An "eagerly awaited" clinical trial has delivered good news: hearing aids may slow the rate of cognitive decline by nearly 50 percent in some older adults.

The US trial involving almost 980 participants, is the first large, randomized trial testing the effect of hearing aids on cognition, which slows normally with age but can recede quickly in dementia.

A large number of people stand to benefit from these results, should they be corroborated. Untreated hearing loss is estimated to contribute to around 8 percent of dementia cases worldwide.

Before this trial, years-long observational studies had found people with hearing loss who wore hearing aids had a reduced risk of cognitive decline compared to those who didn't wear them.

Other studies revealed that people who had trouble hearing in noisy environments were at greater risk of developing dementia, which might manifest as confusion or memory loss.

But the question remained whether using hearing aids could help stave off any drop in cognitive function in older adults – something only a clinical trial could hope to answer.

Participants were recruited in two ways: 739 healthy volunteers aged 70–84 years who responded to advertisements, and another cohort of 238 older adults who had a higher risk of cognitive decline, recruited from an ongoing longitudinal health study.

Randomized into two roughly equal groups, half the participants were fitted with hearing aids; the others received an evidence-based education program on healthy aging.

Wearing hearing aids did indeed reduce cognitive decline during the trial as hoped, but only in participants who had more risk factors for dementia to begin with.

While only aiding a select group, hearing aids had a demonstrable effect: they almost halved the amount of cognitive change seen in at-risk adults over 3 years compared to those on the health education program sans hearing aids.

"These results provide compelling evidence that treating hearing loss is a powerful tool to protect cognitive function in later life, and possibly, over the long term, delay a dementia diagnosis," says otolaryngologist and lead author Frank Lin of Johns Hopkins University.

"But any cognitive benefits of treating age-related hearing loss are likely to vary depending on an individual's risk of cognitive decline."

Those from the health study were typically older, Black, and more likely to have less education, lower income, and live alone, to name a few of their risk factors for dementia. It was this group whose minds benefited from hearing aids, despite having similar levels of hearing to healthy volunteers at the start of the trial.

The health study group also incidentally developed cognitive impairment at nearly three times the rate of healthy volunteers, suggesting people in this group were further down the road toward dementia, if not diagnosed sometime during the trial.

"Hearing aids could really make a difference for populations at risk of dementia," Gill Livingston and Sergi Costafreda, two psychiatrists at University College London, write in a commentary. They were not involved in the trial.

"The findings from this study are hopeful," they add. But "we urgently need more trials in other settings" to see if the "enormous protective effect of hearing aids on cognition" carries over to other at-risk groups and populations.

It's possible that using hearing aids could also affect a person's mood, independence, social connectedness, or involvement in physical activity – factors that may also impact or slow cognitive decline. Future studies could tease apart the effects.

The study has been published in The Lancet.