Hearing loss may push decline in memory, thinking

Older Americans who have hearing loss have an accelerated decline in thinking and memory abilities, compared to those with normal hearing, according to a study published in  JAMA Archives of Internal Medicine .

Those with hearing loss experience a 30% to 40% greater decline in thinking abilities compared to their counterparts without hearing loss, according to the findings published Monday.

Hearing loss is common among old older adults, affecting about two-thirds of adults 70 and older, and about one-third of adults younger than 60, according to lead study author Dr. Frank R. Lin of Johns Hopkins University.  A large number of people with hearing loss are untreated, Lin explained, because they associate hearing loss with the stigma of getting older.

About two years ago Lin and his associates published a paper showing that hearing loss was associated with greater risk for developing dementia. “Fortunately most of us will never develop dementia, but most of us will experience some kind of cognitive decline over time,” explained Lin.

Rather than looking at hearing loss and dementia, the researchers studied people with normal cognitive function to determine whether people with hearing loss had different rates of memory and thinking decline compared to people with normal hearing. Dementia rates are projected to rise as the world's population ages, the study noted; identifying factors that may contribute to cognitive decline and dementia in older adults may lead to ways to slow and treat brain decline.

The researchers studied about 2,000 older adults enrolled in a long-term study which began in 1997. All subjects included in the study had no dementia or cognitive impairment.  Each subject went through an audiometric assessment performed in a sound-treated booth, which Lin described as “the gold standard” for hearing testing. Their memory, thinking abilities and decision-making were also tested. Both tests were repeated at three, five and six years, and researchers looked at average decline in memory and thinking abilities, comparing people with normal hearing to those with reduced hearing.

“We found that people with hearing loss had a faster rate of mental decline compared to people with normal hearing. ... And the greater the rate of hearing loss, the faster the decline of memory and thinking. It was dose dependent,” said Lin. People with hearing loss took 7.7 years to have a five-point drop in their thinking skills, compared to 10.9 years for people with normal hearing.

Why does this happen? Lin said there’s no definite explanation, noting that various explanations may apply. When people suffer from hearing loss, it’s not that they can’t hear. It’s that the cochlea, the part of the inner ear that converts a complex sound to a precise signal that goes to the brain for decoding, isn’t doing a good job converting, so people hear a garbled signal. Lin described it like a bad cell phone connection.

One theory is that "if the brain is dedicating extra resources to try and hear what’s going on, it's probably taking away from other brain resources like thinking and memory, “ explained Lin.

A second explanation, using the cell phone example, is that people experiencing lousy reception end up tuning out, because it’s so labor intensive to try to hear the call. This explanation plays into the idea of social isolation, which has been shown to have negative health effects including increased illness, death rates, and increased cognitive decline and dementia.

A third possible explanation is that some mechanism in the brain is affecting both hearing and brain function. Lin said it's likely that the hearing loss and brain decline are explained by all three factors. He also acknowledged that while his study tried to adjust for other factors affecting hearing and cognitive abilities, they did not account for factors including something in the inflammatory process or the age of mitochondria, the energy factories of cells.

Lin thinks the big public health question is whether treating hearing loss will have an impact on brain function and memory decline.