A week hardly goes by without a new claim about causes of Alzheimer’s disease or cognitive impairment. The New York Times Health Correspondent, Jane Brody, has never missed an opportunity to hype a potential cause or cure, no matter how suspect the evidence is. This past week she wrote about vision and hearing loss as possible causes.
This is an idea that has been around for awhile. And while some scientists make a plausible argument that reduced sensory input might lead to lower cognitive abilities, that is a far step from causing plaques, tangles, strokes, or other dementia-related pathologies. We have yet to hear a plausible explanation for the biological mechanism the connects hearing loss, which occurs within the ear, or vision loss, which occurs in the eye, to dementia, which occurs in the brain. Hearing loss and vision loss definitely limit and distort the quality of information a person is able to take in, but they do not cause brain cells to progressively deteriorate the way we know they do in dementia.
The most plausible explanation is that the findings are an artifact. Think about how cognitive functioning is assessed. We show someone visual images and/or present verbal information that they are to repeat or remember. If you can’t see or hear well, you are more prone to make mistakes on tests. That is a fundamental premise of neuropsychological testing. Whenever Judy saw someone for testing who had hearing loss her first priority was to be sure that conditions were optimal for hearing (quiet room, speaking in a low register and projecting her voice directly to the person), and when that wasn’t sufficient, using written materials to supplement verbal instructions.
Imagine dear old Uncle Bill, who is hard of hearing, being asked a standard dementia screening question: “What’s today’s date?” And Bill answers, “I haven’t been on a date in years.” How might that answer be interpreted? Before jumping to the conclusion that he has dementia, it’s important to consider he didn’t hear the question correctly. Or maybe he’s just a wiseass. Either way, it may not be dementia.
Many of the studies that have reported correlations between hearing loss and cognitive function have been conducted over the phone. It’s hardly an optimal way to assess someone with hearing loss. But even clinical studies may confound hearing loss and cognitive problems, if the person conducting the tests is not well-trained. The studies are also largely correlational, and don’t show decline in cognition over time. Brody cites two large studies, one with 3,000 people, and one with 30,000 people, that present correlations of cognition and hearing loss, but a large sample is not necessarily better. It’s easier to find statistical significance with a large sample, but what is called “effect size,” that is, the size of the association, may be quite small.
We would all like to know that there is something that we could do that might prevent dementia. And if we have hearing loss that affects daily life, it would be a good idea to get evaluated for hearing aids or other hearing devices, because that could make things easier at least in some situations. But that’s a far cry from advising someone to get hearing aids in order to prevent the development of dementia.
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