written by Tara Haelle

It’s a bit ironic that since I began researching and writing the chapter section of our book on SIDS (sudden infant death syndrome) and on bedsharing, I’ve been assigned two articles related to infant sleep deaths and then saw a study last week about a new discovery related to SIDS. It’s an area I’m particularly interested in and well-versed in, and about which I’ve blogged about before here and here.

Despite how much I had already read, however, I was surprised to learn a number of new things, so it seemed a good time to write about the new studies and to provide some hints of what’s to come in the book. The new studies are especially interesting in light of what I’ve learned in my research, such as possible physiological aspects of SIDS involving arousal, some reasons upright sleeping (such as in a car seat) may be a SIDS risk, and why soft bedding *under* a baby is a SIDS risk even if the baby sleeps on her back.

Let’s start with the study in Acta Neuropathologica last week which identified a possible brain abnormality linked to SIDS cases. Researchers inspected tissue from the hippocampus in 153 babies who died suddenly between 1991 and 2012. Some of the deaths were classified as SIDS or otherwise unexplained, and the others were due to infection, suffocation, an accident, homicide or other identifiable reasons. The hippocampus is a part of the brain involved in “autonomic function.” Autonomic function is the body’s internal management system: it’s the part of the nervous system that controls breathing, heartbeat, digestion and other bodily process that we need to survive but which we do not consciously control. The hippocampus in particular is involved in aspects of breathing and heart function.

The autonomic function also controls our ability to arouse from sleep, especially if something is wrong. It’s long been thought that SIDS occurs in part because some infants have an underlying genetic or other biological condition that affects their ability to arouse and/or their autonomic control over cardiorespiratory functions. When they are placed in a difficult environmental circumstance – such as sleeping on their stomachs or being around tobacco smoke – they cannot wake themselves up when necessary, such as if they briefly stop breathing for a few moments or have a heartrate irregularity.

The combination of some kind of breathing or heart episode and their inability to arouse themselves then appears to lead to their death. It’s possible that certain unsafe sleeping environments may even trigger the problem in the first place, such as reducing their ability to breathe well. (HOWEVER – and this is important – remember that it is never possible to remove all risks for SIDS. Even babies sleeping in safe sleeping environments, on their backs, can and do die from SIDS, and it is impossible to make 100% certain that an infant will not die in his or her sleep.)

Evidence has been building for this hypothesis, and the latest study lends even more support. Among those 153 babies, 41 percent of the ones who died for unexplained reasons (including 43 percent of those specifically identified as SIDS cases) had an abnormality in the hippocampus. Only 8 percent of the babies with explained deaths had this brain abnormality.

This is good news because it provides more clues to what might actually cause SIDS, or at least some cases of it. The abnormality was present in less than half the babies with SIDS, but researchers have already suspected that there might be multiple ways that SIDS deaths occur. At the same time, these findings are just one small step toward understanding SIDS better: this was a pretty small sample size (though that’s typical for SIDS-related studies), and the abnormality can only be identified in an autopsy, so it’s not something we could screen for.

So that study dealt with physiological aspects of SIDS. The other two I wrote about – and a third I’ll get to in a moment – dealt with the environmental aspects.

The first was actually from last month and deals with both SIDS and infant sleep suffocation deaths: the dangers of infants sleeping on sofas (the link goes to the article I wrote for HealthDay). The study, from Pediatrics Oct. 13, concluded that the sofa was pretty much among the most dangerous places for babies to sleep. Among just over 1,000 deaths examined for the study, 13 percent occurred on a sofa.

This finding isn’t news, of course, but I’m pointing it out because among the findings, babies who died sleeping on a couch were more than six times more likely “to be found in a new sleep location.” That means they don’t usually sleep on the couch. And that means some of these deaths quite likely could have resulted from parents accidentally falling asleep on the sofa, something that has been documented in other studies and will be more relevant in a moment.

