Infants who die from Sudden Infant Death Syndrome (SIDS) may not be equipped to arouse themselves from sleep the way healthy infants can, according to School of Medicine researchers.
Reporting recently at the Fourth SIDS International Conference, Anna Lijowska, M.D., a fellow in newborn medicine, and Bradley T. Thach, M.D., professor of pediatrics, said they have identified a specific sequence of reflexes that leads to arousal in infants. A problem with that sequence could contribute to accidental death while an infant sleeps.
SIDS is the sudden, unexplained death of an infant younger than 1. In the United States, SIDS kills 5,000 to 6,000 infants each year. Recent studies estimate that up to 30 percent of those infants die from rebreathing exhaled air, causing a form of accidental suffocation. Sleeping face down, infants can rebreathe exhaled air, which is low in oxygen and high in carbon dioxide. This stale air becomes trapped within the bedding around the baby's face.
Rebreathing is not the only cause of SIDS, but pediatricians believe it is significant enough that they now recommend infants be put to sleep on their backs or sides rather than on their stomachs. The Consumer Product Safety Commission also has issued a "Safety Alert" that warns parents to avoid the use of soft bedding products, such as comforters or pillows, that could trap carbon dioxide near a baby's nose and mouth.
The researchers say eliminating these risks is a key to lowering the number of SIDS deaths.
But most infants can startle themselves awake if they are exposed to high levels of carbon dioxide. "Most of the time, the babies adjust their body positions. They sigh, they startle, and they turn their heads," Thach said.
Studying infants from 2 to 7 months of age, Lijowska and Thach have identified a specific sequence of arousal in sleeping infants. By gradually introducing a 10 percent carbon dioxide/90 percent oxygen mix into a hood over the heads of healthy infants in their studies, they found all of the infants aroused themselves the same way. When carbon dioxide levels rose, the infants began to sigh and startle.
The sequence begins with a sigh -- a sound familiar to every parent who has watched a child sleep -- in which the infant breathes in two or three times before exhaling. The sigh is followed by a startle reflex. Then the baby thrashes its head and body. Finally, there is full arousal. The entire sequence takes three to five seconds. Because the events always occur in exactly the same order, the investigators believe the sequence may point to a reflex pathway in the brain stem.
"Infants do this a lot in their sleep," Thach said. "Sometimes the arousal sequences are spontaneous. Other times they occur in response to stimuli."
The investigators believe an abnormal arousal pattern may contribute to the risk of SIDS. If an infant does not have normal reflex-arousal responses, carbon dioxide levels could continue to rise while oxygen levels fall, and the baby could suffocate.
"The fact that all infants react in the same way leads us to think that a fundamental brain stem reflex may be involved. That gives us a reason to look at the brain stem as we investigate SIDS," Lijowska explained.
She said because it is possible for researchers to initiate the arousal sequence, it also should be possible to test infants for abnormal startle reflexes. Knowing this may help doctors assess an infant's risk for SIDS.
-- Jim Dryden
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