Researchers have taken a significant step forward in the quest to identify the cause of sudden infant death syndrome (SIDS), which claims the lives of dozens of Australian babies a year.
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The study, led by University of Adelaide researchers, found that abnormal brain activity involving "substance P" was associated with SIDS - the sudden unexpected death of an apparently healthy baby, usually while they're sleeping.
They said the failure of substance P - neuronal signalling molecules that influence the activity of the brain and the body - to properly bind with certain neuroreceptors may impair a baby's ability to respond to life-threatening challenges during sleep.
"While the exact cause of death in SIDS has not been identified, multiple studies have pointed to a subset of SIDS babies that ... all seem to have some form of underlying vulnerability, exposing them to increased risk," said lead author Dr Fiona Bright.
"[We] discovered a significant abnormality within key regions of the brain stem in SIDS babies, specifically in parts of the brain stem that control breathing and movements of the head and neck."
The researchers, including experts from Harvard Medical School and Boston Children's Hospital, examined 55 cases of SIDS in the United States. The study was published in PLOS ONE.
Substance P and the associated neuroreceptor "neurokinin-1" (NK1R) play an important role in the brain's control of the respiratory system.
The failure of substance P to properly bind with NK1R may hamper the body's ability to respond to hypoxia, the depravation of oxygen.
"While they may be otherwise healthy-looking, there is an inability for that child's brain and body to respond appropriately to an event in which the child is deprived of oxygen in some way," said Dr Bright.
Experts have long been suspicious of substance P, but findings were inconsistent and inconclusive.
The researchers said abnormal NK1R binding was significantly influenced by prematurity and male sex, which may explain the increased risk of SIDS in premature and male babies.
They said the study ultimately identified a subset of SIDS infants with a developmental abnormality, and the next step was to develop screening techniques or biomarkers to identify babies most at risk.
In 2013, 117 babies died suddenly and unexpectedly, and of those deaths, 54 were identified as SIDS, according to the Australian Bureau of Statistics.
Professor Roger Byard, from University of Adelaide, said the abnormality was another strong reason why babies shouldn't sleep on their front.
"If a child has this underlying vulnerability in its brain chemistry, and its breathing becomes compromised by sleeping on its front, that child is at greater risk of death because its body simply can't respond in the normal way," he said.
"The baby can't lift its head, and its breathing and heartbeat will be compromised."
The research was funded by the River's Gift SIDS charity, founded in 2011 by grieving parents who were trying to find the cause of the sudden death of their four month old son, River.
"We are excited about these latest research findings, which show tangible evidence as to the underlying cause of SIDS in a number of cases," said River's father, Karl Waddell.
"We hope this research will eventually assist in the quest to stamp out SIDS."
To reduce SIDS deaths, Australian Children's Education and Care Quality Authority has instructed childcare centres to make sure babies sleep on their back unless there is a medical reason, even if it defies the parents' wishes.
The charity Red Nose, recognised as the national authority on safe sleeping practices for children, said it welcomed the changes to the National Law and Regulations.
"This change will ensure consistency in child care centres across the country so that every child will be slept according to Red Nose's safe sleeping guidelines, which have reduced the rate of sudden unexpected death in infancy in Australia by 80 per cent and saved 9450 lives," said Yvonne Amos of Red Nose.
The federal Health Minister Greg Hunt said he would not overrule the advice of medical experts.
"Ultimately, parents have the right to determine whichever children's care institution they place their child with," he said.
"In terms of the guidelines, those guidelines are about saving and protecting beautiful young babies and toddlers."
