Babies who die from sudden infant death syndrome have substantially lower levels of the brain-signaling chemical serotonin in the part of the brain that regulates breathing than infants who die from other causes, researchers reported Wednesday.
Scientists at Children's Hospital in Boston hope that the findings will eventually help them develop a test that could show which newborns are particularly at risk from SIDS, the leading cause of death in American babies after their first month through the first year.
The findings, published in The Journal of the American Medical Association, are the latest in more than 20 years of SIDS research conducted by a team led by Hannah Kinney, a neuropathologist, and David Paterson, a neuroscientist.
While serotonin is best known for its role in regulating mood, in the brainstem it helps coordinate breathing, blood pressure and sensitivity to carbon dioxide and temperature during sleep.
The researchers studied brain-tissue samples from 35 infants found to have died from SIDS and 10 who died from other causes in San Diego between 2004 and 2008, measuring levels of serotonin and an enzyme that helps make serotonin.
Levels of serotonin in the SIDS babies were 26 percent lower and levels of the enzyme 22 percent lower than among the control cases. And levels of binding to serotonin receptors in the tissue were lower by more than 50 percent among babies who died of SIDS.
Kinney said the findings all reinforce the idea that SIDS in the majority of cases is a result of a disorder of serotonin in the brainstem. But this underlying vulnerability may only prove fatal when the baby's ability to breathe is put under stress by SIDS risk factors such as tobacco smoke, a respiratory illness or being put in an unsafe sleep environment -- sharing a bed, having bulky covers or crib pads, being placed to sleep in any position other than on the back.
"It's no one single factor, but a culmination of abnormalities that result in death," Kinney said. The researchers found that 88 percent of the SIDS babies in the study had two or more risk factors.
But Paterson noted that a minority of SIDS babies who had no apparent risk factors had even lower levels of serotonin receptor binding than did the other SIDS infants, "which may mean some babies might have a worse defect than others that causes some infants to die even when parents and caregivers have done everything right."
Despite local and national efforts to have infants placed in safe sleep environments, and particularly on their backs, some 5,000 sleeping babies still die suddenly and mysteriously in the United States each year.
Many infant-death researchers and investigators have come to believe that unsafe bedding and positioning are responsible for the majority of those deaths, even as research into an underlying physical cause has advanced.
Kinney and others say that finding the brain defect only underscores the importance of safe sleep practices and good prenatal care, including not smoking or drinking alcohol when pregnant.
"We now know that there is a physical reason that many of these babies die," said Dr. Rachel Moon, chair of the American Academy of Pediatrics task force on SIDS. "When babies with this defect are placed to sleep in unsafe places, the results can be disastrous," Moon added.
"As yet, we do not know which babies have this defect, so it's important to protect all babies," said Laura Reno, a spokeswoman for the infant advocacy group First Candle/SIDS Alliance, which helped pay for the research in conjunction with other SIDS foundations and the National Institute of Child Health and Human Development.