Up to one-half of the babies who die of sudden infant death syndrome (SIDS) show an abnormal heart rhythm at birth, according to new research. That observation raises the theoretical possibility that some infants at risk for SIDS could be identified early, and some deaths possibly prevented.
Although the syndrome is relatively rare, about 3,000 infants die of SIDS each year in the United States, making SIDS the leading cause of death of children between the ages of one month and one year.
The incidence of SIDS has fallen by 40 percent since 1992, when pediatricians began recommending that babies be placed on their backs, not their stomachs, while sleeping. Why a back-down sleeping position reduces the risk of sudden death isn't known, although there are numerous theories.
The new research addressed a different issue. It sought to learn whether apparently healthy babies who died suddenly showed abnormalities on electrocardiograms (EKGs) recorded in the first few days of life. The idea that SIDS might have a cardiac origin was proposed more than two decades ago and derived in part from the knowledge that sudden death in adults is usually the result of a disordered heart rhythm.
The researchers report in today's New England Journal of Medicine that half the SIDS victims they studied had unusually long "QT intervals" -- a critical part of the heart-rhythm cycle -- on their newborn EKGs.
Lengthening of the QT interval is a well-established risk factor for sudden death in adults, especially young and otherwise healthy ones.
"The study is resurrecting an idea that we had sort of put to bed," said Marian Willinger, the head of SIDS research at the National Institute of Child Health and Human Development. "It is a tantalizing report that needs to be pursued."
Even if other studies confirm the findings, it's unclear how they might alter the care of newborns.
For one thing, nobody knows if these cases of SIDS can be prevented. Drugs known as beta-blockers markedly lower the risk of death in adolescents and adults with long QTs resulting from genetic defects. Whether the same is true in these infants is unknown.
Furthermore, even though a prolonged QT appears to raise an infant's risk of SIDS, it is far from a death sentence. In the study, 2.5 percent of newborns demonstrated the abnormality, but only a handful died.
The researchers estimate that about 50 infants would have to be treated to prevent one SIDS death. Identifying even that relatively small group would require that all newborns get EKGs, a test that would add at least $100 million to medical bills each year.
But should beta-blocker drugs prove effective, the treatment might be feasible. Drug treatment could be limited to the first six months of life, when about 90 percent of SIDS cases occur. Evidence, although not definite, suggests the drugs are safe and don't alter normal development.
The new research is the product of an unusually long and arduous effort by Peter John Schwartz, an Anglo-Italian cardiologist at the University of Pavia in Italy. "There was only one way to either prove or dismiss the hypothesis," he said yesterday. "It was to study a very large number of infants, and then to wait and see when some of them died."
From 1976 to 1994, Schwartz and his collaborators at several other Italian hospitals got EKGs on 34,442 infants. In the ensuing years, 34 of the infants died. Of those deaths, 24 were attributed to SIDS and 10 to other causes.
The researchers then randomly chose 9,725 of the EKGs of the surviving infants and compared them to the EKGs of the SIDS infants. In each case, a SIDS baby's EKG was put into a group of at least 50 survivors' EKGs, and the entire batch was read "blindly," without the cardiologist knowing which test came from the SIDS victim.
When a QT interval duration of 440 milliseconds was selected as the cutoff for normal, 12 of the 24 SIDS victims were above it. The researchers calculated that having a QT in the abnormal range raised an infant's risk of SIDS 41-fold. Nevertheless, SIDS was extremely rare even in that "high-risk" group, with only 1.5 percent of infants in that category dying.
In the past, much attention has focused on respiratory causes of death among SIDS babies. More recently, research has focused on abnormalities in the control of breathing and blood pressure, and on the body's response to the build-up of carbon dioxide in the blood. Those are controlled by the brain through the autonomic nervous system -- the part of the nervous system not under voluntary control.
One theory for why a back-down sleep position reduces the risk of SIDS is that it alters the position of the structures in the middle ear that help control balance and the sense of orientation in space. That, in turn, affects blood pressure and heart rate.
Ronald M. Harper, a neurobiologist at the University of California at Los Angeles School of Medicine, is a main proponent of this hypothesis.
"I really believe that something is going on in cardiovascular control in these kids," he said. "I think one of the important issues this manuscript raises is that, 'Hey, maybe we ought to be looking at a cardiovascular mechanism of death, rather than a breathing mechanism.' "
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