By Kate Kneisel

Restoring hearing loss with hearing aids or cochlear implants was linked with less long-term cognitive decline and even some gain, a systematic review and meta-analysis showed.

Use of hearing aids by adults with hearing loss was associated with a significant 19% relative reduction in risk of any cognitive decline compared with uncorrected hearing loss across long-term studies with follow-up ranging from 2 to 25 years (HR 0.81, 95% CI 0.76-0.87).

Hearing aid or cochlear implant use was also associated with a 3% improvement in short-term cognitive scores (ratio of means 1.03, 95% CI 1.02-1.04), according to Benjamin Kye Jyn Tan, MBBS, of the National University of Singapore, and colleagues.

"Importantly, this benefit is evident for both normal baseline cognition and baseline mild cognitive impairment," after "adjusting for possible confounders, including age and gender, education, socioeconomic status, and comorbidities," the group reported.

Hearing loss has been identified as a top modifiable risk factor for dementia, the researchers noted. "This study adds to the growing evidence base and serves as an impetus for clinicians treating patients with hearing loss to persuade them to adopt hearing restorative devices, to mitigate their risk of cognitive decline such as dementia."

While the analysis couldn't establish causality, the findings support inclusion of hearing evaluation "as part of a standard workup for patients who may be experiencing cognitive decline," agreed an accompanying editorial by Justin S. Golub, MD, MS, of Columbia University Vagelos College of Physicians and Surgeons in New York City, and coauthors.

There has been a hypothesis that hearing loss and cognitive decline might occur independently as a result of a common mechanism of age-related neurodegenerative processes, Tan's group acknowledged.

While noting that "hearing loss in dementia is likely to be multifactorial," the study authors discussed several theories for how hearing aids might exert cognitive benefits. For example, the "sensory deficit hypothesis suggests that lack of sensory input may lead to structural alterations [in the brain], including atrophy," they wrote. "Allowing hearing restorative devices to provide sensory stimulation before prolonged deprivation may cause cortical changes that could prevent cognitive deterioration."

Another possible mechanism, they added, is that "hearing aid use may prevent social isolation [well-known to accompany hearing loss] and its resultant development of cognitive impairment, although further studies are required to analyze this association."

The meta-analysis looked at long-term associations between hearing aid use and cognitive decline in a pooled analysis of eight studies with a total of 126,903 participants as well as short-term outcomes in 11 studies with a total of 568 participants. Most of the studies were prospective cohorts or other observational designs; the two randomized clinical trials had only short-term data available.

In addition to diagnoses of dementia or cognitive impairment, cognitive outcomes were assessed with commonly used measures of cognitive function such as the Mini-Mental State Examination, the Montreal Cognitive Assessment, or both.

Fourteen studies only enrolled people with minimally moderate hearing loss. Five studies used self-reported hearing loss.

In stratified analysis of subgroups based on type of cognitive decline, all outcomes showed a significant effect with hearing assistive device use compared with controls:

-Incident cognitive impairment: HR 0.79 (95% CI 0.65-0.97)

-Conversion of mild cognitive impairment to dementia: HR 0.73 (95% CI 0.60-0.88)

-Incident dementia: HR 0.83 (95% CI 0.77-0.90)

Subgroup analysis by geographic region also showed significantly lower hazards of any type of cognitive decline among hearing device users across all continents included.

Statistical heterogeneity was low across the comparisons in the meta-analysis, the authors noted. The meta-analysis did not include any low-quality studies; findings were similar in analyses of studies with moderate and low risk of bias.

"This report is a testament to the power of pooling studies in meta-analysis," the editorialists wrote, noting that the effects had not reached statistical significance in most of the individual studies. They called for use of standardized measures of hearing loss and cognitive outcomes to support future research in the field, which should include patients with no baseline cognitive impairment and patients with mild cognitive impairment.

Tan's group noted limitations of their meta-analysis, including the inability to compare severity of hearing loss or to adjust for ethnicity and education level due to lack of data.