WebMD Medical News
Louise Chang, MD
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
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.
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
''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."
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
They found that:
Finding the deficiency in serotonin levels isn't the whole story, Kinney
says. "We think there are probably several neurotransmitter systems involved in
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
"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
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.
SOURCES:Hannah C. Kinney, MD, professor of pathology, Harvard Medical School;
neuropathologist, Children's Hospital Boston.Duncan, J. Journal of the American Medical Association, Feb. 3, 2010;
vol 303: pp 430-437.Marian Willinger, PhD, special assistant for SIDS, Eunice Kennedy Shriver
National Institute of Child Health and Human Development, National Institutes
of Health, Bethesda, Md.Richard Martin, MD, director of neonatology, Rainbow Babies & Children's
Hospital; professor of pediatrics, Case Western Reserve University,
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