Cognitive problems developed faster among older adults with mild hearing loss in a new study. / BananaStock

by Janice Lloyd, USA TODAY

by Janice Lloyd, USA TODAY

Millions of hearing-impaired older adults are more likely to suffer early memory and thinking problems than adults without hearing loss, a study out Monday finds.

Cognitive problems developed 30% to 40% faster when hearing declined to 25 decibels - mild hearing loss, according to the research online in the JAMA Internal Medicine. "That's when you begin noticing trouble hearing and understanding in settings like a busy restaurant,'' says lead author Frank Lin.

Estimates vary on the number of people in the USA who have some degree of hearing loss, but they range from one-third to two-thirds of adults ages 70 and older. Only 15% reportedly get hearing aids, study authors write. About 7 million people in the USA have some form of dementia, and numbers are expected to double every 20 years.

"Our findings show how important it is for physicians to discuss hearing with their patients and be proactive in addressing hearing declines," says Lin, an otologist and assistant professor at the Johns Hopkins University School of Medicine in Baltimore.

The study, funded by the National Institutes of Health, didn't attempt to examine why cognitive skills decline when mild hearing loss occurs. Lin says he hopes to show in future research whether hearing devices could prevent the problem. There was no funding from hearing device manufacturers for his research, Lin says.

The researchers followed 1,984 adults ages 75-84 from 2001 to 2007, accounting for factors known to contribute to loss of brain function - high blood pressure, diabetes and stroke. All participants had normal brain function and hearing when the study began.

They were given two brain tests; one at the start of the study asked them to memorize words, follow commands and answer questions about year, date and time. The other test timed them on how long it took to match numbers to symbols. Both cognitive tests and hearing tests were repeated three more times to gauge decline.

Decline was statistically significant in the hearing-impaired, compared with people with normal hearing, Lin says. Those who suffered hearing loss took 7.7 years to show mental decline, vs. 10.9 years for those with healthy hearing.

Social isolation, one of the risk factors for dementia, is one possible explanation for cognitive decline, Lin says. Another: some underlying brain damage that leads to both hearing and cognitive decline.

"It could also be that if you're constantly having to expend more (mental) energy decoding what you hear, then it comes at a cost,'' Lin says. "Hearing loss doesn't directly contribute to dementia, but leads to cognitive load on the brain."

Arthur Wingfield, chairman of the neuroscience program at Brandeis University in Waltham, Mass., has done research showing mild-to-moderate hearing loss can undermine the cognitive resources of "otherwise healthy young adults" as well as older ones. Wingfield is not associated with Lin's research.

"The link Dr. Lin demonstrates between mild loss of hearing and cognitive skills is a call for alertness to a public health issue that has received less attention than it should,'' says Wingfield. "It should also be a concern because of the increase in hearing loss among younger adults, who are unaware of it when it is relatively mild."