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Sarah Cormier with a photo of her daughter, Quinn, at home in Airdrie, Alta.

A decision by Canadian coroners to stop using the term SIDS to classify mysterious infant deaths has generated confusion among mourning parents, consternation from scientists and raised an unsettling question: Does SIDS exist?

The night of Dec. 28, 2014, Sarah Cormier laid her youngest daughter, four-month-old Quinn, to sleep in her crib before heading downstairs to relax and watch TV. She remarked to her mother, who was visiting for the holidays, that her two daughters made her feel happier than she ever had felt before.

An hour and a half later, when Ms. Cormier returned upstairs to feed the baby, she knew immediately that something was wrong. Quinn didn't appear to be breathing. She called her husband upstairs and they dialled 911. Despite the best efforts of the emergency responders, Quinn was pronounced dead that night at a Calgary hospital.

A "classic case of SIDS" is what one doctor told the Cormiers. It was a devastating, life-changing moment. No one knows what causes sudden infant death syndrome, or SIDS. But the Cormiers had heard of the mysterious condition and knew that, sometimes, babies succumbed to it despite the preventive measures taken by parents. Quinn, for instance, was breastfed, put to sleep on her back in a bassinet and no one in the house smoked – all of which are supposed to help reduce the risk of SIDS.

More than a year later, the Cormiers received a second shock when they opened the long-awaited autopsy report into their daughter's death. Instead of listing SIDS, the report said Quinn died from "undetermined" causes. Ms. Cormier was overcome with anger and disbelief – not to mention guilt – at the confusing, ambiguous finding. And the unanswered questions it left the grieving parents to ponder.

"We were back to December 28 all over again," Ms. Cormier said. "What did we do wrong? Was her bed too soft? Was she too hot? Was she scared?"

It turns out the Cormiers' report came during a major shift in the way mysterious infant deaths are classified. A decision by Alberta's Office of the Chief Medical Examiner to abandon the term SIDS in 2014 followed a similar trend by coroners and medical examiners elsewhere in Canada and other countries, who were phasing out the term altogether, except in some rare instances. Since there is no single identifiable cause of SIDS, nor a diagnostic test to confirm it, death investigators wanted a more accurate descriptor for cases of unexplained infant death. Across the country, that means deaths that would once have been called SIDS are now being classified using a combination of terms such as "undetermined" and "sudden unexpected death."

The decision to eschew SIDS goes well beyond semantics. It's turned into a fundamental disagreement over whether the condition even exists. On one side are coroners and medical examiners who say the term has become largely meaningless because it doesn't provide insight into the cause of death. They say too many infant deaths, even those where accidental suffocation is the likely cause, are being lumped into that broad category.

On the other side are doctors, researchers and parents who point to brain or other genetic abnormalities among infants who have died suddenly as proof SIDS is a real medical condition that must be further explored so a cause can be found. The decision to retire SIDS is creating significant worry among them that the once-ubiquitous term will soon disappear. And along with it, the chance to figure out what is causing these mysterious deaths – as well as how to prevent more from occurring.

"If it's not in the media, if it's not in the news, then people think it's gone," said Ian Mitchell, professor of pediatrics at the University of Calgary and SIDS expert. "The numbers of deaths will increase because we're not tracking them and we're not focused on the prevention that we know we can do."

SIDS, coined in 1969, has always been a diagnosis of exclusion: It's applied when no other specific cause, such as a viral infection or head injury, can be found. However, there are some characteristics common to "SIDS babies": 90 per cent of cases occur before six months of age, with the peak between two and four months. About 60 per cent of deaths occur in boys and cases are more likely in the fall and winter months, as well as in homes where someone smokes. Premature and low-birth-weight babies also have a higher risk of dying of SIDS.

Complicating matters is the fact there are usually some risks present when coroners and medical examiners investigate a baby's death, such as blankets in the crib or an infant that was put to sleep in the same bed as his or her parents. There's no way for investigators to tell whether the baby died of a natural underlying condition, like SIDS, or if he or she suffocated, said Dirk Huyer, Ontario's chief coroner.

"In a percentage of [deaths], and it's a significant percentage, we don't find an answer," Dr. Huyer said. "We don't find a definitive reason why those babies die. We don't have an autopsy finding. We don't have a test finding. We don't have anything positive out of this whole detailed examination."

As a result of the ambiguity, the Canadian Forum of Chief Coroners and Chief Medical Examiners, which has representatives from each province, set out in 2010 to settle the question around SIDS once and for all. The problem was the term had become confusing and all-encompassing, according to Lisa Lapointe, chair of the group. Some investigators would classify an infant death as SIDS even when the baby was found underneath a sleeping parent and the cause was clearly asphyxiation.

Provinces used different terminology, such as sudden unexpected death of an infant (SUDI) while others used SIDS or undetermined, making it difficult to compare cases across provinces or accurately track causes of infant deaths in Canada.

In 2012, the group reached consensus: Coroners and medical examiners would use the term "undetermined" instead of SIDS to describe unexpected deaths of infants under the age of 1 in cases where autopsies and other investigations yielded no apparent cause.

"You're basically left with, 'We have no idea why this child died,'" Ms. Lapointe said.

Sarah Cormier lost Quinn in December, 2014. After being told it was a classic case of SIDS, the medical examiner later ruled the manner of death to be ‘undetermined,’ leaving the family with unanswered questions and the feeling they caused her death.

