The two data sets used different measurements of cognitive abilities like memory, orientation and planning, and the consistency of their findings suggests that the association between vision impairment and compromised brain function is real, the researchers concluded.
The lead author, Dr. Suzann Pershing, ophthalmologist at Stanford University School of Medicine, said that âwhile this association doesnât prove vision loss causes cognitive decline, intuitively it makes sense that the less engaged people are with the world, the less cognitive stimulation they receive, and the more likely their cognitive function will decline.â
An earlier study of 625 elderly people at the University of Michigan by Dr. Mary A.M. Rogers and Kenneth M. Langa found that those with poor vision had a 63 percent greater risk of developing dementia over a period of eight and a half years. Those with poor vision who did not visit an ophthalmologist were five times more likely to experience cognitive decline and nine and a half times more likely to develop Alzheimerâs disease.
âThis is not necessarily a one-way relationship,â Dr. Pershing said. âIf you improve the vision of people with cognitive impairment, they can function better.â
Likewise with hearing, where there is stronger evidence that hearing loss can speed cognitive decline. However, it is not yet known whether wearing properly adjusted hearing aids can decrease the risk of dementia or slow its onset. A study sponsored by the National Institute on Aging should show in five years whether using hearing aids can help preserve brain function in people with hearing loss as they age.
Dr. Frank R. Lin, otolaryngologist at the Johns Hopkins Center on Aging and Health, said âthe potential impact is huge,â especially now. Companies like Apple and Samsung are expected to soon market hearing aids that cost perhaps $50 to $300 instead of $5,000 a pair, making this assist accessible to many more people. (Already, Dr. Lin said, âCostco has made a huge dent in affordability,â lowering the cost of hearing aids fitted and serviced by well-trained audiologists.)
Meanwhile, hereâs what we already know. A prospective study of 1,984 older adults directed by Dr. Lin found that those who initially had hearing loss were 24 percent more likely than their age-mates with normal hearing to experience cognitive decline within six years.
Their cognitive abilities declined up to 40 percent faster than others with normal hearing. They had greater problems with brain functions like thinking and memory, developing them on average three years earlier than people their age with normal hearing. And the more severe their hearing loss at the start of the study, the greater their cognitive loss over time.
âOlder adults with hearing loss face an increased risk of dementia even when you control for diseases like diabetes and high blood pressure,â Dr. Lin said in an interview. âSo we think theyâre causally related.â
He suggested three ways that may explain how poor hearing and dementia are linked. One involves âcognitive loadâ â when you canât hear well, the brain receives garbled signals, forcing it to work harder to derive meaning from the message.
Another mechanism may be that people who canât hear well tend to become socially isolated, which results in diminished cognitive stimulation and cognitive loss.
Perhaps most important is a third possible explanation involving brain structure; hearing loss results in a faster rate of brain atrophy mostly over the hearing portion of the brain, which is also involved in functions like memory, learning and thinking.
At Washington University in St. Louis, Jonathan Peelle and colleagues have shown through functional M.R.I. studies that even people with mild hearing loss ârecruit more of their frontal cortex,â which means that the part of the brain needed for thinking and decision-making is overworked when trying to understand speech.
âEven in young adults with clinically normal hearing,â Dr. Peelle said, âjust a small decline in how well they hear engages more of their frontal cortex.â When hearing loss is advanced, studies have shown that the auditory cortex shrinks, which may diminish the brainâs ability to perform tasks other than hearing, he said.
While the cost-effectiveness of regular screening programs for vision and hearing in older adults has yet to be demonstrated, experts strongly recommend periodic exams to pick up correctable problems that may diminish the risk of dementia and would certainly improve quality of life.
The American Academy of Ophthalmology recommends a comprehensive eye exam every two to four years for people aged 40 to 54 with no symptoms or risk factors for vision loss, increasing to every one to three years for those aged 55 to 64, and every year or two thereafter. The exams can help to preserve vision by detecting correctable or treatable problems like glaucoma, cataracts and age-related macular degeneration.
The American Speech-Language-Hearing Association recommends a hearing test âat least every decade through age 50 and every three years thereafter.â As Dr. Lin explained, hearing loss can occur so gradually that people donât recognize the problem until it is well advanced. Also, there is no harm done in getting tested.