SIDS breakthrough: Scientists find new clue to infant deaths

A research team headed by Dr. Hannah Kinney, a professor of pathology at Harvard Medical School and a neuropathologist at Children's Hospital Boston, found that abnormal levels of the hormone serotonin may make some babies vulnerable to SIDS, the leading cause of death for babies.

SIDS baffles scientists
SIDS, also known as cot death or crib death, is a syndrome marked by the sudden death of an infant between one month and 12 months old. The term cot death is often used in the United Kingdom, Ireland, Australia, India, South Africa and New Zealand.

But why an infant who seems totally healthy falls asleep and just does not wake up cannot be explained even after a thorough forensic autopsy, a detailed death scene investigation, and a review of the medical history of the child and family.

Brainstem serotonin may be the culprit
Now, Kinney and colleagues said their findings suggest that a deficiency of serotonin in the brain stem may help explain most of the deaths.

"It is not going to explain all SIDS deaths," Kinney tells WebMD. However, she adds, "it will explain the majority."

Brainstem serotonin controls vital functions during sleep, such as heart rate, blood pressure, body temperature, and breathing.

According to Kinney, the newly discovered serotonin normally helps babies respond to high carbon-dioxide levels during sleep by helping them wake up and shift their head position to get fresh air.

So, the lower level of serotonin may reduce the baby’s capacity to respond to breathing challenges, such as low oxygen levels or high levels of carbon dioxide.

"Our research suggests that sleep unmasks the brain defect," Kinney said. "When the infant is breathing in the face-down position, he or she may not get enough oxygen. An infant with a normal brainstem would turn his or her head and wake up in response. But a baby with an intrinsic abnormality is unable to respond to the stressor."

"It's no one single factor but a culmination of abnormalities that result in the death," she added.

Study details
To reach their findings, Kinney and colleagues examined brain tissue from two groups of infants--one died from SIDS and the other from other causes.

In the group of 35 infants who died of SIDS, five died unexpectedly of other causes, and five were hospitalized and died for reasons associated with a lack of oxygen.

The tissues examined by the research team came from the medulla, a region at the base of the brain that regulates basic functions such as body temperature, breathing, blood pressure, and heart rate.

Kinney's team found that serotonin levels were 26 percent lower in tissue from babies who died of SIDS than the group of infants who had otherwise died unexpectedly. Levels of tryptophan hydroxylase, an enzyme needed to make serotonin, also were 22 percent lower.

The findings appear in the Feb. 3 issue of the Journal of the American Medical Association.

Researchers hoping for the best
The team hopes their findings will one day lead to a test that measures a baby’s serotonin levels in the blood or other tissues that reflect brain serotonin levels, making it possible to identify those at the highest risk for SIDS.

Also, Dr. Alan Guttmacher, director of the National Institute of Child Health and Human Development, which funded the study, hopes the findings would help explain why some infants are vulnerable to SIDS.

"We have known for many years that placing infants to sleep on their backs is the single most effective way to reduce the risk of SIDS," said Dr. Guttmacher.

"The current findings provide important clues to the biological basis of SIDS and may ultimately lead to ways to identify infants most at risk as well as additional strategies for reducing the risk of SIDS for all infants," Guttmacher added.