HUNDREDS of babies are killed each year by a condition called sudden infant death syndrome (SIDS). And scientists may have finally found the underlying cause of the mysterious deaths.

US researchers found that SIDS, also known as cot death, could be due to a brain abnormality that prevents victims from detecting insufficient oxygen levels in the body, reported The Times.

Because of this, babies who suffer from SIDS can suffocate on their own clothes or bedsheets, especially if they are sleeping on their tummies.

Scientists from the Boston Children's Hospital studied post-mortem samples from the brainstems of 31 infant victims of SIDS and compared them to 10 babies who had died of other causes.

TOO MANY RECEPTORS

The new study, published in the Journal of the American Medical Association, found that the cot-death babies examined had twice as many brain cells that manufacture serotonin compared to babies that died of other causes.

However, they had fewer receptors for the chemical.

Serotonin is a chemical that is best known for regulating mood but, in the brain stem, it is believed to help coordinate breathing, blood pressure and sensitivity to carbon dioxide by relaying messages from one brain cell to the next.

And it is this defect in the serotonin system that may explain why some babies die suddenly in their sleep.

Dr Hannah Kinney, one of the study's authors, explained that when babies sleep face down, or have their faces covered by bedding, they are thought to re-breathe carbon dioxide instead of fresh oxygen.

Nerve cells in the brain stem then react to the lack of oxygen and stimulates breathing and arousal centres in the brain to prevent suffocation.

'A normal baby will wake up, turn over, and start breathing faster when carbon dioxide levels rise,' Dr Kinney said.

But with a baby who suffers from SIDS, the defects in the serotonin system may impair the brain's ability to sense the build-up of carbon dioxide.

Hence, the baby is not roused from sleep and suffocates to death.

The study also showed that boys who died from cot death had significantly fewer serotonin receptors than girls, which would explain why male infants die from the condition twice as often as female infants.

The researchers added that the brain abnormalities likely develop during the early phases of foetal development.

SMOKING COULD BE CAUSE

Environmental factors such as smoking or alcohol use by the mother may negatively affect the development of the baby's brain stem.

However, they suspect that if the babies survive their first six months to a year after they are born, their brains should develop sufficiently to react normally to low oxygen levels.

Dr Kinney hopes that this discovery will help to reduce future incidences of SIDS.

'These findings provide evidence that sudden infant death syndrome is not a mystery but a disorder that we can investigate and some day may be able to identify and treat,' she said.