Women Are the More Sensitive Sex

It has long been known that certain pain-related conditions, likefibromyalgia, migraine and irritable bowel syndrome, are more common in women than in men. And chronic pain after childbirth is surprisingly common; the Institute of Medicine recently found that 18 percent of women who have Caesarean deliveries and 10 percent who have vaginal deliveries report still being in pain a year later.

But new research from Stanford University suggests that even when men and women have the same condition — whether it’s a back problem, arthritis or asinus infection — women appear to suffer more from the pain.

There is an epidemic of chronic pain: Last year, the Institute of Medicineestimated that it afflicts 116 million Americans, far more than previously believed. But these latest findings, believed to be the largest study ever to compare pain levels in men and women, raise new questions about whether women are shouldering a disproportionate burden of chronic pain and suggest a need for more gender-specific pain research.

The study, published Monday in The Journal of Pain, analyzes data from the electronic medical records of 11,000 patients whose pain scores were recorded as a routine part of their care. (To obtain pain scores, doctors ask patients to describe their pain on a scale from 0, for no pain, to 10, “worst pain imaginable.”)

For 21 of 22 ailments with sample sizes large enough to make a meaningful comparison, the researchers found that women reported higher levels of pain than men. For back pain, women reported a score of 6.03, men 5.53. For joint and inflammatory pain, it was women 6.00, men 4.93. Women reported significantly higher pain levels with diabetes, hypertension, ankle injuries and even sinus infections.

For several diagnoses, women’s average pain score was at least one point higher than men’s, which is considered a clinically meaningful difference. Over all, their pain levels were about 20 percent higher than men’s.

Unfortunately, the data don’t offer any clues as to why women report higher pain levels. One possibility is that men have been socialized to be more stoic, so they underreport pain. But the study’s senior author, Dr. Atul Butte, an associate professor at Stanford’s medical school, said that explanation probably did not account for the gender gap.

“While you can imagine such a bias,” he said, “across studies, across thousands of patients, it’s hard to believe men are like this. You have to think about biological causes for the difference.”

An extensive 2007 report by the International Association for the Study of Pain cited studies showing that sex hormones may play a role in pain response. In fact, some of the gender differences, particularly regardingheadache and abdominal pain, begin to diminish after women reachmenopause.

Research also suggests that men and women have different responses toanesthesia and pain drugs, reporting different levels of efficacy and side effects. That bolsters the idea that men and women experience pain differently.

One reason for the lack of information about sex differences is that many pain studies, in both animals and humans, are done only in males. One analysis found that 79 percent of the animal studies published in a pain journal over a decade included only male subjects, compared with 8 percent that used only female animals.

In addition, experiments testing pain in men and women have shown that they typically have different thresholds for various types of pain. In general, women report higher levels of pain from pressure and electrical stimulation, and less pain when the source is from heat.

Melanie Thernstrom, a patient representative on the Institute of Medicine pain committee from Vancouver, Wash., said the newest research “really highlights the need for more treatment and better treatment that is gender-specific, and the need for far more research to really understand why women’s brains process pain differently than men.”

Some researchers believe the pain experience for women may be even more complicated. Women who have given birth, for instance, may have a different threshold for “worst pain ever,” causing them to underreport certain types of pain. The bottom line, Dr. Butte said, is that far too little is known about how men and women experience pain and that more study is needed so that, ultimately, pain treatment can be customized to each patient’s needs.

“If doctors have a threshold for when they give a dose or start a medication,” he said, “you could imagine that the number they are using is too high or too low because a person may be in more pain than they are saying.

“In the end, it comes down to what the brain perceives as pain.”