(Reuters Health) - Kids exposed to tobacco smoke in the womb and early in infancy could have double the odds of developing hearing loss compared with children who were not exposed to tobacco at all, a Japanese study suggests.

While previous research suggests that adult smokers are at greater risk of hearing loss than nonsmokers, less is known about how much smoke exposure during infancy or pregnancy might impact hearing.

For the current study, researchers examined data on 50,734 children born between 2004 and 2010 in Kobe City, Japan. Overall, about 4 percent of these kids were exposed to smoking during pregnancy or infancy, and roughly 1 percent had tobacco exposure during both periods.

Hearing tests done when kids were 3 years old found that 4.6 percent of the children had hearing loss.

They were 68 percent more likely to have hearing loss if they were exposed to tobacco during pregnancy, and 30 percent more likely if they inhaled second-hand smoke during infancy, the study found.

When kids had smoke exposure during both periods, they were 2.4 times more likely than unexposed kids to have hearing loss.

“Patients with the greatest risk of hearing impairment are those who are directly exposed to maternal smoking in the womb,” said Dr. Matteo Pezzoli, a hearing specialist at San Lazzaro Hospital in Alba, Italy.

“Interestingly, the exposure to tobacco in early life seems to further strengthen the prenatal toxic effect,” Pezzoli, who wasn’t involved in the study, said by email.

When pregnant women smoke, it may harm fetal brain development and lead to auditory cognitive dysfunction, Pezzoli added. Tobacco smoke may also damage sensory receptors in the ear that relay messages to the brain based on sound vibration.

Globally, about 68 million people have a hearing impairment that is thought to have originated in childhood, Koji Kawakami of Kyoto University in Japan and colleagues note in Paediatric and Perinatal Epidemiology. Kawakami didn’t respond to requests for comment.

Researchers assessed children’s hearing using what’s known as a whisper test. For these tests, mothers stood behind their kids to prevent lip reading, then whispered a word while kids’ had one ear covered.

While this test is simple and considered an accurate way to assess hearing in adults and older children, there’s some concern about how reliable the results may be in young kids.

It’s considered more reliable when it’s done by trained clinicians and specialists and less reliable when it’s done by primary care providers, researchers note. It’s unclear how accurate study results based on tests administered by the children’s parents would be, researchers acknowledge.

The study also wasn’t a controlled experiment designed to prove whether or how tobacco exposure during pregnancy or infancy might directly cause hearing loss in kids.

“There was no standardized medical evaluation of hearing or examination of the ears by ear specialists,” said Dr. Michael Weitzman, a pediatrician and hearing researcher at New York University who wasn’t involved in the study.

“Moreover, the severity of hearing loss could not be ascertained in this study, and it did not follow up the children throughout their childhood so we do not know if what they found attenuated or got worse over time,” Weitzman said by email.

Still, the results add to the evidence linking tobacco exposure to hearing problems in kids, Weitzman said.

To protect children against hearing problems caused by cigarette smoke, it’s important for women to quit before they become pregnant or as soon as they discover they’re pregnant, said Huanhuan Hu, a researcher at the National Center for Global Health and Medicine in Japan who wasn’t involved in the study.

“To minimize the chance that their baby will be exposed to tobacco smoke in the womb, other family members should also quit, or at least not smoke at home or nearby the pregnant women,” Hu said by email.

SOURCE: bit.ly/2IAPU2W Paediatric and Perinatal Epidemiology, online June 5, 2018.