spinal cord of lab animals is much higher in males than females. "This suggests," says Berkley, "that the serotonergic influences on the processing of information (including pain signals) would be greater in males than females."
Berkley says that there is a large shift in the way the central nervous system chemicals are produced (including serotonin) with response to the different stages of the female cycle. In addition, estrogen causes a cyclic growth of neurons. "We are beginning to see the variability in females that is not seen in males structurally," says Berkley.
Some research might indicate that women are just more likely than men to call a stimulus painful, but Berkley says that this is not the true picture, that there are many other factors involved. She conducted a study involving three sets of people: (1) healthy men, (2) healthy women, and (3) women with dysmenorrhea (painful menstruation). Berkley provided a pain stimulus to the skin of the arms and legs, as well as a deep tissue stimulus to the muscles of the abdomen. The healthy men and women responded the same to the skin tests. So on the surface, there seems to be no gender differences.
What about the abdomen? The pain threshold for women with dysmenorrhea was much lower (e.g., they were more sensitive to the pain) than for those women who were healthy. How did the men respond to the deep muscular stimulus to the abdomen compared to the women? "The men wouldn't let us near their abdomen!" says Berkley. Regardless of whether the examiners were male or female, the men in the study just dashed out of the room at the thought of touching their abdomen. The bottom line according to Berkley is that the presence of a disease condition (dysmenorrhea) made the women in the study more sensitive to pain and enhanced the cyclic nature of the pain all over the body (i.e., more pain when estrogen is low).
Chest Pain and Exercise
At first glance, it would be easy to say: "Well, women just seem more likely to complain than men." While it may appear this way, blood analyses of the body's natural pain killer, beta-endorphin, tell a different story. The beta-endorphin levels at rest were significantly lower for the women in the study.
In addition, Sheps' physiology colleague, David Sheffield, Ph.D., has been analyzing the blood levels of beta-endorphin in both men and women during the treadmill test. Ordinarily, the body pours out beta-endorphins during exercise and other stressful situations as a natural way to help combat the potential onslaught of pain. According to Dr. Sheffield, the amazing finding was that when women exercised, their beta-endorphin levels did not rise as high as their male counterparts-regardless of any diagnosis.
Are women just louder complainers, or is something else going on? Sheffield says that the pain fighting systems are physiologically different between men and women, and that this could explain why women sense more pain.
How this information applies to FMS/CFS was not addressed, but it is interesting to note that multiple chemical sensitivities and difficulty handling bright lights is a problem for many people with FMS/CFS. Although FMS is often thought of as a pain syndrome, the sensory detectors in patients might be more sensitive, not just for detecting pain, but also for sensing odors and other inputs to the nervous system.
"In addition to there being sex differences to naturally occurring behaviors, there are also sex differences to drug induced behaviors," says Becker. She points out that women are far more sensitive to stimulants such as amphetamines. So if you are a women with FMS/CFS and you need something to boost your energy level (who doesn't?), then you might want to think twice before consuming too many stimulants, such as coffee, tea, and other caffeine-containing beverages.
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