Amanda Ruđeri
BBC Future
It was three days before the twins' second birthday.
When their mother, Carmel Herrington, put them to bed, she expected Charlotte to wake her up several times during the night.
Her brother Damien - a sweet, playful child with brown curls and a passion for motor toys - always slept more soundly.
Instead, Herrington woke up the next morning feeling rested.
But when she entered the twins' room, she encountered a parent's worst nightmare. Damien died during the night.
There are hardly any more terrifying scenarios for a parent.
Despite 30 years of public health efforts, it is still a reality for thousands of families around the world every year.
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In Australia, where Herrington lives, sudden infant death syndrome (SIDS) remains the leading cause of infant death.
About 100 babies a year die suddenly and unexpectedly, usually in their sleep (this falls into a category called SUDI, or "sudden unexpected death in infancy," and includes SIDS, accidents such as suffocation or strangulation, and unknown causes).
In Great Britain, around 300 babies die suddenly in their sleep every year.
In the United States (US), SIDS is the leading cause of death for infants between one month and one year of age.
So in 2020, about 1.389 babies died in the USA.
That same year, another 1.062 babies died of unknown causes, and an additional 905 deaths were ruled accidental suffocation or suffocation in bed.
Despite the personal tragedies each of these cases represents, the overall numbers are often interpreted as relative success stories - they used to be much worse.
In the United States in 1990, for example, for every 100.000 live births, 155 infants died suddenly in their sleep from causes including SIDS.
Ten years later, that rate dropped to 94 per 100.000 live births.
A similar decline was recorded around the world.
But over the past 20 years, the decline has stopped. Thus, in 2020, the rate was 93 newborns per 100.000.
And while in many countries, advances in health care, vaccines and improved medicines have helped to solve and largely eradicate many other conditions that once killed thousands of young children - measles, mumps, polio and whooping cough, to name a few - sudden overnight deaths mostly remain a mystery.
Not only is there no "SIDS vaccine," but SIDS itself is a diagnosis that remains after doctors have ruled out all other possibilities.
If there is no clear cause of death, SIDS is often found on the coroner's certificate.
We also don't know what causes SIDS.
"For many years we thought there was something called SIDS.
"That's not the case now," says Richard Goldstein, a leading SIDS researcher and specialist in pediatric palliative care at Boston Children's Hospital and Harvard Medical School.
He adds that SIDS is a description of the outcome.
"The outcome is that an apparently healthy infant falls asleep and dies during sleep for no apparent reason," he says.
The truth is that we know many ways we can reduce the risk of sudden unexpected infant death.
For this reason, parents are advised to vaccinate their child regularly, not to smoke, to share a room with the baby, to have babies sleep on their back and to never sleep with the baby on a couch or chair, in soft bedding or under the influence of alcohol or drugs.
Among other guidelines to reduce the risk of SIDS, parents are also advised to ensure that the baby is never in a chin-to-chest position as this narrows their airway (which can happen in a carrier, car seat or sleep nest ), and not to swaddle the baby too much as this can lead to overheating.
Inclined sleep products, such as sleep nests and crib (or bassinet) bumpers, have recently been banned in the US due to the risks they pose, but remain on the market in other countries.
One reason mothers are also encouraged to breastfeed is that it is associated with a lower risk of SIDS.
Some of these guidelines come down to basic baby physiology.
"The most important thing for your baby is to consider that what she needs while she sleeps is very important.
"It's different from what adults need when they're sleeping," says Anna Pease, a research associate at the University of Bristol in the UK who researches SIDS prevention options.
"Babies prefer nose breathing. For the first few weeks, they only breathe through their nostrils while they sleep.
"They're little, tiny holes and they have to get all the oxygen they need to live through those little nostrils. That's why they need a clean face.
"Okay, that's nice, they want to lean on a teddy bear, or you covered their face with a blanket - put it away. It may all look sweet to you, but these nostrils keep them alive. Make sure they are not covered in any way," he says.
Babies and young children also overheat more easily than adults, which is why families are advised not to overdress children, but also to avoid common behaviors such as covering strollers for shade.
But when it comes to SIDS, in particular, minimizing the risk is not the same as eradicating death—and understanding what increases the risk of SIDS is not the same as knowing what causes it.
