Why do some infants with all or most risk factors live?

I don't know the answer to these questions. One problem is that we don't
yet know the role of these 'risk factors' in SIDS. By 'role' I mean we
don't yet know *how* prone sleeping or smoke exposure (for example) might
contribute to the death of a child. All these studies provide is
associations, not cause-effect answers. So, we know that smoke exposure
is associated with a higher SIDS rate but don't know why. There are
studies looking into whether it's nicotine exposure prenatally or exposure
of the lungs to the smoke postnatally... but there are no clear answers
yet. Same with sleeping position; we have numerous studies and some
patterns are emerging but it is not really clear yet. Same also with the
brainstem research. Studies have shown associations between SIDS and
certain brainstem findings but no cause-effect relationship has been
demonstrated. So, a major problem is that we don't yet know what role
these factors play or how they might interact with each other.

In a sense many studies already published and going on today already do
what you're asking about. That is to say, studies compare infants that
died versus infants that didn't and ask "what was the difference?" So,
the 'controls' in these studies are children that didn't die of SIDS and
the question being asked is 'how are these different from the infants that
died?'

The answer to why some infants without any *known* risk factors die anyway
is unknown but may have to do with 1) that there may be risk factors as
yet unknown, and 2) that perhaps all infants are vulnerable during early
infancy and no 'risk factor' is a necessary condition for SIDS to occur.
In other words, risk factors are just that - factors that are associated
with an *increased* risk of SIDS. However, they are not necessary
conditions for SIDS to occur.

An example of an unknown risk factor might be a subtle abnormality of
respiratory control (like Dr. Harper's group and others have described).
It is possible that some infants have it but we just don't know how to
diagnose it. So, an apparently healthy baby with this subtle disorder but
no known risk factors dies of SIDS... there may be no way of knowing
about it after death, so it would appear that a healthy baby with no risk
factors died. In the future, we might be able to diagnose this condition
in infants and then it would be identified as a known risk factor.

It may even be possible that some normal variations in developmental
cardiorespiratory control are risk factors... we just don't know how to
detect them yet.

John L. Carroll, M.D.
The Johns Hopkins Children's Center
Baltimore, MD