[MOL] Study suggests pain changes body..... [01628] Medicine On Line

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[MOL] Study suggests pain changes body.....

. Health News

Study Suggests Pain Changes Body

July 28, 2000

WASHINGTON (AP) - For years, doctors operated on premature babies without anesthesia in the belief that even if the infants felt the pain, they would not remember it. New research with rats suggests that the body does remember the pain and is forever changed.

A study using newborn rats at the National Institutes of Health found that painful trauma that mimics medical procedures commonly performed on premature infants caused the rats to become much more sensitive to pain as they grew older.

The reason is that pain causes the developing nervous system of the very young to grow more nerve cells that carry the sensation of pain to the brain, NIH researcher M. A. Ruda said.

"We found that there are more nerve endings that fire and transmit the (pain) information," said Ruda, the first author of a study appearing Friday in the journal Science. "These animals later were more sensitive and had a greater response to pain."

Ruda said the study only suggests what may happen in premature infants.

"This is animal research so one has to be cautious in extrapolating to humans," she said. "But we use these animals as models of pain and they have been very valid for predicting the responses that one sees in humans."

The study is part of a continuing effort by medical science to understand how and when the nervous system develops and how the growth of nerve tissue is affected by stimulation, such as pain. Such research has a direct bearing on efforts to save and improve the lives of infants born prematurely, before the normal 40-week gestation.

Survival of babies born up to 15 weeks premature is now not unusual, but it takes a major medical effort and many painful procedures, including countless needle sticks, breathing tubes and even surgery.

Just how much pain such babies feel has been uncertain, said Dr. Patricia A. McGrath, a pain researcher and professor of pediatrics at the University of Western Ontario, who was not a member of the Ruda research team.

Ten years ago, she said, "there was a real belief that the pain system in premature babies was not developed and these infants would really not feel as much pain."

More important, McGrath said, doctors had concerns that premature infants given powerful anesthetics "would not be able to clear the drug as well as older infants." The choice sometimes was between skipping anesthesia during lifesaving surgery or passing on any therapy. Most doctors chose life and hoped that the pain caused no lasting effect.

But Ruda said that the study in rats suggests there is a lasting effect, a change in the nervous system caused by the trauma. If this conclusion is the same in humans, she said, it could mean that premature infants who endured pain of major medical therapies may have a sharpened sensitivity to pain all their lives.

In the study, Ruda and her colleagues injected an irritant that causes pain into the left rear paw of day old rats. Lab rats at that age are in the same development stage as human babies born at 25 weeks of gestation.

The paws were sore for several days before healing. Later, when the rats were grown, the researchers tested their perception of pain by exposing the treated paw to heat. In effect, the test was to see how fast the animals would withdraw from a hot surface. Their reaction time was compared with that of control rats that had not been treated.

Ruda said the treated rats reacted to the heat about twice as fast as the control rats.

When the heat test was administered on the right, or untreated paw, of the test rats, they reacted just as did the control rats. This suggested that the painful trauma early in life had made the treated paw much more sensitive to pain.

Microscopic examination of nerves in the animals showed why, Ruda said.

Nerves leading from the treated paw were much more dense than were the nerves in the untreated paw or in control animals, she said. The increased density means there were more nerve circuits to carry pain stimuli to the brain, Ruda said.

Ruda said other studies have shown that premature babies tend to report more pain in their childhood years and their parents report that these children's pain response is greater than their siblings'.

Dr. Jonelle C. Rowe, an NIH doctor who specializes in treating premature babies, said surgery without anesthesia was rare. Even now, procedure such as insertion of a breathing tube, which require painkillers in adults, are done without anesthesia in premature babies.

"We use anesthesia as well as we can in these babies," Rowe said. A major research effort is under way to find the best way to safely relieve medical procedure pain in the very young, she said.

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