When the ability to hear fades in older adults, it may well be a signal that declines in memory and thinking ability are not far behind.
Several studies have noted that older adults with greater hearing loss were at substantially increased risk for cognitive impairment compared with individuals with normal hearing.
The latest findings were published online this week in JAMA Internal Medicine, part of a long-term aging study that began in 1997, and involved 1,984 men and women between the ages of 75 and 84 who got their hearing and brain function checked beginning in 2001.
Led by Dr. Frank Lin of the Johns Hopkins Center on Aging and Health in Baltimore, the team did repeated cognition tests over six years. Among those with hearing loss, memory abilities declined some 30-to-40 percent faster than in those for whom hearing was not impaired.
Lin said the results underscore how important it is for physicians treating the elderly to address hearing declines. As many as 27 million Americans over 50 -- and two-thirds of men and women aged 70 and older -- suffer from hearing loss. Only about 15 percent of those who need a hearing aid get one.
While Lin's study is one of the largest and most comprehensive looks at hearing loss and dementia risk, it's not the first. Dr. George Gates, a hearing specialist at the University of Washington, published results from a similar trial involving 271 patients in 1996. The four-year follow-up identified 21 people with Alzheimer's disease among those who had hearing that made it difficult for them to follow a conversation in a crowded room.
Gates says it's impossible to separate hearing and cognition. We hear with our brains. And other studies have shown that we see with our brain in much the same way. Some scientists think there may be some common, underlying damage in the brain that contributes to both hearing and cognitive problems. Other recent studies indicate that dementia spreads along nerve cells linking different regions of the brain.
Curiously enough, a team of Boston researchers recently reported that it succeeded in restoring some hearing function to deafened mice using an experimental drug first developed for Alzheimer's disease. Called gamma secretase inhibitors, the product was ineffective against dementia, but helped mice re-grow about 20 percent of the hair-like cells that convert sound into electrical signals in the brain.
Lin, who has reported on dementia risks for people with milder hearing impairment, says it's not clear how hearing loss impacts mental function. While there may be some common structures in the brain, he said hearing loss could simply lead to social isolation, which in itself contributes to dementia.
In fact, diagnosing Alzheimer's can be more difficult in people who are hard of hearing, since they may miss questions or verbal cues during testing.
It's also possible that parts of the brain are overtaxed when people with hearing loss try to compensate in other ways. The researcher said the brain may actually shift resources to help with hearing at the expense of cognition, just as the brain is known to respond to other sensory losses such as sight.
Lin's team is now working on another study that will try to determine if hearing aids and or other interventions can delay cognitive decline among people experiencing hearing loss.
More than 5.4 million Americans have Alzheimer's and related dementias, a number that's expected to more than double by 2050 if no preventive measures or cures are identified.