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Study Demonstrates Possible Biological Basis for Pain Tolerance - 2003-06-26

When stung by a bee, some people say it does not hurt much, while other people cry out in pain. Why is there a difference in the amount of pain people say they feel? No one knows. A new study is indicating a biological basis for pain tolerance.

"I am afraid to go the dentist. I can not even stand to listen to, say, a TV commercial that has a dentist drill in it," says one person. "And I can not stand to take even the needle they give you to get rid of the pain when they drill you."

While another person says, "I have a very high threshold with pain. I remember I was very young, and I was riding a motorcycle, I mean a cycle, and I fell down, and a horse cart ran over my cycle, and that cycle was on my leg, and I had my bone being pressed down. And I got up and I told the horse cart, 'Now stop it. And you are to take me home now.'"

People tolerate varying amounts of physical discomfort, crying out over something others would find no big deal. While scientists have not figured out why some people feel pain more acutely than others, a new study in the journal, Proceedings of the National Academy of Sciences, has found what appears to be a biological basis for pain tolerance.

The study by researchers at Wake Forest University School of Medicine in Winston-Salem, North Carolina, involved healthy men and women between the ages of 21 and 40. Scientists touched the eight women and nine men on the right lower leg with a hot probe for 30 seconds. The temperature, 49 degrees Celsius, was considered uncomfortable by all of the participants. Investigators then used a form of magnetic resonance imaging to look at the response in two pain regions in their brains.

Six of the 17 volunteers found the experience extremely uncomfortable, according to neurobiologist Robert Coghill, the study's lead researcher.

"We found that our six most sensitive subjects had activation that was much more frequent and much more sensitive, and much more intense, basically, than our least sensitive individuals," he explained.

What investigators saw was that the brain regions associated with pain were much brighter and more extensive in the most sensitive individuals than in those who were most tolerant of the heat.

"What the subjects are telling us is in very good agreement with what their brain activation is telling us," he said. "So, it really leads us to the conclusion that more activation in pain-related brain regions does in fact correlate with a greater experience of pain, and that these differences in pain experiences do occur across individuals.

According to Professor Coghill, the study is the first to demonstrate a possible biological basis for pain tolerance.

Some researchers think a person's pain threshold may be determined by genetics. Others believe it has to do with psychological factors. That is, a past experience with an extremely painful event might make people more sensitive to physical discomfort in the future. That might include chronic pain.

"I was pleased with the findings," said Professor June Dahl, a professor of pharmacology at the University of Wisconsin in Madison, who is interested in the management of pain in cancer patients. She says the study affirms complaints about pain in such patients.

"We do not have any pain-ometer, for example, that we can use as a thermometer is used to measure body temperature," she said. "We do not have a device that is similar to one that is used to measure blood pressure. So, we have to believe the patient's reports of pain. And this study really validates the patient's subjective report."

Because there has never been a way to measure pain other than reports on a pain scale, Professor Dahl notes, there has been a tendency in the medical profession not to believe cancer patients, in particular.

"To think that they are making up their report; that they are really drug seekers; that they do not really have a pain problem," she said. "And this has been a fundamental challenge to everybody who is trying to bring about improvements in the management of pain. You have to get an appropriate assessment. If you do not assess pain, you cannot develop an appropriate treatment plan."

And that is important, according to Professor Dahl, as it is often hard to pinpoint the causes of pain in cancer patients she says are living longer as a result of improved treatments.