One evaluation of 4.929 cases of SUDI in the US, which includes SIDS, found that nearly three-quarters occurred in sleep-related circumstances with at least one risk factor.
But it means that more than one in four infants who died were in environments that appeared to fully follow safe sleep guidelines.
Similarly, a review of child protection practices in all SUDI deaths, including SIDS, in England and Wales in 2018 and 2019 found that "modifiable factors" were identified in 60 per cent of 325 cases.
In 40 percent of cases, such risk factors were not found (which, it is worth noting, may also be due to the absence of information).
It is also worth keeping in mind what is considered a risk factor.
In the US, for example, this means any deviation from a certain set of circumstances, where a child is found on their back in a crib or bassinet with nothing but a safety-approved mattress and fitted sheets.
Some cases categorized as risk factors may be babies co-sleeping with a parent who has abused alcohol or drugs while others may simply be a baby on their back in a crib with a teddy bear nearby.
There is another problem.
"We don't have comparative data to show how many children with the same risk factors don't die - we don't understand the denominator," says Goldstein.
Herrington, for example, followed all recommendations on safe sleep.
To her, as to most SIDS researchers today, there had to be a reason why some babies in distress could wake up while others, like Damien, could not.
(Technically, the diagnosis of SIDS is now only used for babies under one year old. Damien's death today would be considered "sudden unexplained death in childhood," or SUDC. But many researchers think the phenomena and causes may be similar.)
"I couldn't understand why this baby wouldn't wake up.
"We have really powerful survival mechanisms. "A baby wakes us up pretty quickly when it's not happy," Herrington says.
Damien died in 1991.
By 1994, Herrington, who already had a degree in biochemistry, quit her job as a lawyer to devote her career to SIDS research.
Last year, after more than 25 years, she was ready to give up.
In December 2021, she was certain that her latest study, which examined whether there was any association between a potential biochemical marker called butyrylcholinesterase (BChE) - an enzyme that plays a key role in autonomic functions such as breathing and sleep - and SIDS -a, she did not indicate that she would find anything.
She thought about retiring, to spend her time painting or improving her French.
Then she checked the numbers. In a sample of about 700 babies, including 26 who died of SIDS, infants with SIDS had, on average, reduced BChE activity -- measured in dried blood spots two or three days after birth -- compared with infants who did not. died of SIDS.
Fueled by an enthusiastic press release, the research became known around the world.
Headlines consistently trumped Herrington's study, claiming that she had found the "cause" of SIDS—or, alternatively, that a "cure" was imminent.
Unfortunately, neither one nor the other is true.
"This is just a biomarker. It's not the cause.
"At this stage, what it shows is increased vulnerability. The study has yet to be confirmed by an independent laboratory. But the world is so hungry for answers because we just don't know which babies are going to die," says Herrington.
A modern epidemic
The sudden death of babies overnight is a tragedy as old as history itself.
One early literary reference dates back to an Old Testament story in which King Solomon settles a dispute between two mothers over whose baby is theirs.
One of the mothers lost her child during the night because she "covered" him.
"Suffocation" has remained a strong fear for centuries.
During the 1800s, in Western countries, more families began to put infants in their rooms.
But the babies continued to die.
Not only that, but during the 20th century rates in many countries rose sharply - despite being lower in countries like Japan, where families often shared a bed.
It was clear that something more than a "cover-up" was going on.
(In fact, we now know that a newborn who sleeps in his own room is associated with a higher risk of SIDS, which is why room-sharing is generally recommended.)
"In the late 1980s, when SIDS was much more common, there were around 1.500 deaths a year in the UK.
"Everyone seemed to know someone, or knew someone who knew someone whose baby died, for no apparent reason, in their sleep," says Pease.
At first it seemed that the answer to what was happening was how the babies were put to sleep.
In the Netherlands, for example, SIDS deaths more than doubled after parents were told to put babies to sleep on their stomachs.
It is now clear that putting babies to sleep on their stomachs, also known as the stomach position, is associated with a far greater risk of SIDS.
This may be because it seems to disrupt the baby's wake-up response - which would normally upset them if, for example, they were having trouble breathing - and also makes them more likely to overheat.
Various countries have begun to run public health campaigns, emphasizing the importance of safe sleep guidelines.
