Hearing aids slow cognitive decline in people at high risk

I think a more helpful way to look at research is to look for a convergence of outcomes across the evidence, like a bunch of needles of various sizes pointing in the same direction (or not) on a gauge. And where there are divergences, and there always will be, which differences in methodology can explain them.

In this example, it would be particularly hard to establish a causative relationship that said "hearing aids inhibit dementia" or "hearing loss causes dementia" if instead all you had was a population, some of whom used hearing aids, and some of whom had varying levels of hearing abilities, and some of whom developed dementia. In all the noise, you'd be very fortunate to find that a control group of people with undiagnosed (how would you ever measure that!) hearing loss who failed to get hearing aids had cognitive decline in excess of that experienced by those who did get hearing aids. Here, instead, the intervention is "the coin flip said you're in the hearing aid group/you're in the health education group."

Of course, the murky ethical issue here is that this randomized intervention resulted in some people having poorer hearing, perhaps permanently (use it or lose it!) and some people having more rapid cognitive decline. This randomized intervention directly caused real suffering. Nearly 125 people in the heart-health group with increased risk of dementia onset were assigned to a study that just received health education. For at least three years, they had reduced hearing and a reduced ability to communicate, and after three years, they had significantly greater cognitive decline.

Caution is good, but in general I have no problem personally with the 'shut up and multiply' application of ethics in this case. There are 8 billion people all getting old, and more being born every day who will eventually get older. It's very important as a species to know that hearing aids slow cognitive decline, and if that meant that 119 people were placed at a slightly increased medical risk, 119 divided by 8 billion is a pretty small number, and 7.999999881 billion who are now more likely to get hearing aids and have slower cognitive decline is a lot of positive utility. But it wasn't me or my mom who can no longer hear and who is suffering from dementia, and some will draw a hard line and say that you can't perform studies that will put even a small number of people at increased risk.

We should be careful to evaluate merit and limit harm, but we should perform more randomized intervention studies.

Unfortunately it suffers from at least two problems.

First, generalisable effects from specific interventions often fail to replicate, so for example subjects tend to get better at a specific task but it doesn't necessarily carry over to real-world contexts. There are a few promising potential directions, although even those results are modest and uncertain.

Second, people with even mild/early signs of dementia are extremely unlikely to continue with an intervention. Even if they do they often modify it, e.g. unplugging a smart device to save energy, or taping over buttons to prevent accidentally pressing them.

I would hunt out references but I'm on mobile, they shouldn't be hard to find.

Did their friends and family learn sign language as well? People who are deaf young tend to get a social circle who can communicate with deaf people, while old people are unlikely to rebuild all of that.

If you don't need to use the language except for a one week tour (where you are still mostly with people who speak your language) this is good enough. However if you are suddenly without ability to communicate you probably can't suddenly learn fast enough to combat decline.