The second study came out yesterday and found that more than half – yes, more than HALF – 55 percent, to be exact – of parents still use soft bedding in their infants’ sleep environments. The good news is that this number is a big drop from the 86 percent using soft bedding back in 1993. The bad news is that the proportion hasn’t budged much in the past decade, but soft bedding is still as much a risk factor for SIDS as it ever was. Clearly this message isn’t getting out as much as it needs to.

I wrote about the study for HealthDay and was impressed as I read it that the data set was so large: 19,000 parents interviewed between 1993 and 2010. But I also remember reading that the use of blankets *under* babies had increased from 26 percent to 32 percent over that time and wondering about the mechanisms for SIDS with soft bedding under sleeping babies. It makes sense that sleeping on pillows provide opportunities for babies to roll or sink into the pillow – a dangerous situation – but why would softness under a baby sleeping on her back and not yet rolling be a problem?

That wasn’t answered for me until this weekend, when I read an older study from New Zealand which explained how infants’ underdeveloped jaws, when pressing down on their chests, might restrict their airways. This position explains why studies have shown reduced blood oxygen when babies sleep upright in car seats, for example. Their chin falls to their chest, slightly restricting their ability to breathe easily. (Hence the reason for car seat challenges for preemies, actually.)

A baby sleeping on a particularly soft surface may sink just enough to collapse inward the tiniest bit, pushing their chin toward their chest and preventing fully open airways. This is also the reason firm mattresses, particularly in bedsharing arrangements, are emphasized for safe sleep. (I can’t help but think that if this mechanism were better explained to parents that they would take the firm mattress/no soft bedding recommendations more seriously.)

And that brings us to the finding that about 70 percent of the babies sleeping with soft bedding – either with blankets (the most common item), comforters, pillows or similar items – were on adult beds or sharing their sleep surface with someone else. Once again, the authors in this study emphasized the dangers of bedsharing as a risk factor for SIDS – the very issue I’ve previously written about.

But here’s the thing: as I read study after study after study the past few weeks and pored over the AAP’s Technical Report for safe infant sleep, I was struck by a maddening realization: there is not one study – not one – that I could find in which bedsharing was found to increase the risk of SIDS *after controlling for all possible confounders*. There were plenty finding associations, frightening large associations. But, like the past ones I’ve analyzed, they didn’t adequately control for all the other known SIDS risk factors. It’s possible I missed some (there are a LOT and I haven’t read every single one), but the only studies I found that actually controlled sufficiently for confounders showed no increased risk once those factors were considered.

In fact, one such study just came out a few months ago. I found out about it through Science of Mom’s excellent post on it and then read it myself in PLOS ONE (freely available). I won’t go into the details because Science of Mom has already done that nicely and this post is already running long, but here’s the key sentence from the abstract: “The multivariable risk associated with bed-sharing in the absence of these hazards was not significant overall, for infants less than 3 months old, and was in the direction of protection for older infants.”

Multivariate risk refers to the risk after controlling for protective factors such as breastfeeding and risk factors such as sofa co-sleeping (rather than bedsharing), parents who smoked or drank alcohol and premature birth or low birth weight. This finding is pretty much what I’ve seen in past studies going back more than a decade (several of which included the same lead author, Peter Blair out of the U.K., also quoted in my soft bedding article linked above).

Basically, if babies were bedsharing with a parent without at least three risk factors present – ie, in non-hazardous conditions – the risk of SIDS dropped to the same rate as if the child were sleeping alone in a crib or bassinet. This study fits with my critiques of past bedsharing studies and even fits with the data in sofa study discussed above. Remember I suggested that parents might be accidentally falling asleep on the sofa? Past studies by Blair have found that to be the case – and they found it in some cases among parents who took their child to the couch because they were trying to avoid sharing a bed, as they had been instructed to do.

So, these studies and dozens more have been percolating in my brain as I finish up the chapter section in the book on SIDS and bedsharing and are helping in piecing together the puzzle of how we can hopefully eventually begin further reducing the risk of SIDS and other infant sleeping deaths.