These campaigns are largely responsible for halving SIDS rates, and the rate of infants found to be prone to SIDS has dropped significantly.
But although the campaigns were known as "Back to Sleep", this is too simplistic.
In addition to telling parents to put babies on their backs, they emphasized factors such as smoking, which is associated with an increased risk of SIDS.
It's also notable that SIDS rates have fallen at the same time as infant mortality from other causes—a signal that systemic changes, such as more universal prenatal care, may have played a role.
Identifying risk factors for SIDS is important.
It saved thousands of lives. But it does not lead to the cause of SIDS, nor does it provide a cure.
"My day job is dealing with families that have gone through this.
"On their behalf and on my behalf, I would say, 'Do whatever you can,'" Goldstein says.
"But as researchers who want to eradicate SIDS, we want to know what the terminal cascade is so we can address it directly," he adds.
What you need to know about SIDS
Sudden infant death syndrome is defined as the sudden, unexplained death of an apparently healthy baby, usually during sleep, which includes napping.
It remains the leading cause of infant death worldwide.
SIDS falls under a broader category called SUDI, or "sudden unexpected death in infancy," which includes SIDS, accidents such as suffocation or strangulation, and unknown causes.
Because determining whether a baby died from SIDS or an accident such as suffocation can be complex, it is common to look at the overall SUDI rate to see if the rates are improving or worsening.
In some countries, including the USA, after a sharp decline in the SUDI rate during the 1990s, the SUDI rate has stagnated.
Researchers are still determining what causes SIDS.
Hopefully, the more we can understand about what causes SIDS, the more we can prevent it from happening.
The SIDS conundrum
There are a number of reasons why SIDS has been so difficult to unravel.
One is that it happens during sleep, where the baby is usually not observed.
Another is how SIDS is categorized.
In the earlier days of tracking SIDS deaths, medical examiners often categorized the death of a baby during sleep as SIDS.
The classification is now more precise, meaning that some of the percentage drop in SIDS rates could be an artificial statistical effect (although the researchers point out that, again, the drop in sudden unexpected death reflects overall infant mortality, so this effect is unlikely to play itself a big role).
Currently, Pease says, in countries including the UK and the US, a SIDS death is defined as one in which - even after an autopsy, assessment of the clinical history and investigation of the circumstances - there is no specific cause of death.
But even in those cases, many pathologists feel more comfortable writing the term "undetermined," saying that they believe SIDS implies a single, unknown pathologic factor.
This means that SIDS rates could be underestimated.
There may be more cases of SIDS than we think, for other reasons.
If a baby is found lying on its stomach, for example, many pathologists will diagnose the cause of death as something like "positional asphyxia" rather than SIDS — even though most babies in those circumstances will wake up and change positions, says Torliv Ole Rognum, a forensic pathologist and lead SIDS researcher at the University of Oslo, Norway.
Or, as Goldstein and others wrote in a recent article, medical examiners often say the cause of death is "suffocation" if they see a sleep-related risk factor in place — such as a blanket or pillow — even though there's no physical evidence the child had difficulty breathing.
The investigation process also varies from country to country.
In Norway, says Rognum, a complete autopsy is performed within 48 hours of death, which includes one particularly important element - microbiology.
In the UK, by comparison, it takes a week to carry out a medical autopsy and a month to complete a forensic autopsy.
"That's strange.
"What can you find after a month's autopsy? We've shown that you can't trust microbiology more than 48 hours after death," he says.
Moreover, as Goldstein and others write, sudden infant deaths are evaluated outside institutions that investigate undiagnosed diseases.
Often initially treated as criminal investigations, they receive forensic autopsies, which do not include techniques such as advanced phenotyping and molecular investigations used for clinical diagnoses.
This is despite the fact that SIDS is "the ultimate undiagnosed disease," says Goldstein.
"In addition to failing to address the emotional pain and confusion of families, we allow the autopsy to focus primarily on the legal issue of manner of death rather than the medical issue of biological and environmental causes."
"The current approach can answer questions about maltreatment, but leaves any effort to learn more about etiological factors, including possible risks to siblings, bereaved family members. Medicine does not attempt to exhaustively explain these deaths, although this would be routine in other disease areas," they write.
All of this made it difficult to obtain a representative sample of SIDS deaths.
