Infants who die of Sudden Infant Death Syndrome (SIDS) have abnormalities in the brainstem, a part of the brain that helps control heart rate, breathing, blood pressure, temperature and arousal, report researchers funded by the National Institutes of Health. The finding is the strongest evidence to date suggesting that innate differences in a specific part of the brain may place some infants at increased risk for SIDS.
The abnormalities appeared to affect the brainstem's ability to use and recycle serotonin, a brain chemical that also is used in a number of other brain areas and plays a role in communications between brain cells. Serotonin is most well known for its role in regulating mood, but it also plays a role in regulating vital functions like breathing and blood pressure.
The study, which appears in the November 1 Journal of the American Medical Association, was conducted by researchers in the laboratory of Hannah Kinney, M.D., at Children's Hospital Boston and Harvard Medical School, as well as other institutions.
"This finding lends credence to the view that SIDS risk may greatly increase when an underlying predisposition combines with an environmental risk—such as sleeping face down—at a developmentally sensitive time in early life," said Duane Alexander, M.D., Director of the NIH's National Institute of Child Health and Human Development (NICHHD).
SIDS is the sudden and unexpected death of an infant under 1 year of age, which cannot be explained after a complete
autopsy, an investigation of the scene and circumstances of the death, and a review of the medical history of the infant and his or her family. Typically, the infant is found dead after having been put to sleep and shows no signs of having suffered.
In previous studies, researchers have hypothesized that abnormalities in the brainstem may make an infant susceptible to situations in which they rebreathe their own exhaled breath, depriving them of oxygen. This hypothesis holds that certain infants may not be able to detect high carbon dioxide or low oxygen levels during sleep and do not wake up.
To conduct the current study, researchers examined tissue from the brainstems of 31 infants who died of SIDS and 10
who died of other causes.
The lower brainstem helps control such basic functions as breathing, heart rate, blood pressure, body temperature, and arousal. The researchers found that brainstems from SIDS infants contained more neurons (brain or nerve cells) that manufacture and use serotonin than did the brainstems of the control infants, explained the study's first author, David Paterson, Ph.D., a researcher at Children's Hospital in Boston.
Although the brainstem tissue from the SIDS infants contained more serotoninusing neurons, these appeared to contain fewer receptors for serotonin than the brainstems of control infants. In their study, the researchers tested the infants' brainstem tissue for a serotonin receptor known as subtype 1A.
Tissue from both the SIDS and the control infants contained roughly equal amounts of a key brain protein, serotonin
transporter protein.
This protein recycles serotonin, collecting the neurotransmitter from the surrounding spaces outside the neuron and
transporting it back into the neuron so it can be used again.
Dr. Paterson explained, however, that because the SIDS infants had proportionately more serotonin-using neurons than the control infants, they would also be expected to have more serotonin transporter protein. So even though they had equal amounts of serotonin transporter protein, the levels were nevertheless reduced—relative to the increased number of serotonin-using neurons—and, for this reason, unlikely to meet the needs of these cells.
"Our hypothesis right now is that we're seeing a compensation mechanism," Dr. Paterson said. "If you have more serotonin
neurons, it may be because you have less serotonin and more neurons are recruited to produce and use serotonin to correct this deficiency."
"These findings provide evidence that SIDS is not a mystery but a disorder that we can investigate with scientific methods and, some day may be able to identify and treat," said Dr. Hannah Kinney, the senior author of the paper.
A large body of research has shown that placing an infant to sleep on his or her stomach greatly increases the risk of
SIDS. The NICHHD-sponsored "Back to Sleep" campaign urges parents and caregivers to place infants to sleep on their backs to reduce SIDS risk. The campaign has reduced the number of SIDS deaths by about half since it began in 1994.
The campaign also cautions against other practices that increase the risk of SIDS, such as soft bedding, smoking during
pregnancy, and smoking around a baby after birth.
Information and free materials on ways parents and caregivers can reduce the risk of SIDS are available on the Back to Sleep Campaign website at www.nichd.nih.gov.
A large proportion of the SIDS cases in the study by Drs. Paterson, Kinney and their coworkers were correlated with known SIDS risk factors: 48 percent were found sleeping on their stomachs, 29 percent were found face down, and 23 percent were sharing a bed, at the time of death. (Click image to enlarge)
