Researchers at the Hopkins School of Medicine and the National Institute on Aging have discovered a link between hearing loss and the development of dementia including Alzheimer’s disease.
The study focused on 639 patients who were part of the voluntary Baltimore Longitudinal Study of Aging (BLSA), which began in 1958 and is still ongoing. Changes in hearing were only measured in participants up to 1994.
These audiometric measurements were compared to more recent ones taken starting in 2006. They found that persons with a greater level of hearing loss were more likely to be male, older and hypertensive.
The patients were then examined for neurological symptoms of dementia, and this was compared to the 1994 BLSA data on these symptoms.
Changes in these symptoms were then compared to changes in hearing. All measurements were adjusted for age, sex, race, education, smoking, diabetes and hypertension, the last two of which have been determined to be risk factors for dementia in previous studies.
Using a measurement known as pure-tone average (PTA), hearing levels in the participants were taken. This measurement is determined in units of decibels/year (decibels are the standard unit used to measure sound).
In the more recent set of samples taken by Hopkins researchers, participants were asked to return for new PTA measurements. The frequency of visits was related to age: PTA measurements were taken more frequently in older persons.
In 1994, when the last audiometric measurement was taken by BLSA, approximately one-quarter of the volunteers had developed hearing loss and none had been diagnosed with dementia.
In the group of volunteers that would later develop dementia, PTA loss of hearing was measured as 0.52 dB/yr, almost twice the 0.27 dB/yr measured in volunteers who did not develop dementia.
This was compared to data obtained when only participants age 65 or older were considered, or when participants with a history of stroke were included. These restrictions did not yield any significant changes in the relation between hearing loss and dementia.
Hearing loss was further categorized according to other commonly accepted levels. They found that the severity of hearing loss had a direct, positive relationship with the risk of developing dementia.
Persons with moderate hearing loss were 1.6 times more likely to develop dementia than persons with mild hearing loss. Furthermore, this number increased to 2.6 for those persons with severe hearing loss.
The researchers proposed several theories for the relationship between these two conditions.
The first possibility is that the two conditions share a similar pathway of development. However, the researchers did not consider this a viable option since the methods of measuring hearing loss did not require an activation of high levels of sound processing in the brain, the kind of high levels that are investigated in diagnosing dementia. Another proposal is that the increasing strain of decoding sounds that comes with age could put a strain on the patient’s brain, thereby increasing the risk of developing dementia.
Finally, the researchers speculated that the social isolation that is sometimes found in persons with hearing loss could lead to dementia, as it is a known risk factor for this cognitive disorder.