That's a challenge that's heightened by the sensitivity of the research process, especially in studies that look at tissue or blood samples like Herrington's, because parents have to agree to be part of the process.
Because the survey relies on parents who feel "able or interested" to contribute, Goldstein says, white, Western and relatively affluent families are overrepresented.
This is even though the risk of SIDS is far greater among families of lower socioeconomic status, and even though in countries like the UK, the socioeconomic gap in the likelihood of a child dying suddenly in infancy is getting worse.
"The sociodemographics of the children most at risk of SIDS are the sociodemographics of those least likely to participate in the study.
"It's also fair to say that they are the sociodemographics of people most likely to be subjected to harsh treatment and suspicion during the response to their child's death," he says.
Like all sudden and unexpected deaths, SUDI cases are initially treated as suspicious from the outset, meaning the first step is to rule out any fault by the guardian or parent.
Then there is the relative lack of research funding.
At the US National Institutes of Health, the world's largest source of funding for medical studies, SIDS is one of the least funded areas of pediatric research.
However, even with these obstacles, researchers are getting closer to understanding SIDS.
Getting closer to the answer
Despite widespread coverage of the topic, Herrington was not the first researcher to find a potential "SIDS biomarker."
Nor are we completely ignorant of what the possible causes could be.
First, there's another reason why SIDS deaths have dropped over the years—we've discovered why some of these babies die.
It has not eradicated what is categorized as SIDS. This means that in some cases we managed to prevent those deaths.
Take fatty acid oxidation disorders (FAOD), genetic defects that disrupt the body's ability to produce energy and can be fatal within hours.
We now know that up to five percent of deaths once categorized as SIDS are caused by FAOD.
Today, newborns are routinely screened for the most common FAOD, medium-chain acyl-CoA dehydrogenase deficiency (MCADD), which affects one in 20.000 infants of Northern European descent.
Newborn screening is not a cure-all. But in the case of MCADD, a rare condition in which the body cannot efficiently break down fat for energy and which can be especially dangerous if a person goes without food, it saved lives.
Rognum remembers one small child who died after vomiting for just a few hours.
An MCADD mutation was discovered during the autopsy, as well as liver disease, a likely result of MCADD.
"It shouldn't happen anymore. If we know about that (MCADD) diagnosis, we'll make sure that child doesn't get hypoglycemic and die.
"I think we will find more such explainable causes. Then they need to be removed from the 'cake' that makes up SIDS," he says.
Most researchers, however, believe that even if more accurate diagnoses of diseases like MCADD help offer explanations for some deaths that would otherwise be classified as SIDS, it will never be the whole story.
There is an essential phenomenon shared by many infants who die of SIDS. And it has to do with their awakening from sleep.
This theory emerged in the 1980s, when hospitals began using medical monitors for newborns, including those who died of SIDS under medical care.
Looking at this new data, researchers began to notice that infants with SIDS had different sleep patterns.
Specifically, babies who died of SIDS moved less and had fewer spontaneous awakenings, or episodes of fussing and waking.
When they looked back at the babies' data in the moments before they died of SIDS while on monitors, the researchers also saw something else.
Safer sleep for babies
While we can't be sure what exactly causes SIDS, we do know that certain sleep practices are associated with a higher risk of babies dying during sleep.
This is why the British Lullaby Trust recommends the following for every sleep, including naps:
- clean space without crib bumpers, blankets or pillows
- a mattress that is firm, flat and not tilted
- no sleep positioner or nest
- nothing within range that could pose a threat, such as shoelaces or loose blankets
- always in the same room as you for at least six months (US guidelines say 12 months)
- put the baby on his back
- don't wrap her in too many layers or too heavy a sleeping bag
- never sleep with your baby on the sofa or armchair - remember that this often happens by accident, so move to a safer sleeping location if you are sleepy
- never share a bed if someone in the bed smokes, drinks alcohol, takes any medication that can cause drowsiness or inhibit the reaction, if there are pets or other children in the bed, if the baby can get trapped in any crevice (e.g. mattress and wall), or if the baby was born before 37 weeks of pregnancy or weighed less than 2,5 kilograms at birth
When a healthy baby has too little oxygen or too much carbon dioxide, Goldstein explains, their breathing stops ("pause apnea") before they start gasping.
