Feb. 2, 2010 -- Lower levels of the hormone serotonin may help explain why some infants succumb to sudden infant death syndrome (SIDS), according to a new study.
In the U.S., SIDS deaths have declined by more than 50% since 1990. Experts say that's partly due to practices believed to minimize the risk, such as putting infants to sleep on their backs rather than their stomach and avoiding soft bedding, which could lead to asphyxiation.
But SIDS is still the leading cause of death among infants age 1-12 months, accounting for about 2,750 U.S. deaths annually. It's defined as the death of an infant before his or her first birthday that can't be explained even after a complete autopsy, investigation of the death scene and circumstances, and a review of the medical history of the child and family.
Now, the new research suggests that a deficiency of serotonin in the brain stem (which controls vital functions during sleep, such as breathing, heart rate, and blood pressure) may help explain most of the deaths, says study researcher Hannah Kinney, MD, a professor of pathology at Harvard Medical School and a neuropathologist at Children's Hospital Boston.
"It is not going to explain all SIDS deaths," Kinney tells WebMD. However, she adds, "it will explain the majority." Her study is published in the Journal of the American Medical Association.
Explanations for SIDS
SIDS research is a ''controversial area," Kinney says. Many experts look to the "triple risk" model to explain it, believing that SIDS results from an underlying vulnerability, a critical developmental period, and an outside stressor.
''The real risk period is the first six months," Kinney says of the critical period in which most deaths occur.
But experts disagree on what the vulnerability is. Kinney's research suggests low levels of the neurotransmitter serotonin, and probably other brain chemicals yet to be identified, are what make infants vulnerable. Other experts suspect other vulnerabilities, such as infections.
Yet others say SIDS is due simply to suffocation, she says. "We say, yes, some babies will die if they are severely asphyxiated," Kinney tells WebMD. But she adds, "what we are saying is in the majority of cases, the babies have an underlying defect that puts them at risk that makes them unable to respond to a stressor, such as having their face compressed [while sleeping on the stomach or becoming tangled in soft bedding]."
In previous research, Kinney and her colleagues found defects in the serotonin system of SIDS babies, including defects in the serotonin receptors, which are crucial for serotonin to work.
''But we never knew if there was too little or too much serotonin," she says. "In this study, we actually measured the levels of serotonin and the enzyme that makes serotonin."
Measuring Serotonin Levels
Kinney and colleagues evaluated serotonin and tryptophan hydroxylase (TPH2), the enzyme that helps make serotonin, in 35 infants who died from SIDS.
They compared these measurements with those from two groups -- five infants who died suddenly for whom a cause of death was established and five infants who died while hospitalized for having insufficient oxygen to the tissues.
They retrieved tissue samples from the autopsies to measure the enzyme and hormone levels.
They found that:
- Serotonin levels were 26% lower in the SIDS babies than in the infants who died suddenly with a known cause of death, and the enzyme levels were 22% lower. The serotonin and enzyme levels were also lower in the SIDS babies than in the group of hospitalized infants.
- Levels of binding to the serotonin receptors were also lower in the SIDS babies.
Finding the deficiency in serotonin levels isn't the whole story, Kinney says. "We think there are probably several neurotransmitter systems involved in SIDS."
The new research seems to verify the suspicions of many experts who have thought abnormalities of respiratory control contribute to SIDS, says Richard Martin, MD, director of neonatology at Rainbow Babies and Children's Hospital and professor of pediatrics at Case Western Reserve University in Cleveland.
"Serotonin is important for arousal," he says. "If serotonin production is diminished in patients at risk for SIDS, that all makes sense.
"I think the message is there is something inherently wrong in some of these babies," he says. "On the other hand, in no way should that subtract from the public health message -- to avoid prone position, avoid fluffy bedding and pillows, avoid exposure to cigarette smoking, and don't overheat your baby."
While researchers have been focusing on brain stem abnormalities to explain SIDS for many years, the new research ''tells us more about what is wrong with that part of the brain, the brain stem," says Marian Willinger, PhD, special assistant for SIDS at the Eunice Kennedy Shriver National Institute of Child Health and Human Development at the National Institutes of Health.
She calls the finding an important development. "This is another piece in the puzzle in terms of understanding what is wrong in the brain."
Eventually, she says, it may help researchers develop ways to intervene or to develop screening tests for SIDS.