Researchers in Seattle think they may be one leap closer to understanding why some infants die mysteriously in their first year of life.
They reported that all babies in a small study group who died of sudden infant death syndrome - the exact cause of which still eludes doctors - shared the same abnormality in their right inner ear in a hearing test administered at birth.
This knowledge might someday enable doctors to identify newborns at risk for so-called crib death through a routine exam, the authors wrote in this month's issue of the journal Early Human Development.
"Everything about SIDS has been elusive, but there's definitely something going on here," said Dr. Daniel D. Rubens, a pediatric anesthesiologist at Seattle Children's Hospital who led the study. "There's a new way of looking at SIDS."
SIDS experts elsewhere said they were excited by the prospect of this new avenue of research. But they cautioned that the study was small and needs replication to gauge whether there was truly a link between a simple hearing problem and the risk of unexplained baby deaths.
"If this turns out to be true in other studies by other people, it's very important," said Dr. Robert Meny, a Towson pediatrician and chairman of the American Association of SIDS Prevention Physicians. "It would be a big breakthrough, but there have been a lot of 'breakthroughs' in SIDS, and a lot of them turned out to be not true over the long term."
Still, he said, "I've never seen anything like this before."
Said Dr. Ann Halbower, director of the Johns Hopkins Pediatric Sleep Disorders Program: "It gives us a whole new ballpark to play in. If this ends up being real and is able to be duplicated, it's huge."
Parents of newborns worry about SIDS from the moment they bring their infants home from the hospital, and many have changed the way they raise their children as a result of SIDS research.
Where previous generations put their babies to sleep on their stomachs, for the past 15 years doctors have encouraged parents to put their babies "Back to Sleep," a campaign that has decreased the number of SIDS cases.
Although doctors don't know what causes SIDS, they know that having a smoker in the house increases the risk. They know that boys are more at risk than girls and that African-American babies die at a greater rate than other infants.
Researchers have investigated various possible causes, including disturbances in respiratory control and infant overheating, but nothing has proven conclusive.
One of every 2,000 newborns dies of crib death, typically between the second and fourth month of life. They die in their sleep, with no apparent warning.
In the Seattle study, researchers looked at data collected in Rhode Island, a state with a particularly detailed database of hearing tests conducted on newborns.
Rubens theorizes that the inner ear - which is involved in hearing and balance - is important in transmitting to the brain information about carbon dioxide levels in the blood. He says that injury to cells in the inner ear could keep the brain from getting the message that there is too much carbon dioxide - enough to disrupt breathing function in the brain and cause death.
Doctors have long believed that when the cause of SIDS is confirmed, it will turn out to be a combination of small problems or conditions that add up to death.
But, in their research paper, Rubens and colleagues theorize that the ear damage associated with SIDS might be the result of an injury during childbirth. Whatever injury occurs would be healed by six months, when the incidence of SIDS typically falls off.
Rubens noted that when they're given hearing tests at birth, healthy babies typically have better hearing in the right ear than the left. Yet in each of the SIDS cases studied by Rubens' team, the left ear tested better than the right.
The finding was the same in each of the 31 SIDS babies, on hearing tests at three different frequencies. The differences were large enough to be statistically significant, he said.
Researchers don't know how many babies with similar hearing issues in the left ear do not die of SIDS. The hearing abnormality is not enough to cause serious hearing loss, Rubens said.
Rubens agrees that much more research is necessary.
Dr. Henry Krous, a pathology professor at the University of California-San Diego medical school and director of the San Diego SIDS Research Project, called the work "provocative."
He said researchers should now conduct a large, forward-looking study that tracks hearing results on tens of thousands of infants as they're tested just after birth. Since most newborns already receive hearing tests, pulling together a study of that size would not be as daunting as it sounds, he said.
"It's very hard to believe it's a 1:1 ratio" between the hearing anomaly and SIDS, Krous said.
Rubens said doctors need to standardize the way hearing tests are performed and recorded. He predicted that the results he found would be even more pronounced if infants were tested with sounds at a frequency higher than the ones at which most hearing tests are performed.
If parents knew their babies were at elevated risk for crib death, they could be warned to take very special precautions, the SIDS experts said.
"I have no doubt this will stimulate [more] research," Krous said.
stephanie.desmon@baltsun.com