Monday, October 13, 2008


A SIDS study

The idea of babies dying in their sleep, unexpectedly and without explanation, frightens and horrifies us. Of course, the idea of adults dying that way does too, or should... but when it’s babies, few-month-old bundles of cuteness and love, we find it especially jarring. So we give it a name — SIDS, Sudden Infant Death Syndrome — and we grope for explanations, trying to find out what the problem is and to stop it.

One problem with doing that is, of course, that we’ve just given a name to something vague. We may talk of a SIDS “diagnosis”, but it’s not so in the same sense as “liver cancer”. A diagnosis of SIDS means, “Your baby died, and we don’t know why,” a non-diagnosis, really. We assume that there are enough cases that are from the same cause that we can isolate at least one cause of at least some cases.

But it makes studying it difficult. And, of course, making it more difficult are other factors, such as the relative infrequency of SIDS and the fact that there are serious ethical issues preventing many of the ways medical issues are typically studied.

The result is that the studies we have are anecdotal, and we’re using a trial-and-error approach to it. Or better to call it a trial-and-progress approach, because progress has been made: there was a 56% reduction in the incidence of SIDS between 1992 and 2003.

There’s a recent study, published in the Archives of Pediatrics & Adolescent Medicine that adds itself to the mix. Unfortunately, one needs a subscription to read it, so I have to go by the news articles, which are notoriously spotty on reports of scientific studies. The New York Times says this, for example:

Since 1992 the rate of SIDS deaths has dropped by more than half, to about one death per 2,000 live births from 2.4 per 1,000. The decline is linked to a national “Back to Sleep” campaign that promotes putting babies on their back instead of their stomach, which has been shown to lower the risk of sudden death.
That’s a reduction from .24% to .05%. In what sense is that merely “more than half”? I suspected that it’s an error, and that at least one of the numbers is wrong.

Checking other reports turns up that the Times article tried to normalize the numbers and made an error in the process: it should have said 2.4 per 2000, not 1000. So the correct numbers are .53 SIDS deaths per 1000 live births in 2003, and 1.2 per 1000 in 1992, .053% vs .12% eleven years earlier.

Putting that in other terms, it means that in 1992 the rate was one SIDS death in about 833 live births, and in 2003 it was one in about 1887. We’ll come back to this after looking at what WebMD says about the study:

One theory is that SIDS is caused by the buildup of carbon dioxide when infants with inadequate sleep arousal responses re-breathe exhaled air trapped by bedding or proximity to other sleeping family members.

With this idea in mind, Li and colleagues from the research division of the California-based managed health care group Kaiser Permanente hypothesized that increasing airflow near a sleeping baby would help protect against sudden infant death syndrome.

To test the theory, they interviewed the mothers of 185 babies who died of SIDS in Northern California and Los Angeles County from 1997 to 2000. The average length of time between the SIDS death and the interview was 3.8 months.

The mothers of 312 children matched for age, area of residence, and socioeconomic and ethnic background to the SIDS victims were also interviewed.

Sleeping in a room with an open window was found to reduce the risk of SIDS by 36%, while sleeping with a fan in the room was associated with a 72% reduction in risk.

And the NY Times adds that the benefits of a fan along with the previously identified “safe sleeping environment” are “not statistically significant”, though there’s a 16% reduction in risk. How can that be?

It’s that statement that really points out what the problems with this sort of study are, and where its limitations lie. Let’s go back to the numbers I computed earlier: in 2003 you’d have to look at 1887 live births to statistically expect one SIDS death.

The researchers interviewed parents of 185 SIDS victims in the late 1990s. Even if we assume that the rate of death was 1 in 1000 then (somewhere between the 1992 and 2003 rates), that means we’d need 185,000 live births to expect that many SIDS deaths. If we looked at 18,500 live births, we’d expect to find only 18 deaths — still enough to do statistics on this. With 1,850, two deaths would be against the odds.

But that’s still six times the number of survivors they actually looked at.

If you talk with only 312 parents of living children, even if they had ignored all the “safe sleeping environment” recommendations, even if that group had the 1992 death rate applied to it, you’d only expect a 37.5% probability of a single SIDS death.

That means that things have to be pretty glaring before they have any statistical significance with a sample that small, and even then I wouldn’t call it a “reduction in risk”. At best, we can say that there’s a correlation — but we have to be careful about confusing correlation with cause.

So, should we be running fans in babies’ rooms when they sleep? Sure, it seems harmless and there’s some small indication that it might help. Mostly, it needs more study, but the only way to study it is to recommend it, have parents do it widely for a few years, and then see what happens to the SIDS rate by, say, 2011.

We’ll get there... but until we can actually confirm causes of this, it’s going to remain difficult to study and hit-and-miss to prevent.

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