Untreated hearing loss linked to dementia.
The updated 2020 Lancet report on dementia finds an untreated hearing loss in midlife as the largest modifiable risk factor to prevent dementia.
Worldwide around 50 million people live with dementia, and this number is projected to increase to 152 million by 2050. An update to the 2017 Lancet Commission on dementia, prevention, intervention and care expands the number of modifiable risk factors from 9 to 12.
Forty per cent of dementia cases could be prevented or delayed by targeting these 12 modifiable risk factors.
The 12 modifiable risk factors for dementia are: Hearing impairment, Physical inactivity, Education, Diabetes, Hypertension, Low social contact, Smoking, Excessive alcohol consumption, Obesity, Brain injury, Depression, Air pollution.
Of these 12, hearing loss in midlife remains the largest modifiable risk factor, estimated to account for 8% of the risk.
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30367-6/fulltext
Hearing impairment
Hearing loss had the highest PAF for dementia in our first report, using a meta-analysis of studies of people with normal baseline cognition and hearing loss present at a threshold of 25 dB, which is the WHO threshold for hearing loss. In the 2017 Commission, we found an RR of 1·9 for dementia in populations followed up over 9–17 years, with the long follow-up times making reverse causation bias unlikely.
A subsequent meta-analysis using the same three prospective studies measuring hearing using audiometry at baseline found an increased risk of dementia (OR 1·3, 95% CI 1·0–1·6) per 10 dB of worsening of hearing loss.
A cross-sectional study of 6451 individuals designed to be representative of the US population, with a mean age of 59·4 years, found a decrease in cognition with every 10 dB reduction in hearing, which continued to below the clinical threshold so that subclinical levels of hearing impairment (below 25 dB) were significantly related to lower cognition.
Although the aetiology still needs further clarification, a small US prospective cohort study of 194 adults without baseline cognitive impairment, (baseline mean age 54·5 years), and at least two brain MRIs, with a mean of 19 years follow-up, found that midlife hearing impairment measured by audiometry, is associated with steeper temporal lobe volume loss, including in the hippocampus and entorhinal cortex.
Hearing aids
A 25-year prospective study of 3777 people aged 65 years or older found increased dementia incidence in those with self-reported hearing problems except in those using hearing aids.
Similarly, a cross-sectional study found hearing loss was only associated with worse cognition in those not using hearing aids.
A US nationally representative survey of 2040 people older than 50 years, tested every two years for 18 years, found immediate and delayed recall deteriorated less after initiation of hearing aid use, adjusting for other risk factors.
Hearing aid use was the largest factor protecting from decline adjusting for protective and harmful factors.
The long follow-up times in these prospective studies suggest hearing aid use is protective, rather than the possibility that those developing dementia are less likely to use hearing aids. Hearing loss might result in cognitive decline through reduced cognitive stimulation.
