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U.S. researchers have moved one step closer to solving the mystery of sudden infant death syndrome, which claims the lives of seemingly healthy babies while they sleep.

The new work revealed that the brainstems of these babies had lower than normal levels of serotonin, a chemical that transmits messages between nerve cells.

Earlier research had already indicated there was a serotonin problem, but scientists weren't sure if there was too little or too much of the chemical. "We now know there is a deficit of serotonin," said one of the authors of the new study, David Paterson of Children's Hospital Boston.

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The findings, published in the Journal of the American Medical Association, are based on a comparison of tissue samples from the autopsies of 41 infants who succumbed to SIDS and 12 who died of other causes.

The brainstem, in combination with serotonin, helps to regulate basic functions such as body temperature, blood pressure, heart rate as well as breathing.

The researchers believe that a low amount of serotonin may inhibit an infant's ability to respond to breathing challenges, such as inappropriate oxygen and carbon dioxide levels.

"The baby experiences some kind of stress during sleep, such as rebreathing carbon dioxide in the face-down position or increased temperature from over bundling, that cannot be compensated for by the defective brainstem circuits, and the baby then goes on to die," the lead researcher, Hannah Kinney, speculated in a statement.

"In a normal baby rebreathing carbon dioxide, serotonin pathways in the brainstem would stir the baby awake long enough to turn its head, allowing it to breathe fresh air," she added. A baby with low serotonin levels in the brainstem may never stir.

Dr. Paterson said if their hunch is correct, the researchers may be able to develop a diagnostic test to identify children born with the serotonin defect. But, he cautioned, it could take years before doctors can accurately pinpoint high-risk infants. In the meantime, he said it's critically important for all parents to adhere to the safety recommendations known to reduce the risk of SIDS - such as putting babies to sleep on their backs and not their bellies.

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