Despite the agreement on the need for an overall change, there's still little uniformity in the way provinces classify mysterious infant deaths. Ontario, Newfoundland and Quebec continue to use the term SIDS, while others have adopted a combination of classifications, including undetermined and sudden unexpected or sudden unexplained deaths.

For instance, Alberta recorded its final SIDS deaths in 2013 and now uses "undetermined," while Manitoba stopped using SIDS after 2010 in favour of sudden unexplained infant death. Ontario recorded only two SIDS deaths from 2011-15, compared to the 13 recorded from 2006-10.

Undetermined is preferable because many people mistakenly believe SIDS is a cause of death, according to Ms. Lapointe. The term was created to give a name to unexplained, unexpected infant deaths, but it had taken on a life of its own, so an update was necessary, she said.

"One of the challenges is that people say SIDS is a disorder and we don't know that," said Ms. Lapointe, who is also chief coroner with the British Columbia Coroners Service. "SIDS was made to define unexplained deaths. There's no proof that it's a disorder."

In Manitoba, Mark O'Rourke, a spokesperson for the coroner's office, was more blunt: "It's just an acronym. It's not a disease."

It's a point of view that deeply troubles Ernest Cutz, one of Canada's foremost SIDS experts who recently retired as a pathologist from Toronto's Hospital for Sick Children.

"You call it undetermined [and] the file is closed, parents are left without having the answers," he said in a recent interview. "[SIDS] is a rare pediatric disease which needs to be addressed."

Dr. Cutz argues the shift to "undetermined" is largely the result of high-profile cases of infant deaths that were initially ruled as SIDS, but later found to be homicides.

One of the best known examples is Waneta Hoyt, a mother from a small town in New York State whose five babies died suddenly, of no apparent cause, in the 1960s and '70s. They were classified as SIDS deaths, but in the 1990s, a suspicious district attorney reopened the case. Ms. Hoyt confessed to smothering her children. While she later recanted, she was eventually convicted of murdering all five.

Cases such as hers led forensic investigators to adopt a more cautious approach, Dr. Cutz said. Therefore, using undetermined in place of SIDS means that "in case these parents may confess down the line," no one will "be caught with pants down."

Dr. Cutz said public-education campaigns that helped dramatically reduce SIDS deaths in recent decades may have also unwittingly contributed to the idea it's not a real condition.

The most well known is the "back to sleep" campaign, which began in the early 1990s after research showed babies put to sleep on their stomachs were more likely to die suddenly from no apparent cause.

Parents were told repeatedly to put their babies on their backs to sleep alone in a crib with no loose blankets or toys and, in the ensuing years, SIDS deaths dropped between 50 per cent and 90 per cent in countries around the world, according to the Public Health Agency of Canada. But the relentless focus on sleep position and blankets eventually led to the widespread belief that SIDS was just another word for suffocation, Dr. Cutz said.

"The assumption was made, these babies die of suffocation and there is nothing wrong with them," he said. "You change the sleeping position and you prevent SIDS."

The problem is the suffocation theory doesn't explain why so many babies survive despite being placed on their stomachs or other unsafe sleep positions, Dr. Cutz argues.

"Thousands, millions, sleep like this. Only some of them will die from SIDS," he said.

Dr. Cutz and many other researchers theorize that babies who die suddenly and unexpectedly have underlying heart, brain or other abnormalities that cause them to go to sleep and never wake up. In other words, while they may look, sound and act like healthy, normal babies, they have a condition that puts them at greater risk of sudden unexpected death.

According to that line of thinking, placing an infant younger than 12 months on his or her stomach to sleep in a hot room with a fluffy blanket doesn't cause death. Rather, it establishes the ideal conditions for the critical biological defect to manifest itself, known as the "triple risk model" in the research community. For instance, SIDS infants may have problems regulating their breathing, which is fatally exacerbated by being put to sleep on their stomachs.

Daniel Rubens, an anesthesiologist and SIDS researcher at Seattle Children's Hospital, has developed one such theory into the underlying mechanism of SIDS. He led a small 2007 study published in the journal Early Human Development that compared newborn screening tests of a group of infants that died of SIDS to a group that didn't. They found the babies who died had mildly suppressed hearing in their right ears at birth, which he believes could compromise their ability to rouse themselves and regulate breathing.

"That's not nothing. That is really unusual," Dr. Rubens said. "There is a reason these babies are dying."

He's now conducting a larger study of SIDS infants in England to see if the hypothesis holds up.

In Ontario, Dr. Huyer said he understands the objections of those who say "undetermined" is too dismissive. Like them, he believes SIDS is a real condition, but said the system could benefit from better terminology and stricter definitions. Rather than squelch research, Dr. Huyer said this new approach can help by ensuring fewer deaths are classified as SIDS without taking into account how other factors, such as the temperature of the infant's room, could have contributed to the death.

To that end, Ontario began a new initiative in January to use genetic testing on all cases of sudden infant death to look for cardiac abnormalities.

"We feel it is the right thing to do," Dr. Huyer said. "We should be supporting research of unexpected deaths in infants."

For Ms. Cormier, not a day goes by that she doesn't think about her daughter, Quinn, who would have turned 3 this summer. Now that more time has passed, she said she can understand why some medical examiners want to call deaths such as Quinn's undetermined. But she will never accept it because someone needs to find the reason these infants died, she said.

"You can't just let these babies be forgotten," Ms. Cormier said. "They lived. They were here. They were part of our family."