Many cases of SIDS are thought to be caused when babies sleeping on their stomachs are suffocated by their bedding. Other causes, however, are largely a mystery, though experts have speculated that some deaths are a result of heart problems, breathing difficulties, even murder.
The latest research, conducted in Italy, singles out a heart defect that can trigger cardiac arrest. It raises the possibility that screening with an ordinary electrocardiogram soon after birth might spot babies at risk so they can be treated to prevent their deaths.
Still, these researchers and others said it is too soon to make any sweeping changes in the way doctors deal with this devastating condition, since the risks of treatment could outweigh the benefits.
"It's tantalizing, but it's just not settled," said Dr. George Lister of Yale University.
The study involved 33,043 infants and took 19 years to complete. Doctors checked babies with electrocardiograms a few days after birth, then followed them for a year.
Babies with a heart rhythm defect called a prolonged QT interval were 41 times more likely than usual to die from SIDS. Half of the 24 babies in the study who died of SIDS had this condition, compared with none of those who died of other causes.
The study was conducted by Dr. Peter John Schwartz and others from the University of Pavia. It was published in Thursday's New England Journal of Medicine.
"This is the largest study ever, so it can't be discounted," said Marian Willinger, special assistant for SIDS at that National Institute of Child Health and Human Development.
However, she and others had technical concerns about how the study was done. Earlier, much smaller studies found no link between QT interval and the risk of SIDS.
The largest of these earlier studies, done in England, involved 7,254 babies. The Italian researchers said that when their findings were combined with that study, the results conservatively suggest that "probably no less than 30 to 35 percent of infants who subsequently die of SIDS can be expected to have a prolonged QT interval in the first week of life."
In the United States, about one of every 1,000 babies dies of SIDS. The rate dropped about 30 percent during the 1990s after doctors began warning parents to put babies to bed on their backs.
The QT interval is the distance between two particular spikes on an electrocardiogram. It is the time between when the left ventricle begins to beat and when it is ready to beat again. When this interval is too long, life-threatening rhythm disturbances can result.
An editorial in the journal by Drs. Jeffrey A. Towbin and Richard A. Friedman of Baylor College of Medicine said screening infants for prolonged QT interval ijustified if they are considered to be at high risk, such as having a family history of SIDS.
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