"Those breaths will usually, in a healthy baby, cause the heart rate to increase.
"Those babies get excited and wake reflexes occur: they bend over, they yawn, they turn over, they wake up and they cry, and that clears most babies of relatively modest obstacles and they survive.
"And babies with SIDS didn't do that. They didn't wake up and remained 'untethered' between these agonal breaths, which are triggered by certain brain centers, and the cardiac response," says Goldstein.
That means a "vicious cycle" in which the feedback system doesn't work, ending in coma and death, Rognum says.
Why? In Norway, Rognum, together with pediatrician and neuroscientist Ol Didrik Saugstad, came up with the theory of the "fatal triangle", which they defined as "a vulnerable period after birth, some genetic predisposition and a trigger event".
In the US, around the same time, a team led by Goldstein and Hannah Kinney from Boston Children's Hospital came up with a similar idea, the "triple risk model".
The latter designation became famous and it is this theory that is now the leading explanation among SIDS researchers.
It gets to the heart of what scientists have suspected since at least the 1970s - SIDS is not caused by a single event, but by several factors coming together.
"There is not just one reason.
"We put it more in the category of expressing a rare undiagnosed disease where at least some of the time, in its initial presentation, it is incompatible with survival," Goldstein says.
Rognum noted that the period of greatest risk for dying from SIDS, between the second and fifth months after birth, is also a period in which the immune system develops rapidly.
"When something develops very quickly, it is also unstable," he says.
It is a vulnerable period after birth.
The trigger can be a seasonal respiratory infection or a tendency to sleep, or both together - a pairing that increases the risk of SIDS by 29 times.
(Rognum points out that the risk of SIDS in the earlier, not the later, months of life is less dramatic than it once was, and death remains a risk even after the fifth month of life, contrary to popular belief.)
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However, what the "predisposition" is may be the most enduring puzzle at the heart of SIDS.
In recent years, however, this aspect has also become less of a mystery.
Researchers, including China, thought it might be a problem with the serotonergic system - neurotransmitters centered in the brainstem that regulate a number of automatic processes, including sleep and breathing.
Over the past 20 years, Kin's team has refined the hypothesis through multiple studies.
Specifically, elevation of serotonin (5-HT) in the blood is a biomarker for SIDS in about 30 percent of cases. And their findings were confirmed by other teams.
One study of autopsies, for example, found that serotonin levels were 26 percent lower in SIDS cases than in healthy infants—a biomarker discovered before Herrington's findings.
Similarly, Rognum thought that the genetic element could be due to variants, or polymorphisms, in the genes that make interleukins - which can be either anti-inflammatory or pro-inflammatory molecules.
They are usually produced in response to damage caused by infection or injury, so variants in these genes can make this part of the immune response weaker or stronger than it should be.
“We found in the cerebral spinal fluid that SIDS cases had significantly higher levels of interleukin-6. It's the interleukin that gives us a fever.
"Half of the SIDS cases have levels in the same range as children who died of meningitis and septicemia without having those illnesses," Rognum says.
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In one study, Rognum, Kinney and other researchers examined how both of these findings might come together, looking at whether infants with SIDS were more likely to have a compromised serotonergic system that altered the way the interleukin-6 receptor is expressed.
The answer was yes.
Specifically, the study showed that - in the part of the brain stem that includes protective responses to carbon dioxide accumulation - the babies showed abnormal expression of the interleukin-6 receptor.
The accumulation of carbon dioxide, of course, can be the result of rebreathing due to, say, the tendency to sleep.
“Are SIDS infants 'normal' infants who accidentally die from suffocation?” Kinieva and other researchers wrote in another study.
On the contrary, brainstem research suggests that they have a basic vulnerability in the serotonergic alarm system that makes them susceptible to sudden death, i.e. the triple risk model.
Researchers have even demonstrated the same effect in mouse pups, if serotonergic transmission is inhibited, the pups are less likely to recover from apnea - where breathing suddenly stops during sleep - caused by a lack of oxygen and are more likely to die.
It's a sequence that's identical to what researchers saw in sleep monitor recordings of infants who died of SIDS.
Other DNA defects have also been implicated.
A variant of the SCN4A gene, for example, which affects respiratory muscle function and is seen in some neuromuscular disorders, is associated with a higher risk of SIDS.
