The most common cause of death of U.S. infants before their first birthday is the nebulous complication known as sudden infant death syndrome (or SIDS), according to the Mayo Clinic. The underlying causes of this condition, in which no immediate cause of death is revealed in an autopsy, remain unknown, vexing scientists and parents alike.

Recent research has linked abnormal production of the neurotransmitter serotonin to the occurrence, and a new study underscores that link, reporting that infants who have died of SIDS have about a quarter less serotonin in their brainstems than infants who have died suddenly of other causes or those who have been hospitalized for low oxygen levels. The findings were published online February 2 in Journal of the American Medical Association.

Babies with this deadly deficit might not show any differences during waking hours, but in sleep, serotonin plays an important role in regulating temperature and breathing. "Our research suggests that sleep unmasks the brain defect," Hannah Kinney, an associate professor at Harvard Medical School and a senior researcher on the study, said in a prepared statement. "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 head and wake up in response. But a baby with an intrinsic abnormality is unable to respond to the stressor."

For the study, researchers analyzed brainstem tissue samples from 41 infants who had died from SIDS and compared them to tissue from seven infants who had died suddenly from other known causes and five infants who had been hospitalized for hypoxia-ischemia (lack of oxygen).

In addition to the low levels of serotonin, most of the infants in the study who had died from SIDS (95 percent) had at least one risk factor at the time of death—including stomach or side sleeping (63 percent), previous minor illness (44 percent) or sharing a bed (20 percent)—and 88 percent of the SIDS infants had two or more risk factors. The babies who died without one of these known risk factors had an even lower amount of serotonin than the others, "suggesting that additional risk factors are necessary to precipitate death when the [serotonin] system is less compromised," noted the authors.

A study published July 4, 2008, in Science found that in mice that had been bred to have too many serotonin receptors, about three quarters of the specimens died before the age of four months. But other research had not revealed whether an overabundance or lack of the neurotransmitter might be to blame.

Although public awareness campaigns encourage parents and caretakers to put babies to sleep on their backs—a first-defense against SIDS—rates of the syndrome remain relatively high, killing about 2,500 infants a year in the U.S, according to the American SIDS Institute.

"The current findings provide important clues to the biological basis of SIDS," Alan Guttmacher, acting director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, said in a prepared statement. Having found these deficits "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," he said.

Image courtesy of Wikimedia Commons/Paul Goyette