Dec 2020 The Hearing Journal
Dementia is a prevalent and costly public health concern, with more than 24.3 million people living with this condition worldwide, and approximately 4-6 million new diagnoses made each year. The underlying cause of dementia is not fully understood, but in recent years research has highlighted the association between declining sensory systems and cognitive function. For instance, declines in the olfactory, visual, and auditory systems are all associated with an increased risk of developing dementia. In particular, the auditory system has received substantial attention in recent years. Some have estimated that hearing loss contributes to as much as 9.1 percent of all dementia cases. However, there remains no clear evidence of a causal association nor a clear understanding of the underlying mechanism.
Figure 1: The differences in the temporal area in a patient who has had asymmetric hearing loss for 15 years. The right side of the temporal area is atrophic when compared with the contralateral side, in which the auditory threshold is normal.
Figure 2: The difference in working memory performances without hearing aids (blue box) and with hearing aids (HA) or bone-anchored hearing implant (BAHI) in noisy conditions. Audiology, paediatrics, brain health.
Several MRI studies have been conducted to identify the link between hearing loss and cognitive decline (Fig. 1). For instance, hearing loss is associated with brain atrophy in regions critical for cognitive function such as the temporal and cingulate cortices. Another hypothesis that has gained some traction is that cognitive decline in the elderly is fuelled by behavioural consequences that emerge from hearing loss. For example, those with hearing loss are more apt to socially isolate and display a higher prevalence of depression, both of which have also been linked to a greater risk of cognitive decline. However, if cognitive decline emanates from the behavioural consequences of hearing loss, one would expect that treating the hearing loss would slow the rate of cognitive decline and also reduce the severity of social isolation and depression—a trend that some have failed to observe. Nevertheless, treatment of hearing loss does appear to aid in the defence against cognitive decline in older adults. One possible reason for this is the amelioration of working memory capacities by using a hearing prosthesis.
Taken together, research suggests a strong association between cognitive decline and hearing loss in older adult populations, but the underlying mechanism remains not fully understood. Moreover, it is extremely difficult to disentangle the link between hearing loss and cognitive decline since age-related hearing impairment often occurs alongside other sensory deficits. Therefore, our recent studies aimed to explore cognitive deficits in the paediatric population with unilateral hearing loss (UHL). Testing paediatrics with UHL provides a unique opportunity since it avoids age-related comorbid sensory deficits and removes confounding effects of hearing loss on neuropsychological evaluations. In this population, we tested working and short-term memory abilities prior to hearing loss treatment and again at one and six months post-treatment. Tests of working and short-term memory were conducted using a normalised neuropsychological evaluation (PROMEA) and compared to age-matched normal-hearing controls. Paediatric patients with unilateral hearing loss showed significantly poorer working memory and short-term memory abilities than their normal-hearing counterparts. However, following treatment of the unilateral hearing loss working memory and short-term memory scores improved significantly at both 1-month and 6-month post-treatment evaluations (Fig. 2). Our findings in paediatrics support the notion that age-related hearing loss likely contributes to cognitive decline.
Evidence on the causal mechanisms of dementia and the impact of hearing loss on cognitive decline is still too preliminary to be considered definitive. However, the improvement of memory functions by hearing restoration is clearly and strongly confirmed by case-control studies. Despite observations that cochlear implants can restore auditory brain connections, the effect of hearing rehabilitation on the involution of brain atrophy has not yet been demonstrated. Studies on memory function of adults with age-related hearing loss with and without hearing prosthesis are necessary to confirm the results observed in children. But noting the positive effects that hearing restoration could have on cognition, we strongly suggest hearing rehabilitation in adults with hearing loss.