WASHINGTON, USA (AFP) -- Sudden infant death syndrome (SIDS) could be caused by low levels in the brainstem of serotonin, a neurotransmitter which controls functions such as heart rate and breathing, a study showed Tuesday.

Researchers at Children's Hospital Boston measured the levels of serotonin and tryptophan hydroxylase, the enzyme that helps make the key neurotransmitter, in 36 infants who died from SIDS and two control groups -- babies who died suddenly of other causes and infants who were hospitalised with chronic oxygenation problems.

What they found is that serotonin levels in the lower brainstem of the SIDS babies were 26 per cent lower and tryptophan hydroxylase levels were 22 per cent lower than in the control groups. They also found that SIDS babies had fewer serotonin receptors in the brainstem.

"The receptors are what serotonin interacts with to produce an effect," David Paterson, one of the lead authors of the study published in the Journal of the American Medical Association (JAMA, told AFP.

"In SIDS cases, in the brainstem, they actually have fewer of these receptors, and if there's a lower number of receptors, that's usually an indication that there's something wrong with serotonin," he said.

Serotonin in the brainstem controls functions such as heart rate, blood pressure, body temperature and breathing.

"When the baby is awake, the brainstem works together with other systems in the forebrain, the cortex to control these functions," said Paterson.

"However, when the baby is asleep, the higher functions in the brain switch off so the brainstem system itself is solely responsible for controlling all these functions," he said.

If the infant has a serotonin deficiency, and if there are other SIDS risk-factors, such as the baby sleeping on its stomach where it rebreathes its own breath, with higher carbon dioxide, the serotonin system in the brainstem could fail to detect a problem and tell the baby to take action, Paterson explained.

"If this system is not working properly, the baby might not respond to this challenge and die," he said.

SIDS is the leading cause of death in infants younger than one year in the United States.

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