Mysterious deaths of babies during sleep have been recorded since antiquity.
But the term sudden infant death syndrome, or SIDS, wasn't coined until the 1960s when it was declared a medical disorder. Some facts about SIDS:
What is SIDS?
Often called "crib death," SIDS is the sudden death of an infant under age 1 that remains unexplained after a thorough investigation, including autopsy, examination of the death scene and review of the medical history.
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How is it diagnosed?
SIDS is a diagnosis of exclusion. An autopsy is used to rule out other medical causes. Authorities rely on a death scene investigation to provide evidence of causes, such as suffocation, that aren't detectable by autopsy.
Has SIDS really been around since ancient times?
Yes, some say there's a mention in the Old Testament, 1Kings 3:19. The earliest explanation was accidental suffocation, or "overlaying," by the mother. "There were penances that the mothers would do to expiate the sin of having killed their child," says Dr. John Butts, N.C. chief medical examiner. Devices were invented and built to hold nursing babies, to prevent the mother from asphyxiating the child.
What causes SIDS?
Scientists don't know. There have been many theories over the years.
One in the 18th century focused on the possibility that an enlarged thymus gland produced a mass of tissue at the back of the neck that could close off the baby's windpipe. This and other theories, such as allergies to milk and "poor mothering," were discarded over the years.
Since the early 19th century, some experts theorized that infants who died unexpectedly had "faulty constitutions," according to the textbook "Crib Death."
Early researchers also considered the possibility that sleep apnea (short periods of not breathing during sleep) or other abnormalities of breathing control were related to unexpected infant deaths.
In 1972, the journal Pediatrics published an article by Dr. Alfred Steinschneider, who theorized SIDS was caused by apnea and that the syndrome could also be genetic. His study referred to a "Mrs. H" who lost five children to SIDS. His work was discredited in 1995 when a jury convicted Waneta Hoyt - Mrs. H - of killing all five of her children.
Then what's the latest theory?
Today, many experts espouse the "triple-risk" theory, believing SIDS can result when three factors come together in a sort of perfect storm. The factors are: (1) a critical period in the baby's development, (2) an underlying vulnerability, and (3) an outside stressor, such as sleeping face down or becoming entangled in soft bedding.
The critical stage is the first six months of life, when babies' immune, cardiovascular and respiratory systems are developing. Experts disagree on what the vulnerabilities might be. But some ideas are respiratory infections, genetic susceptibility and defects in the brain stem.
The most recent finding, announced in February, suggested that brain stems of SIDS babies contain low levels of the hormone serotonin, which controls vital functions during sleep, such as breathing, heart rate and blood pressure. Researchers from Children's Hospital Boston think this would explain why some infants are at risk.
Infants in certain sleep environments are more likely to re-breathe exhaled carbon dioxide, which can be lethal. Normally, babies in that situation would turn their heads to get fresh air. But babies who die of SIDS don't respond or cannot wake up when they experience breathing difficulty.
What risks are linked to SIDS?
SIDS deaths have been considered unpredictable and unpreventable. But it has become clear over the years that certain factors put infants at higher risk.
They include: stomach- and side-sleeping; maternal smoking or drug use during pregnancy; second-hand smoke; overheating; soft sleeping surfaces; bed-sharing; little or no prenatal care; prematurity or low birth weight; and being male, African-American or Native American.
What are experts doing to make a difference?
In 1992, the American Academy of Pediatrics recommended that babies sleep on their backs. After a nationwide "Back to Sleep" campaign launched in 1994, SIDS deaths nationally dropped about 50 percent.
By 2005, the academy for the first time warned parents against bringing infants into their adult beds. That recommendation remains controversial because many parents' groups support bed-sharing, in conjunction with breastfeeding, as a way for parents to bond with and stimulate their babies. Some parents also believe that if they're sleeping with or near their babies, they're more likely to wake up if the babies stop breathing.
After years of looking for other causes and trying to protect parents from unnecessary guilt, researchers have returned to the idea that many deaths previously ruled SIDS could, in fact, have resulted from unintentional suffocation.
Today, many SIDS experts are intent on identifying and reclassifying potentially preventable suffocation deaths so that researchers can focus on finding the medical cause of remaining unexplained infant deaths.
Staff writer Lisa Hammersly and researcher Maria David contributed.