BABIES who die from SIDS have brain stem abnormalities regardless of whether they were exposed to risks such as suffocation or co-sleeping, new research has found.

The international research team, involving a Melbourne scientist, will now look at whether a blood test can be ­developed as an early screening tool to identify infants at risk of sudden and unexpected death in their first year.

A collaboration between the Boston Children's Hospital and Florey Institute of Neuroscience and Mental Health's Dr Jhodie Duncan analysed the brain stems of 71 infants who had died suddenly and unexpectedly over 11 years

They all had abnormalities of four neurochemicals in the brain stem, which is at the base of the skull and connects the brain to the spinal cord.

It keeps bodily functions such as blood pressure and oxygen levels constant in ­response to changing factors in the environment.

Dr Duncan said the ­research, published in the journal Pediatrics, suggested this abnormality left the infants unable to adequately respond when faced with a stressor while asleep.

"In a healthy infant, if a pillow goes over their face, the brain normally detects changes in oxygen levels and initiates arousal responses. The baby would turn its head and continue breathing,'' Dr Duncan said.

Infants with the abnormality did not "respond properly" in the same situation, which lead to death, she said.

"There is some evidence that there might be lots of little episodes that occur, and it ­finally gets to the stage where the infant just can't respond adequately - and that's the night they die.''

Dr Duncan said research would now focus on finding a way to identify these at-risk ­infants, potentially through a blood test to detect the neurotransmitter serotonin, or through infant hearing tests.

UK research found babies who died of SIDS had worse hearing than other babies.

brigid.oconnell@news.com.au

Originally published as Breakthrough: Cause of SIDS identified