Australian researchers believe they know what causes sudden infant death syndrome.

In a study published in the The Lancet’s eBioMedicine journal, they used dried blood spots taken at birth as part of Australia’s newborn screening program to look for what caused some apparently healthy babies to die mysteriously before they were a year old, usually in their sleep.

The culprit appears to be low levels of butyrycholinesterase, an enzyme in the autonomic system, which controls activities like unconscious breathing and the ability to wake up.

“Babies have a very powerful mechanism to let us know when they are not happy,” lead researcher Dr. Carmel Harrington told The Sydney Children’s Hospitals Network. “Usually, if a baby is confronted with a life-threatening situation, such as difficulty breathing during sleep because they are on their tummies, they will arouse and cry out. What this research shows is that some babies don’t have this same robust arousal response.”

The researchers believe the enzyme is present at birth in SIDS babies and “represents a measurable, specific vulnerability prior to their death.” That’s especially important because it could mean that infants at risk of dying from sudden infant death syndrome could be identified. The finding makes room for more research into interventions, the study said.

Known and unknown

The Centers for Disease Control and Prevention counts about 3,400 sudden, unexpected infant deaths each year in the United States. Some of them are attributed to SIDS and some to accidental suffocation or strangulation. And sometimes the cause is simply not known. In 2019, 1,250 deaths were classified as SIDS deaths.

“The Mayo Clinic suspected that SIDS was caused by a defect in the part of the brain that controls arousal from sleep and breathing. Researchers believed that when the baby stopped breathing during sleep, the defect would prevent the child from startling or waking up,” Los Angeles-based KTLA reported.

The findings from researchers at the University of Sydney and the Children’s Hospital Westmead in Sydney bolster that belief.

Researchers have for some time considered a “triple-risk” model of factors that could lead to sudden death: a vulnerable infant, a critical developmental period and an external stressor. “The infant’s vulnerability lies latent unless subjected to an exogenous stressor during the critical period,” the study said.

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Identifying the enzyme would check the box for the vulnerability that puts an infant at risk.

The Australian researchers found that SIDS infants exhibited significantly lower levels of the enzyme, compared to infants who died of other causes than SIDS and compared to levels in the date-of-birth and gender-matched controls.

The researchers believe the vulnerability likely beings during gestation. Smoking in pregnancy is associated with a three-fold increase in risk of sudden infant death, they noted, along with impairment of the autonomic system.

They said they don’t know how the level of butyrycholinesterase changes beyond the point where the infant’s heel is pricked to take blood a day or so after birth because they used the blood from those heel pricks for their finding.

The American Academy of Pediatrics said that infant deaths declined somewhat after parents started following advice to lay babies on their backs to sleep, in an area without thick blankets or a crib bumper or other clutter. But there are still sudden, unexpected infant deaths and the declines have since leveled off.

Harrington lost her own child, Damien, to SIDS nearly three decades ago and has spent years trying to unravel the syndrome’s mystery, she told the Australian Broadcasting Corporation. She hopes the finding that babies have low levels of the enzyme will not only open the door to developing ways to address the enzyme levels, but will also comfort parents who feel that they might have prevented their child’s death if they’d done something differently.

“These families can now live with the knowledge that this was not their fault,” she said.