Hearing Difference May Be SIDS Marker

SEATTLE, Aug. 1 -- Newborn hearing tests may provide a marker for infants at risk for sudden infant death syndrome (SIDS), researchers found.

In a small retrospective case-control study, infants who later died of SIDS had a distinctive hearing loss in the right ear at birth, compared with controls, according to a report published in the July issue of Early Human Development.

The difference in cochlear function is a "unique finding that may offer the opportunity to identify infants at risk of SIDS," said Daniel Rubens, MBBS, of Children's Hospital and Regional Medical Center, here, and colleagues.

"This discovery opens a whole new line of inquiry into SIDS research," Dr. Rubens said.

"For the first time, it's now possible that with a simple, standard hearing test babies could be identified as at risk for SIDS, allowing preventive measures to be implemented in advance of a tragic event," he said.

In the U.S., the annual incidence of SIDS is about 0.55 per thousand infants, the researchers noted. Pathogenesis is unknown, but disturbed respiratory control is thought to play a key role.

Dr. Rubens and colleagues hypothesized that perinatal damage to the inner ear -- resulting in impaired vestibular function and poor respiratory control -- might play a role in the predisposition to SIDS.

To test the idea, they examined medical records of 31 infants born in Rhode Island between 1993 and 2006 and who subsequently died of SIDS. They looked specifically at the results of transient evoked otoacoustic emissions (TEOAE) hearing tests performed at birth.

The SIDS cases were matched to healthy controls on the basis of sex, full-term versus premature birth, and care in a neonatal intensive care unit or a well-baby unit.

The hearing test results - now part of standard care for newborns - were measured as signal to noise ratios at four frequency levels: 1,500, 2,000, 3,000, and 4,000 Hertz.

Compared with the controls, all of the SIDS infants showed a lower response on the right side, which was statistically significant at 2,000, 3,000 and 4,000 Hz, the researchers found.

Specifically, the average differences in signal to noise ratio between controls and cases were:

• minus 4.04 decibels at 2,000 Hz
• minus 3.96 decibels at 3,000 Hz
• minus 3.59 decibels at 4,000 Hz

The differences were significant at P=0.018, P=0.0017, and P=0.034, respectively.

Hearing on the left side was also lower for SIDS babies than for controls, but not significantly so.

Although the study used a case-control approach, the hearing tests were evaluated blindly by two audiologists who did not know the subsequent history of the children, Dr. Rubens and colleagues said.

Newborns usually have a better response on the right side, but the SIDS infants had a better left-side response, the researchers noted.

More research is needed, Dr. Rubens and colleagues said, but one possible mechanism that could damage the inner ear is high blood pressure in the umbilical vein.

Such an insult would affect not only the cochlear organs-explaining the hearing differences-but also the vestibular hair cells, which have been linked to the control of breathing, the researchers said.

Limitations noted by the authors included the observational retrospective design, sample size, the fact that the hearing screening tests were available only in the 1500-4000 Hz range, and only limited demographic data on the subjects.