Gender: Some Painstaking Differences

Any mother will tell you that if men had to go through the excruciating pain of childbirth the human race would have become extinct long ago.

According to feminine lore, guys simply don't handle pain well. The tiniest twinge of discomfort is enough to reduce most men into helpless, whimpering heaps. Women, on the other hand, can handle the tough stuff. In fact, you can ratchet up the old pain-o-meter to agony and beyond and most women will soldier on without flinching.

The trouble with that theory is -- it's wrong. And now men have the science to prove it.

"The laboratory research seems to indicate that for many kinds -- but not all kinds -- of stimuli, women have a lower tolerance for pain," says Linda LeResche, ScD, a professor of oral medicine at the University of Washington, in Seattle.

But what exactly that means is a puzzle LeResche and other pain researchers across the country are trying to work through. The fact is, no one really knows for sure if women and men perceive pain differently, let alone how they react to it.

Untangling the Mystery of Pain

"There are [so many] overlays of societal and cultural norms and other factors that go into the reporting of pain that it may not have a biological basis at all," says Robert Gear, an assistant professor of oral and maxillofacial surgery at the University of California, in San Francisco. "It certainly could have a biological basis, but there is no way to test it so far."

Researchers have been trying for decades to untangle the mystery of pain. An important breakthrough came in the mid-1960s when scientists discovered that the brain could change or modulate the nervous system in response to pain. Until then it was believed the nerve-brain-pain connection was fixed and inflexible.

"There is some experimental research that suggests the pain modulation systems between men and women may be different," LeResche says. "Females may also have an additional system that uses estrogen."

The 1990s were a fertile time in pain research. What we now know is that pain begins when nociceptors -- small, thin fibers located throughout the body in peripheral nerves -- register trauma to nearby tissue. During surgery, for instance, nociceptors fire as soon as the initial incision is made. They release a bath of chemicals that induces the nerves to carry the pain message to the brain. The chemicals collect in a part of the spinal cord called the dorsal horn and are released to the brain -- where pain is registered.

Even with general anesthesia, a patient's central nervous system remains alert and ushers pain messages back and forth from the nociceptor, through the nerves, into the spinal cord, and on to the brain. The brain responds by washing the spinal cord with glutamate, a chemical that rewires the central nervous system and creates a physical memory of pain.

Feeling Your Pain

The challenging part will be teasing out the rest of the information -- including how men and women perceive pain and the physiological/social aspects of pain. But in financial terms it certainly will be worthwhile. The annual cost of pain in the U.S. is roughly $100 billion, including 515 million lost workdays, according to the American Pain Foundation. Approximately 25 million Americans suffer from acute pain due to injuries or surgery.

Laboratory studies show a clear difference in pain tolerance levels between men and women. When healthy men and women are subjected to heat and other types of pain tests, women almost always report feeling discomfort first.

"It takes a lower temperature for a women to tell you that this feels painful," says Roger Fillingim, PhD, associate professor in the college of dentistry at the University of Florida, in Gainesville. "The laboratory studies show rather convincingly that women have a lower pain threshold and pain tolerance than men. That has been fairly consistently shown in the experimental studies that have been done."

To measure the differences in pain tolerance between men and women, Fillingim uses something called effect size, which compares the differences between the groups to the differences within each group. On a scale of small, moderate, and large, the pain tolerance difference between men and women is considered moderate. In degrees centigrade that translates to a difference of one degree to a degree and a half.

"So they are not so great that you would say, 'Here comes a women and she will have more pain no matter what else is going on,'" he says. "It is also not so small that they should be ignored for other factors."

Those findings actually prompt as many questions as they answer. For instance, researchers want to know, what role does a woman's menstrual cycle play in her perception of pain? It must play some part, says Sherry Marts, scientific director for the Society for Women's Health Research in Washington, D.C. For example, she says, women know not to get their legs waxed right before their period because it is much more painful than at other times in their cycle.

"Something in the hormonal factor is affecting the perception of the pain," says Marts.

Fillingim agrees, adding that laboratory research suggests that during the premenstrual phase women are more sensitive to most types of painful stimuli than during other phases of their cycle.

"So there are a lot of complicated interactions among different systems of the body rather than just estrogen going up and down," he says.

Gender-Specific Treatment?

Another question that researchers would like to answer is if the laboratory differences have any clinical meaning? In other words, how can physicians use the data to help their patients?

"Those are some of the questions that are starting to be addressed, but we still need a lot of information," Fillingim says.

As this data starts trickling in, a patient's sex could ultimately change the way he or she is treated. Today, when a man or woman walks into a doctor's office they are evaluated and treated for pain in exactly the same way. But that could change in the very near future, says LeResche.

"We are getting better-designed research," she says. "My sense is that there is quite a bit going on that should yield some answers in the next five years."

Bob Calandra is a freelance writer whose work has appeared in several magazines, including People and Life. He lives in Glenside, Penn.