Hearing aids did not significantly slow the slide to dementia in a two-year trial

Hearing loss is often listed as the single biggest risk factor for dementia that people could in principle change. The evidence for that comes mostly from observational studies, which follow people over time and can show links but not causes. The obvious test is a randomized controlled trial: give hearing aids to one group, not to another, and see what happens to their thinking. An earlier large trial in people with normal cognition found no overall effect over three years. A team in Shanghai has now run such a trial in people who were already showing early problems.

Those early problems are called mild cognitive impairment, or MCI: memory and thinking are measurably worse than expected for a person's age, but daily life still works. People with MCI are at higher risk of developing dementia, yet some of them return to normal over time.

What they did

The researchers screened 21,908 people aged 60 or older, recruited from three hospital ear clinics and from an ongoing community study of hearing. They enrolled 703 who had moderate to severe hearing loss and MCI, defined by a score of 0.5 on the Clinical Dementia Rating (CDR). The CDR is a structured interview, lasting 30 to 45 minutes, that rates memory and everyday functioning on a scale where 0 means no impairment, 0.5 means questionable or very mild impairment, and 1 or more means dementia-level impairment.

Participants were randomly split into two groups. Half (353) were fitted with free hearing aids for both ears by audiologists, with check-ups and adjustments at each visit. The other half (350) received a hearing-care education program of about 45 minutes per visit, covering hearing loss, chronic disease and healthy habits, so both groups had similar contact time with the study staff. Everyone was assessed at the start and at 6, 12 and 24 months.

The main question, set in advance, was how many people would progress to a CDR score of 1 or more within 24 months.

What they found

• Main result: no significant difference. After two years, 3.09% of the hearing-aid group and 4.74% of the education group had reached dementia-level impairment. That gap was not statistically significant; the range of plausible effects ran from a clear benefit to a slight harm.
• Secondary result: more people improved. In the hearing-aid group, 15.34% were rated as fully unimpaired (CDR 0) at 24 months, against 2.36% in the education group. The researchers call this finding exploratory and say it needs cautious interpretation.
• Hearing got better. In the hearing-aid group, average hearing thresholds improved from about 53 to 32 decibels while wearing the aids. People wore them for 5.9 hours a day on average.
• No safety problems from the devices. There were 20 serious adverse events, 10 in each group, none related to the study.
• Checks with different ways of handling the 93 people who did not finish gave similar results.

What to keep in mind

The researchers list several limits themselves. Far fewer people progressed to dementia-level impairment than they had expected when planning the trial, so it was too small (underpowered) to detect a modest effect on the main outcome. Two years is a short time for dementia to develop. The rule that everyone had to score exactly 0.5 on the CDR was not written into the trial registration in advance, which the team says could introduce selection bias, and a CDR of 0.5 is not the same as a full clinical diagnosis of MCI, so the results may not apply to everyone with MCI. The CDR, a rating based on interviews, was used both to choose participants and to judge the outcome.

The trial was open-label: participants knew whether they had hearing aids, which cannot be hidden. The team tried to limit bias by having assessors who did not know the group assignment and by removing the hearing aids before testing, but they say they cannot fully rule out that better everyday hearing affected the scores. Receiving free hearing aids may also have influenced how people answered. The daily wearing time was lower than in the earlier trial in people with normal cognition. And the study was in Mandarin-speaking older adults, more than 90% of whom lived with family; the authors caution against applying the results directly to people who speak non-tonal languages or have less support.

Why it matters

This is one of the largest trials so far to test hearing aids in people who already have early memory and thinking problems, and its main answer is a modest one: over two years, hearing aids did not measurably reduce progression to dementia-level impairment. The higher rate of return to normal ratings is intriguing but is the kind of secondary finding that needs to be confirmed in other trials before it can be trusted. Several such trials in people with hearing loss and MCI are under way.

Whatever the effect on dementia turns out to be, hearing aids help people hear and communicate, and the team notes that their use among older adults in China remains below 10%.