A relationship between dementia and hearing loss was proposed decades ago. A paper published in 1989 reported a case-control study of 100 people with dementia or 'cognitive dysfunction' and 100 age, sex and education-matched controls, finding that the "prevalence of a hearing loss of 30 dB or greater was significantly higher in cases than in controls (odds ratio, 2.0; 95% confidence
interval, 1.2 to 3.4), even when adjusted for potentially confounding variables." Hearing loss was correlated with severity of cognitive impairment as well, with more profound loss being more frequent in those with more severe dementia.
But, which came first, the hearing loss or the dementia? This is a question that Frank Lin and co-authors set out to answer with a prospective study of 639 people aged 36-90 and dementia-free in 1990-1994. Hearing was tested when people were enrolled in the study, and they were followed for 11 or so years and tested for the onset of dementia yearly or biennially for most of that time.
Fifty-eight cases of dementia from all causes were diagnosed, of which 38 were considered to be Alzheimer disease; 4.4% of these had normal hearing. The group with normal hearing was younger at the start of the study (mean age of those with no hearing loss was 59.9 compared with 71-77 for those with mild, moderate or severe loss), though the researchers did control for age in their analysis, so it should not explain the difference in the fraction of each hearing category that were subsequently diagnosed with cognitive impairment. Though, prevalence of dementia was about 14% of the US population in 2002 at age 71 and older (5% for those 71-79 and 37.4% for those 90 and older) (Plassman et al., 2007), so the number will surely increase in the normal hearing group as the population ages. Almost 17% of those with mild loss developed dementia, and 28.3 and 33.3% (2 people) of those with moderate and severe hearing loss. The difference was statistically significant.
As Lin at all report in a 2013 paper:
The magnitude of these associations is clinically significant, with individuals having hearing loss demonstrating a 30% to 40% accelerated rate of cognitive decline and a 24% increased risk for incident cognitive impairment during a 6-year period compared with individuals having normal hearing. On average, individuals with hearing loss would require 7.7 years to decline by 5 points on the 3MS (a commonly accepted level of change indicative of cognitive impairment17-19) vs 10.9 years in individuals with normal hearing.This still doesn't answer the question of whether, as Lin et al. wrote in 2011, "hearing loss is a marker for early-stage dementia or is actually a modifiable risk factor for dementia," however. Is it the first sign of something going wrong -- arteriosclerosis or some such, which might cause hearing loss and dementia both -- or is the isolation that often comes with hearing loss and aging a cause of dementia? If so, and if hearing loss can be prevented or ameliorated, can dementia also be prevented? Or, as Katherine Bouton suggests in her piece, could the "cognitive load" that comes from struggling to understand speech be causal? It's not a biomechanical connection but a psychosocial one.
There are a lot of issues here to sort out, though, before we can even consider these questions. Not all hearing loss is alike. In fact, it can be neurosensory, having to do with cellular or physiological abnormalities of the structures of the inner ear and the auditory nerve, or conductive, the result of things that can go wrong mechanically -- temporarily or permanently -- with the bones in the middle ear that conduct sound waves to the inner ear.
Age of onset varies widely, from congenital to advanced old age. People don't necessarily lose all of their hearing, but can lose only certain frequencies so they can't hear, say, the highest bird song or musical bass lines, or they can lose the ability to hear conversation, or conversation in a crowded room or, of course, to hear anything. And, hearing loss can be mild to profound.
So, in trying to determine whether hearing loss 'causes' dementia, these kinds of issues should probably be considered. Unless it's the cognitive load, As Katherine Bouton suggests, in which case perhaps comparing the experience of people who've been deaf since birth and are members of the deaf community, and therefore for whom cognitive load wouldn't be an issue, with that of people who lose their hearing in adulthood, and have experiences such as Ms Bouton describes, would help elucidate the connection.
And, not all dementia is alike. There are numerous conditions, symptoms, causes, ages at onset and so forth. So, it's possible that the explanation for the possible correlation between hearing loss and dementia may look different depending on the kind of loss, extent of loss, and age at onset, and of course the kind and extent of dementia. And this means that a study based on only 184 individuals with any hearing loss, most of them old, in a sample of 639, and a total of 38 cases of dementia among those with hearing loss, is probably not a large enough study to tease out the relationship.
But it's an interesting question. Though, if a large majority of people over 80 have some hearing loss, but only a third or so have cognitive impairment, there may be more pressing things to worry about.