Newborn hearing abnormalities may signal SIDS.
"We may soon have the possibility, for the first time, of identifying babies months before they die [of SIDS]," lead author Dr. Daniel Rubens said in an interview. "The utilization of a standard newborn hearing screen to identify susceptibility to SIDS would allow for investigation and preventive measures to be taken in these infants immediately after birth," he said (doi: 10.1016/j.earlhumdev.2007.06.001).
But it is still too early to assess SIDS risk on the basis of standard hearing tests, cautioned Dr. Rubens, an anesthesiologist at Children's Hospital in Seattle. "This study shows there is definitely a connection between SIDS and hearing, and it definitely points to the possibility of using these hearing tests, but we're not at that point yet," he stressed in the interview.
His study reviewed results of the newborn transient evoked otoacoustic emissions (TEOAE) heating screen for 31 babies who died of SIDS, and found they had unilateral right-sided abnormalities, compared with 31 matched controls who survived the first year of life. Specifically, the infants with SIDS had significantly lower signal-to-noise ratios in the right ear at three frequencies, compared with controls. There also was a reversal of asymmetry in the SIDS cohort--with controls showing stronger right-sided hearing versus left-sided hearing.
The findings are "intriguing and exciting," but should be interpreted cautiously, agreed Dr. John Kattwinkel, who chairs the American Academy of Pediatrics" taskforce on SIDS.
"There might be a whole bunch of other babies that have [this same abnormality] who didn't die. We really don't know. The question is, What's the positive predictive value?, and they don't have the data to calculate that," said Dr. Kattwinkel, the Charles Fuller Chair of Neonatology at the University of Virginia, Charlottesville.
Indeed, it is not unusual to find abnormalities on newborn hearing tests, according to Alison Grimes, Au.D., president of the American Academy of Audiology and head of the audiology clinic at the University of California, Los Angeles, Medical Center. "'If you take a thousand well babies, you will probably find that about 2% of them fall their newborn hearing screening test," she said in an interview, explaining that several different tests are performed across the country. 'A certain number will fall the screening test but will subsequently be found to have normal hearing and no other issue," she noted.
Dr. Rubens' theory is that the association of abnormal hearing tests with SIDS might be related to cochlear and vestibular, and possibly brain stem, damage, which could disrupt the brain's ability to control respiration. "I propose that not every baby that gets this injury will die of SIDS, but every baby that dies of SIDS will ultimately be shown to have this inner-ear injury," he said in the interview. "That's a theory and has yet to be proved."
Such injuries could be caused by high venous pressure during labor, he suggested.
"There is no evidence for this, but also none against it," he stressed. "The placenta squeezes blood at very high pressure into the baby's veins at the time of birth. I'm proposing that in certain rare instances--and certain babies will be more predisposed than others--that surge of pressure may cause an injury in the small blood vessels, which creates a predisposition for SIDS.'" The hypothesis is strengthened by evidence of an increased incidence of SIDS in labors lasting longer than 16 hours (Am. J. Epidemiol. 1991;133:900-6), and in prima gravida pregnancies (J. paediatr. Child Health 1993;29:215-8), he noted.
Dr. Rubens said his findings of hearing abnormalities, and his theory that they could be caused by a perinatal insult, fit with findings from another recent study that reported brain stem abnormalities found on autopsy in SIDS infants (JAMA 2006;296:2124-32). Theoretically, both abnormalities could be caused by high venous pressure during labor, he said, adding that the cochlear and vestibular hair cells share an extreme sensitivity to pressure and, along with the brain stem, may be particularly vulnerable to pressure.
In addition to the potential of using hearing tests to identify this injury, Dr. Rubens envisions there may be other signs on physical examination. "What I am considering is you may potentially see signs of bleeding in the small veins and end organs, and it would be in areas we don't normally look: bleeding points in the gums, or potentially in the eyeballs or the fingernails." The hearing test and targeted physical examination could together be used as a SIDS screening tool, he suggested.
"If you could really identify a high-risk group, you could be a lot more aggressive with preventive measures," Dr. Kattwinkel said. He mentioned cardiorespiratory monitoring, but said that "the only preventive measure that's really been shown to work is sleeping on the back."
BY KATE JOHNSON
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|Title Annotation:||Clinical Rounds; sudden infant death syndrome|
|Date:||Sep 1, 2007|
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