But while there is a genetic element to SIDS for some babies, there is no "SIDS gene".
One study found that genes associated with known diseases that appear to contribute to sudden infant death syndrome account for about 11 percent of known cases, while another found that it could be 20 percent.
Both are significant numbers, but far from explaining every death. Also, there appears to be a link between SIDS and epilepsy.
While babies who die of SIDS are more likely to show these conditions or biomarkers, not all of them do.
And while these biomarkers are sometimes present in children who die from SIDS, they can also be found in children who do not die.
The triple risk model remains more of a description than an actual explanation of SIDS.
It is still unclear why some babies are born with these problems and others are not. And, perhaps most importantly for parents, in addition to reducing risk factors in the sleep environment, it's still difficult to know how to translate this broader information into saving babies.
"Serotonin testing during autopsy has never been generalized into clinical care.
"No one really knows what to do with it. It's complicated science — it hasn't been translated into clinically applicable testing," Goldstein says.
This raises the question of what to do with SIDS biomarkers in general.
The researchers, including China, wrote that they hope the information can be used to screen for and prevent death using "neural biomarkers and brain-related treatment strategies."
But we're not there yet. Take Herington's prospective finding of lower levels of BChE activity in infants who died of SIDS.
This can be an indication of problems with breathing or other autonomic functions.
While the average level of BChE activity differed between the babies who died of SIDS and the control group, the BChE ranges between the groups overlapped, meaning it may be difficult to determine exactly what level of BChE may indicate a problem, if any.
Even assuming that there is a level that could be used to identify a potential problem, what to do with that information is an ethical dilemma.
All parents have already been told to follow safe sleep guidelines.
The "brain-related treatment strategies" that Kinney mentions do not yet exist.
In the absence of further advice for parents whose children may be vulnerable, and with the fact that most children with these biomarkers will not die of SIDS, you may be unnecessarily frightening families.
Without a single cause, SIDS remains a tragic combination of factors.
But SIDS is not a complete mystery either.
Not only that, but the more we learn about it, the more we know that safe sleep habits - while crucial - aren't the whole story.
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That knowledge is important, not only so we can move closer to eradicating SIDS, but also for families who often face shame, embarrassment and guilt on top of their grief, says Goldstein, who has worked with hundreds of grieving families during his career.
He points to the fact that only about 18 percent of SIDS deaths have sufficient evidence to support suffocation, or that SIDS deaths are much more likely to occur in day care centers than in family homes—although they are no more likely to infants in daycare will be prone to or be on unsafe sleeping surfaces than those cared for by their parents.
"There is an insinuation of neglect, and perhaps that is sometimes the case.
"But SIDS is not a story about parents who don't listen to doctors and ignore advice," says Goldstein.
Even when cases involved a situational risk factor where carers did not follow all safe sleep guidelines, according to a review of SUDI incidents in England by a child protection panel, “in none of the (14 reviewed) cases was there any suggestion that parents intended to harm the child - on the contrary, most of these parents seem as devoted, loving and caring as any other parent."
The panel, however, recommended that, in addition to telling parents what to do, the guidelines should explain why.
For example, explaining how a baby's small airway can be narrowed if it sleeps in a chin-to-chest position.
Peter Sidebottom, the report's panel leader, added that factors beyond the family's control could also be risk factors.
The health visitor may not have given the parents safe sleeping tips, for example, or the family could have been evicted from their home and ended up in temporary or inappropriate accommodation.
But blaming caregivers is a story as old as the story of sudden infant death syndrome.
Take the Old Testament story of the two mothers mentioned earlier.
"There is no sympathy for that other woman.
"You could easily imagine that explaining the depth of despair at the loss of your child. But she's just dropped in the story," says Goldstein.
This goes back to a central argument he and other researchers share: It's important to continue educating parents about safe sleep.
But identifying risk factors will never be a magic solution. In order to find a cure for SIDS, science must first discover the cause.
"We are not there yet. We're not even close," Herington says.
But she can now count her contributions to SIDS research and the long road to finding answers.
“If you ask me what I'm hoping for, my hope has always been, since Damien died, to be able to do something in the infant phase, and we can screen and protect.
"That was always my hope. This discovery gave me more hope," says Herington.
"Losing a baby and burying it is indescribable"
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