More Evidence for Impaired Hearing to Contribute to Cognitive Decline
There is some debate over the mechanisms involved in the bidirectional relationship between hearing loss and cognitive decline in aging. While both must, logically, arise from the same underlying causes of aging, the increased burden of a variety of forms of cell and tissue damage that produce many different forms of dysfunction, it appears that (a) loss of function in the brain can itself contribute to hearing loss, while (b) hearing loss can in and of itself accelerate the pace of cognitive decline.
Thanks to the existence of electromechanical means of improving hearing, meaning hearing aids, cochlear implants, and the like, there is a growing body of evidence to show that people with these devices suffer a slower pace of cognitive decline than those without, at a given level of age-related hearing loss. This is a strong argument for portions of the brain to require the exercise of processing sound in order to better resist decline. "Use it or lose it" isn't just for muscles, and this isn't the only line of evidence to suggest that a well exercised mind will lose functional capacity more slowly in later life. Similar effects are observed for blindness and cognitive decline, for example, implying that the exercise of processing of visual information helps to resist loss of brain function.
Interestingly, the study noted in today's open access paper suggests that the effect of maintained processing of audio information only goes so far. It only helps for a few years. More is needed if we are to be the masters of our own destiny when it comes to the aging of the brain. Rejuvenation therapies must directly address the underlying mechanisms of cell and tissue damage, and the research community must aim at greater goals than a mere slowing of aging.
Cochlear implants (CI) are the gold standard intervention for severe to profound hearing loss, a known modifiable risk factor for dementia. However, it remains unknown whether CI use might prevent the age-related cognitive decline. Recent studies are encouraging but are limited, mainly by short follow-up periods and, for ethical reasons, lack of appropriate control groups. Further, as age-related cognitive decline is multifaceted and not linear, other statistical approaches have to be evaluated.
Immediate and delayed recall as measures of cognitive function were assessed in 75 newly implanted CI users (mean age 65.41 years ± 9.19) for up to 5 years (mean 4.5 ± 0.5) of CI use and compared to 8,077 subjects of the same age range from two longitudinal cohort studies, the Health and Retirement Study (HRS) and the English Longitudinal Study of Aging (ELSA). Linear and quadratic changes in cognitive trajectories were analyzed in detail using mixed growth models, considering possible confounders.
For CI users, the linear time slope showed a significant improvement in the specific domains (recall and delayed recall) over time. The quadratic time slope clearly indicated that the predicted change after CI provision followed an inverted U-shape with a predicted decline 2 years after CI provision. In the hearing-impaired group, a significant decline over time was found, with steeper declines early on and the tendency to flatten out in the follow-up. In conclusion, cochlear implant use seems to boost cognitive trajectories in the first years after implantation. However, long-term prevention of dementia seems to need far more than restoration of hearing loss.