Hear, Hear

A study of twins shows that genes play a role in ear infections

Like many physicians, Margaretha L. Casselbrant suspected that some children have a hereditary predisposition to middle ear infections. Unlike most other physicians, she set out to prove it systematically.

Because twins share their environment, scientists compared how often middle ear infections occur simultaneously in identical twins and in fraternal twins to learn how much genes matter in this disease. Laurie Painter

In 1983, the otolaryngologist began a painstaking, 17-year routine of collecting ear-infection data on same-sex twins and triplets. Casselbrant examined 168 sets of twins and 7 sets of triplets at least once every 2 months, plus whenever they had an earache. She recorded occurrences of excess fluid in the ear and cases of middle ear infection—otitis media—and prescribed a standard course of antibiotics when needed.

At a time when there was little evidence that a person’s genes influenced their risk of this or other infections, Casselbrant made heredity the basis of her study. She examined twin and triplet babies because each set represents a little bundle of comparisons. Identical twins present a near-perfect genetic match. Fraternal twins have less similarity. In both groups, however, the members of each pair lived in the same home and, therefore, experienced the same environment.

Now, after plowing through the first 14 years of data, Casselbrant and her colleagues at Children’s Hospital of Pittsburgh have established that identical twins and triplets are twice as likely to get ear infections simultaneously as are fraternal ones.

Although the study is limited to twins and triplets, it shows for the first time definitively that genetics matters in ear infections, says Peter J. Casano, a practicing otolaryngologist who teaches at the University of Mississippi Medical Center in Jackson. Therefore, a predisposition to otitis media will show up in some families more than others, he says.

Knowing that, physicians may soon treat some patients differently. For instance, surgeons today often implant tiny tubes in the eardrums of children with frequent infections to ventilate the middle ear and relieve pressure. Surgery, however, is a hard choice for parents and a last resort for physicians.

“Now, if you see someone whose older brother or sister needed tubes, you may not have to wait quite as long [to decide] that they need aggressive treatment,” Casano says.

Physicians can also counsel parents in susceptible families to take precautions against otitis media, such as breast-feeding infants, keeping them away from second-hand cigarette smoke, and avoiding large day care centers, where many viruses are passed around, Casselbrant says.

Someday, genetic screening at birth may even predict who is at greatest risk of ear infections, Casano says.

Costly infections

Meanwhile, otitis media remains the most common reason in the United States for taking a baby to a doctor. Ear infections cost up to $5 billion a year when medication, complications, and lost work time are factored in, says James F. Battey, director of the National Institute on Deafness and Other Communication Disorders in Bethesda, Md. Untreated otitis media can lead to more-severe infections.

The most common problem, however, is temporary hearing loss. “Kids who don’t hear well lose interest in speech and language,” Casselbrant says. “It has an effect on learning.”

Most ear infections strike in the first 2 years of life, when the eustachian tubes, linking the ears and upper throat, are horizontal. As children grow, their faces elongate and ear fluid drains better.

By directly observing infants through these high-risk years, Casselbrant gained an advantage over researchers who had pursued a genetic link to otitis media by relying on parents’ recollections of their children’s health. Some studies found a connection, others didn’t. The only other major study of twins estimated that genetics accounted for half to three-quarters of a child’s susceptibility.

Casselbrant didn’t ask parents whether the twins and triplets were identical or fraternal, so in most cases she didn’t know the children’s genetic status when they were first examined. Her colleagues used blood samples taken at age 1 to determine whether the sets were fraternal or identical.

Casselbrant and her colleagues report in the Dec. 8, 1999 Journal of the American Medical Association that a baby whose identical twin or triplet has an ear infection faces a risk of also having one that’s six to eight times that of an average child of similar age.

Part of this risk stems from living with a sibling who has an infection. Nonetheless, among fraternal twins and triplets, when one child has an ear infection, a sibling has only three to four times the risk of infection that a child outside the house might have, Casselbrant says.

From the data, the researchers calculated that three-quarters of susceptibility to ear infection is due to genetics. Casselbrant also looked for fluid in the children’s ears, a symptom that frequently accompanies otitis media. Although the presence of fluid doesn’t always prove the ear is infected, it dulls hearing. An identical twin or triplet of a child with fluid in the ear is much more likely to have fluid there than a fraternal twin or triplet is.

“This work provides a solid scientific foundation for something that was previously suspected, based on anecdotal evidence,” Battey says. The gene or genes responsible for heightened susceptibility to otitis media remain unidentified, however.

“The genetics of otitis media are most likely complex, meaning that many genes each probably contribute to the overall [disease],” say Garth D. Ehrlich and J. Christopher Post of Allegheny General Hospital in Pittsburgh, writing in the same journal issue. Candidate genes might be those affecting the immune system or the ear’s anatomy, they suggest.

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