Sudden Infant Death Syndrome, or SIDS, is the leading cause of death in babies under the age of one. Yet why thousands of seemingly healthy infants die each year in their sleep is still not understood. Dr. David Mokler of the University of New England College of Osteopathic Medicine is among the researchers who say some babies are more at risk for SIDS than others. Mokler co-authored a new article published today in the Journal of the American Medical Association that finds SIDS babies had lower levels of the chemical seratonin in the brain.

Dr. David Mokler: What we think is going on is that the cells that regulate our breathing are not working properly in children with SIDS. And it just so happens that seratonin is the neurotransmitter that's involved with these cells. Carbon dioxide as we breathe it out, if it increases in the blood, the levels of carbon dixoide -- our brain senses that and increases our breathing and wakes us up if we're asleep. In these children that have SIDS, they don't wake up and so they don't move in order to breathe properly.

Josie Huang: I know SIDS prevention has really centered around placing a baby on their back while sleeping, as opposed to their belly. This sounds like it's particularly critical then for a baby with lower seratonin levels.

DM: Absolutely. What we found from these studies and from previous studies in the Back-to-Sleep campaign is that we could reduce the number of children dying of SIDS if they were sleeping on their back, if they're sleeping in a crib, not in the parents' bed. The crib has a firm mattress. There's no play toys in the crib while they're sleeping and they're not covered with lots of blankets because overheating and getting too hot can also be a risk factor."

JH: So your research looked at the autopsy results of 41 SIDS babies. How did their seratonin levels compare to babies who had died from natural causes, your control babies?

DM: Their seratonin level was reduced by 26 percent. We also looked at another indicator of seratonin and that was down 22 percent."

JH: Why do some infants have lower levels of seratonin in their brain tissue than others. Is it just something that they're born with?

DM: That's something we're going to definitely be asking that question in the future. Why do these children have lower seratonin levels? We don't have an answer to that as yet. We'll be looking for that.

JH: And, do we know what percentage of children have abnormally low seratonin levels that make them more at risk?

DM: At present, there is no way of really telling that, because we don't have any tests that can assess the level of seratonin in their brain. These are new findings, that the seratonin levels are actually low. The next step is to try figure to out what other children may have. Can we test for that? Can we find a way of testing for it and then treating that?

JH: If it's genetic, what can parents do with this information, short of testing, short of doing what they're already being told to do, which is placing their baby on their back while they're sleeping?"

DM: I think the main thing that parents can do is to go ahead and take those precautions about having the child sleeping on their back, not in the parents' bed, a firm mattress, and at the present time, there's not much else that can be done.

JH: Do you feel this research that you worked on about decreased seratonin levels, does that solve a lot of mystery around SIDS?

DM: It's the beginning of understanding what's wrong. When you have something as mysterious -- and it has been mysterious for a number of years -- as SIDS, where you have what looks like a perfectly healthy child, there's no indication this child has any problems whatsoever, and then they're found dead in their crib in the morning by their parents, that's tragic. So you have to try to figure out what's wrong to begin with in order to then step forward and start thinking about how you might be able to determine which child might have that risk and how to treat it.

JH: It seems at the very least it can maybe help parents who are looking for answers. Because I imagine it's unbearable for any parent to lose a child, but to not have answers may make it that more difficult.

DM: Absolutely, it makes it very difficult. And I think that that is probably one of the things that I admire about doing this work. The parents whose children have died of SIDS who are willing to give those children's brains to us to be able to do the work to determine what might be going on. There is some benefit to that, in the sense that they might be able to help find answers for other parents.

JH: Thank you very much for talking with us.

DM: You're welcome.

Dr. David Mokler teaches pharmacology at the University of New England. He is co-author of a paper on how low seratonin levels in babies are associated with an increased risk of sudden infant death syndrome. It appears in the latest Journal of the American Medical Association.