By Jeremy Laurance
London - Scientists believe they have solved the mystery of cot deaths which claim the lives of hundreds of babies every year.
A study of victims of Sudden Infant Death Syndrome (Sids) has revealed that they were suffering from a brain abnormality that prevents the detection of insufficient oxygen levels in the body.
The effect of the abnormality is that babies may suffocate if they become smothered by bedclothes, especially if sleeping on their fronts.
The finding "takes the mystery away from Sids", said Marian Willinger, a Sids researcher at the National Institute of Child Health and Human Development, which funded the study.
"It should take the guilt away from any parent who has lost a baby because they always wonder,'What did I do wrong?' Now, they need to really understand, 'My baby had a disease'."
The United States researchers from Boston Children's hospital said it was the strongest evidence yet of a common cause for cot death, the causes of which have puzzled specialists for decades.
Some mothers have been jailed for murder after the deaths of their babies because of uncertainty about the cause.
Three hundred babies died of cot death in Britain last year, 16 per cent down on 2004 and less than a third of the number of victims 20 years ago. The Back to Sleep campaign in the early 1990s which urged parents to put their babies to sleep on their backs is credited with bringing about the dramatic fall.
Greater alertness and better ways of determining causes of death have contributed to the decline.
The researchers, led by David Patterson, studied post-mortem samples from the brainstems of 31 babies who had died of cot death and compared them with samples from 10 babies who died of other causes.
They found abnormalities in the brain stems of the cot death babies affecting the way they used the brain chemical serotonin.
Serotonin is best known for its role in depression and regulating mood but it also influences breathing, body temperature and arousal from sleep.
The study is published in the Journal of the American Medical Association.
"This finding lends credence to the view that Sids risk may greatly increase when an underlying predisposition combines with an environmental risk - such as sleeping face down - at a developmentally sensitive time in early life," said Dr Duane Alexander, director of the National Institute of Child Health.