Hello, my name is Jennifer Deal. In this section, we'll talk about the relationship between hearing loss and dementia. When we think about what it means to age in a healthy way, there are a number of different domains that we might want to address. Keeping active, socially being engaged, cognition and avoiding dementia, avoiding injury, things like falls, being able to maintain physical function, to be able to move around as much as we'd like, and then there is also some important things that we can think about in relationship to economic outcomes like health care utilization, as well as mortality. All of these domains we think hearing loss may impact. Specifically today, we're going to focus on talking about the relationship between hearing loss and dementia. In 2017, The Lancet Commission on dementia prevention, intervention, and care made this statement, that I think it's a bold statement, but one that is true. They said that dementia is the greatest global challenge for healthcare and social care in the 21st century. Part of the reason they made that claim is just because there are so many people in the world now who have dementia. As our population is aging, that's both in high income as well as low and middle-income countries, the number of individuals with dementia will only continue to grow. This graph shows dementia prevalence around the world. This is the number of individuals who have dementia. On our y-axis, we have the number of people in millions, and along the x-axis we have years. Starting in 2015, it was right around 47 million adults. Older adults had dementia, estimated to be 66 million by the time we hit 2030, and over 130 million individuals around the world living with dementia in 2050. Just an astounding number. What's important to note here in this graph, the blue color is high-income countries. We can see that there's an increase in dementia over time in high-income countries. But really this growth is being driven by the number of individuals expected to develop dementia in low and middle-income countries, so it really is a global problem. Another reason why the Lancet claimed that dementia is such an important disease for us to consider is its very high cost. This was a study that was conducted several years ago now and it was looking at the relationship between cost of care for individuals with different chronic diseases towards the end of someone's life. They were looking at the burden of health care costs for patients with different conditions. They actually hypothesized that they would see the greatest cost associated with something like cancer, because cancer has such a high cost in terms of the drugs and the treatment involved. But what they found was actually the largest amount of money that needed to be spent was for individuals with dementia. On the right-hand side we have an article from the popular media. This is from the New York Times. They have the headline costs for dementia care far exceeding other diseases, study finds. Then we have on the left-hand side the conclusion from the abstract from the actual manuscript. What they found was that healthcare expenditures among people with dementia were substantially larger than for other diseases. Importantly, most of these costs were not covered. They were uninsured. They conclude that this places a large financial burden on families and that these burdens are particularly pronounced among groups that are at least prepared for the financial risk. This was a surprising finding, but something that's very important. Overall, it's been estimated that annually, $1 trillion are spent for caring for individuals with dementia across the world every year. A very high cost associated with dementia. In addition to the fiscal cost, there is also a very real personal and family cost to dementia. This article was taken from the New York Times. It followed a woman who was newly diagnosed with dementia and then very bravely allowed a reporter to come into her home, visit with her and her husband as they were really starting to grapple with this diagnosis and what it meant. The author, I think put it much more eloquently than I would be able to. The author of this article said, "The disease," meaning Alzheimer's disease, "with its thundering implication, moves in worsening stages to its ungraspable end. That is the familiar face of Alzheimer's, the withered person with the scrambled mind marooned in a nursing home, memories sealed away, aspirations for the future discontinued." It is culturally dependent, but oftentimes family members will say that they feel like they're losing that person. It's not just that they're dealing with the physical ailment, but actually with the mental and cognitive decline that comes with dementia, with individuals unable to remember family members and who they are. There's a great personal cost to that individual as well as to the family. Dementia is such an important condition and the question is, what can we do about it? Unfortunately, at this time, there are no treatments that alter the natural history of dementia and we do not have a cure. There are some drugs that can slow cognitive decline for at least a little bit. But over time the individual catches up to where they would have been, had they never started taking the drugs. These drugs also have some very nasty side effects associated with them as well. Because we don't have an effective treatment and we don't have a cure, that really points to the need for us to focus on prevention efforts. We want to try to prevent and delay cognitive decline as much as possible. This is the current state of dementia prevention. This comes from a systematic review and meta-analysis that was conducted by the Lancet Commission on dementia prevention, intervention, and care. This is from a 2020 report. What they did was look out all of the potentially modifiable risk factors for dementia. They ranked them in order for what proportion of dementia globally could potentially be prevented, if that risk factor were either treated or eliminated. What they reported was that hearing loss actually has the highest proportion of risk that potentially could be prevented, and so that's 8 percent of dementia cases globally. That's compared to a lot of other risk factors that we've known for a long time, like less education is estimated to be 7 percent, smoking is 5 percent, depression, social isolation both are 4 percent. We have traumatic brain injury, air pollution, physical activity, hypertension, obesity, diabetes, and heavy alcohol use. All of these are modifiable risk factors for dementia, but hearing loss really is the greatest. The interpretation of that 8 percent, is that 8 percent of dementia in the world is due to hearing loss, and could potentially be prevented if no one in the world had hearing loss. This has some really important assumptions. It assumes that hearing loss causes dementia, which is something that we don't know right now. It assumes that all of our studies that we've done so far, really give us a good understanding of the nature of the relationship between hearing loss and dementia. But really that's still is very incredible to think about 8 percent of dementia potentially we could prevent it if we were just able to prevent people from getting hearing loss or effectively treat hearing loss. An important question is, why does hearing loss contribute the greatest proportion of risk compared to all of those other risk factors that have been studied? That's really for two things. First of all, hearing loss is incredibly prevalent. It impacts a large number of older adults. Two out of three adults who are 70 years or older, have a clinically significant hearing loss that can impact their everyday lives. This graph is showing the prevalence of hearing loss in the United States. It comes from a nationally representative dataset, the National Health