Hearing Loss Associated With Accelerated Brain Tissue Loss
By Heather Hogstrom
While both hearing loss and memory loss are common aspects of aging, a recent study published in NeuroImage has demonstrated a connection between the two.
“We’re increasingly understanding that hearing impairment is clearly associated with probably increasing a person’s risk for cognitive decline and dementia,” says study author Frank Lin, MD, PhD, an assistant professor in the Johns Hopkins University schools of medicine and public health. “One of the hypotheses for why that may be the case is that hearing impairment may lead to changes in brain structure and, hence, brain function.”
Lin says this association is somewhat supported by cross-sectional studies showing that people with hearing loss also have a smaller part of the brain that handles sound processing, as well as animal studies showing that, in those with hearing loss, parts of the brain atrophy faster. “So what we did in this study was test that hypothesis,” he explains. “So we looked at a large group of older adults with MRI scans every single year, and we looked at whether people with hearing loss really had faster rates of brain atrophy over time.”
The researchers studied MRI scans of individuals aged 56 to 86 with normal or impaired hearing, tracking brain changes over several years. They found that, compared with those with normal hearing, those with hearing impairment had accelerated volume declines in whole-brain and regional volumes in the right temporal lobe, demonstrating that peripheral hearing impairment is independently associated with accelerated brain atrophy. Shrinkage occurred in regions such as the superior temporal gyrus, which processes sound and speech; the middle and inferior temporal gyri, which are involved in cognitive processes such as semantic memory, language, visual perception, and sensory integration; and the parahippocampus, which plays a role in memory creation and recall.
According to Lin, the reason for the acceleration in brain tissue loss “probably has a lot to do with the fact that when we have bad hearing loss ... the brain is constantly receiving a much more degraded signal from the ear piece. There’s reduced stimulation of parts of the brain. So what we think now may be happening is that hearing loss serves for what we call sort of a second hit on the brain, namely leading to accelerated atrophy in certain areas that are chronically sort of understimulated. And, in turn, that increased atrophy may increase the risk for dementia over time.”
Lin explains that hearing loss is another hit on the brain in addition to other brain changes that happen over time, such as microvascular disease or pathological changes associated with Alzheimer’s disease.
If hearing loss accelerates brain tissue loss, the next question is whether remedying hearing loss would halt the process. And it’s one for which Lin doesn’t yet have an answer. “In terms of the bigger questions of when, if you treat hearing loss, can you reduce the risk of cognitive decline? Can you reduce the risk of dementia? Those studies, in terms of the impact of hearing loss treatment, just have never been done before,” he says. “Those are studies that we’re just beginning to plan now, very large, definitive randomized clinical trials that will test exactly, if you treat hearing impairment, does that mean you can reduce the risk of cognitive decline or dementia and brain atrophy or reduce the risk of accelerated brain atrophy? That’s something that we just don’t know yet. Those are the exact studies we’re planning right now.”
While hearing loss can occur with aging, Lin believes it should not be treated as inconsequential. “I think the key things for geriatricians is that there’s very much ... this perception that hearing impairment is just a usual process of aging, which is true,” he says. “But increasingly from the literature, we’re understanding that hearing impairment probably does play a significant role in increasing the risk of physical functional decline and cognitive decline over time. And the key thing is, while we don’t know yet if treating hearing loss will make that much of a difference, we know for the time being it definitely can’t hurt at all. And anything geriatricians can do to raise awareness among their patients that hearing impairment is probably playing a significant contributing role to their overall health and functioning over time, the better.”
For geriatricians, there are a variety of measures to screen for hearing impairment, such as the watch tick test, finger rub test, or just asking a simple question. “A lot of times a simple question’s enough,” Lin notes. “Mainly because if you look at how common significant hearing impairment is in older adults, basically about 55% of 70-year-olds have a significant hearing impairment and about 70% of 80-year-olds. So in other words, if you think the person has a hearing impairment, they probably do.” He says that while screening is important, a lot of patients suspected of having hearing loss probably do, and the next step should be to refer them to be formally evaluated by an audiologist.
When treating older adults, geriatricians should keep in mind that hearing impairment affects cognitive function. If hearing loss is accelerating brain tissue loss, it should be treated as soon as possible before structural brain changes occur rather than dismissed as a normal part of aging.
— Heather Hogstrom is an editorial assistant for Today’s Geriatric Medicine.