and Nutrition Examination Survey. The prevalence or proportion of individuals who have hearing loss is on the y-axis, and then we have age categories on the x-axis. You can see this dramatic increase as we get older. Once you're over 70, once you're over 80, it really is almost that everyone has some degree of hearing loss. The other reason why hearing loss is associated with such a high proportion of dementia risk around the world, is that there's a strong relationship between hearing loss and dementia. This is the literature that was synthesized for that Lancet Commission review. I will note there are only three studies here, now certainly there have been a lot more studies that have been conducted. But these were the studies that were strong enough and rigorous enough to be included in the review that the Lancet conducted. There's three studies. and overall, they estimated that the increased risk of dementia that's associated with hearing loss is 1.9, or 90 percent increase in the risk. I mentioned that the Lancet Commission Report that estimated of a 1.9 increase in the risk of dementia associated with hearing loss. That's based on three studies, this is the first of those three studies. This was looking at hearing loss and incident dementia. In a study called the Baltimore Longitudinal Study on Aging. It's the longest-running study of aging in older adults in the United States. There were 639 adults included in this analysis and they were followed for over 10 years. Hearing loss was measured using pure-tone audiometry, so was measured objectively at baseline and then individuals were followed for up to 10 years to determine who developed dementia. In the graph on the left, you can see this incredible dose-response. The line at the top represents the number of individuals who are developing dementia among individuals who had normal hearing at baseline. The line below is a mild hearing loss. The line below that is moderate, and the line below that is severe. Every time you see a little step, looks like a little stair-step down, that indicates that someone has developed dementia at that point in time. We can see actually that there are very few number of individuals in the severe category, but the risk of dementia associated with having severe hearing loss compared to normal hearing was really large in terms of the estimate. The hazard ratio or the risk over time associated with a mild hearing loss compared to normal hearing, was 1.89. Another way we could say that, was that it was associated with a 90 percent, an 89 percent increase in the risk. We see that moderate hearing loss compared to normal hearing is associated with even greater risk, it has a ratio of 3 or 300 percent increase in the risk, and severe hearing loss compared to normal hearing was the highest increase of risk overall. That was 4.94 was the hazard ratio or a nearly 500 percent increase in the risk. This was after accounting for important things like age, sex, race, education, smoking, diabetes, and hypertension. Important things that we know are both related to hearing loss as well as related to risk of dementia. The second study that was included in the Lancet Commission Report was conducted in a cohort called the Health, Aging and Body Composition Study. There were nearly 2,000 adults, 1,889, followed for up to nine years. This was a cohort of both black and white individuals from a couple of different study sites in the United States. The graph on the left shows the risk of dementia associated with hearing loss for three separate categories. The solid line at the top is normal hearing. The line below that, that's a dashed line that is mild hearing impairment. Then the moderate or greater loss is the line below that. You can see by looking at the graph that there are big differences between the normal hearing group and the moderate or severe hearing impairment group. When we model that, that's exactly what we found. That for individuals who had a moderate or severe hearing loss compared to normal hearing, the estimated increase in the risk was 55 percent over those nine years. The hazard ratio was 1.55. Again, this was after adjusting for a number of important risk factors: age, sex, race education, study sites, smoking status, hypertension, diabetes, and stroke. Again, a strong relationship between hearing loss and incident dementia. The third and final study that was included in the Lancet's report included over 1,000 men followed for nearly 17 years in the carefully prospective study. This was a study in the United Kingdom. Here they were looking at pure tone average and they modeled pure-tone average continuously. They were looking at the relationship between pure tone average and risk of all cause dementia, as well as with different dementias of different etiologic subtypes like a vascular dementia or a non-vascular dementia, which would have included something like Alzheimer's disease. What they found was that for every increase, for 10 decibel increase in hearing loss for pure-tone average, the risk of all cause dementia was increased by 2.67. That was the estimate. Then we also saw associations with vascular dementia in particular, as well as non-vascular dementia too. Across the board, relationships with hearing loss, with increased levels of pure-tone average, with the association with a number of different etiologic subtypes of dementia. You may have noticed that those three studies were in high-income countries; two were from the United States, one was from the UK. When we think about the relationship between hearing loss and dementia in different parts of the world. The risk associated with that is going to vary depending on the prevalence of the risk factors in that location. Here is the same type of analysis also conducted with the same Lancet Commission group looking at the relationship between hearing loss and dementia in low and middle-income countries. What they found was that hearing loss was still important, still associated with dementia. But the relative importance differ because the prevalence of some of these other risk factors was higher among middle and low-income countries. For example, in Latin America, this is all the way over on the left. In the orange and red color, we see that the relationship between hearing loss and dementia is estimated to be eight percent. But hypertension and obesity also just as important, nine percent and eight percent and low education is really important in Latin America, with about 11 percent of the risk of dementia estimated to be due to low education. We see a similar story when we look in India, and this is in the middle part of this figure. Low education was associated with a 14 percent increase in the risk of dementia. Hearing loss is still important, but the relative importance is a little last so it's estimated to be about six percent of dementia in India is due to hearing loss and can potentially be prevented if no one had hearing loss in India. Similar in China, where we have an estimated four percent in terms of the relationship between hearing loss and dementia. Again, early life, low education, really important risk factor. Just something for us to keep in mind when we're evaluating what types of interventions we should be thinking about in different parts of the world. In summary, there have been quite a few studies now in multiple populations that have looked at the relationship between hearing loss and dementia. For the most part, they're consistent. We're seeing an association where hearing loss is associated with increased risk of dementia over time. Overall, that estimated association is strong and it gets stronger as the severity of hearing loss increases. But what's important and there are studies currently ongoing. We don't yet know if treating hearing loss can prevent or postpone dementia. That's a really important area of future research.
