The cost of dementia in Canada in 1994 dollars including the cost of care in long-term institutions and medical/social assistance was estimated to be 3.9 billion dollars (US: 5 billion dollars) (1). This number increased to 33 billion dollars (US: 42.3 billion dollars) in 2011 according to the Alzheimer's Society of Canada. More staggering is that this cost is estimated to climb to 293 billion dollars (US: 375 billion dollars) by the year 2040. This dementia epidemic is now being felt worldwide yet our knowledge of the pathogenesis of the disease continues to be insufficient at this moment to provide any major clinical treatments that may either prevent or clinically delay the onset of this disease. However, with ongoing research there is hope that major breakthroughs in both diagnosis and treatment of dementia are closer to fruition than only a few years earlier.
ROLE OF SENSORY ORGANS IN THE PATHOGENESIS OF DEMENTIA
Our sensory organs provide constant and fundamental information that our bodies constantly process for spatial awareness and interaction with others. Specifically, our visual and auditory senses not only provide afferent information for important reflexic responses that are essential for our day to day functioning such as the vestibular-ocular-reflex for visual stabilization during motion, but also information that is required for higher cognitive function. Inputs from auditory and visual perception in the posterior brain regions provide crucial information to the frontal brain areas higher executive control networks to guide goal directed behavior or permit us to understand nuances of a conversation or the intricacies of artwork. This link between our sensory organs and their role in the maintenance of our cognitive abilities is only starting to be partially understood. Their importance to our cognitive system is highlighted in studies that demonstrate that a decline in sensory organs such as touch and smell are highly associated with some neurodegenerative conditions. Presence of these deficits is also correlated with disease progression. In some instances when the sensory deficit can be restored or improved, the affected individual's cognitive status can be improved via reduction of their cognitive load, for example, when trying to understand and participate in a conversation. Because we now have excellent clinical and surgical treatments for hearing loss such as hearing aids and cochlear implants, we can counteract the loss of auditory inputs that can accelerate cognitive decline, increase social withdrawal and isolation, and contribute to increased symptomatology of dementia (2).
HEARING LOSS AND ITS RELATIONSHIP WITH DEMENTIA
The WHO International Classification of Functioning and Disability in 2011 developed a broad bio-psycho-social framework in which a complex condition like hearing loss can be understood. Hearing loss when framed as a biological condition affects physical functioning via pathology in the ear. When considered from a psychological aspect, it impacts on related activities like speech comprehension. From a social perspective, hearing loss results in the loss of participation in activities like employment, receiving further education, and attending social events (3). In these multiple contexts, the true impact of hearing loss can be demonstrated and its potential implication for both contributing to and complicating dementia can be appreciated.
Over 5% of the world's population has disabling hearing loss measured as thresholds worse than 40 dB HL in the better hearing ear. By the year 2030, age related hearing loss is estimated to be within the top 15 causes of burden of disease (4). Otolaryngologists and audiologists are intimately aware of the enormous social, economic, and personal burden associated with hearing loss. Patients with hearing loss are more likely to experience social isolation, depression, and loss of self-esteem (5). Work by Mohr et al. (6) estimated the lifetime socioeconomic cost of hearing loss in prelingually deafened individuals to be in the range of one million dollars. Work by Lin et al. (7,8) have demonstrated an association with an increased risk of dementia, depression, poorer balance, and falls. This work also demonstrated that the risk of developing dementia also increased linearly with the severity of baseline hearing loss (1.27 times increased risk per 10 dB of hearing loss) (7).
The rising incidence of dementia and its associated social and medical costs are also well studied. Hearing loss is currently the 3rd most prevalent chronic condition in adults. Hearing aids have been clearly demonstrated to be efficacious in patients with sensorineural hearing loss. However, the role of routine screening for improving patient outcomes has not been studied (9). It is also not known whether hearing loss is simply a marker for early-stage dementia or is actually a modifiable risk factor (7).
Furthering the association between dementia and hearing loss that was established by Dr. Frank Lin's research, Gates et al. (10) in 2011 also demonstrated that central auditory dysfunction (CAD) was a harbinger of Alzheimer's disease. He followed 274 volunteers for 4 years with audiometric testing as well as three auditory tests for CAD including dichotic sentence identification (DSI), dichotic digits (DDT), and synthetic sentence identification (SSI-ICM). Those with incipient dementia also had significantly lower scores on CAD testing. In these incipient dementia cases, it appeared to be evident that CAD was a precursor to Alzheimer's disease.
A large meta-analysis was recently published in 2016 by Taljaard and Olaithe (11). Over 33 studies analyzing the relationship between hearing impairment and cognitive function were included. Many previous associations between hearing loss and dementia were confirmed in this study. The degree of impact in all domains of cognition was significantly associated with the degree of hearing impairment in treated and untreated hearing impaired individuals. However, most interestingly it also noted that treating hearing impairment significantly improved cognitive function.
Going beyond the clinical realm, there have also been recent animal studies that link deafness to cognitive dysfunction. Park et al. (12) exposed young mice to 110 dB white noise for 1 hour daily for 20 days. These mice developed moderate hearing loss at 6 months and when compared with their control counterparts with no hearing loss, they had decreased spatial working memory and recognition. Specifically, the hearing loss group at lower scores in several trial points in the radial arm maze. These results suggest a causal relationship between hearing loss and cognitive performance in these mice.
Due to the association between hearing impairment and age-related neurodegenerative diseases, it was speculated that auditory sensory cells may possess commonalities with neurons so they could potentially act as a more accessible tool to study Alzheimer's disease. Omata et al. (13) produced a new transgenic Alzheimer mouse line that also was demonstrated to express amyloid-β (Aβ) in auditory hair cells. These mice were then tested with auditory brainstem responses (ABR) and noted to have impairment in high frequency sound perception at 4 months, similar to patients with presbycusis with hair cell loss in the basal cochlea. In additional experiments, overexpression of the protein tau which is also associated with the development of Alzheimer's disease, synergistically enhanced Aβ-induced hearing loss at 4 months.
CHALLENGES AND OPPORTUNITIES
Despite the significant potential for positive clinical impact that can be envisioned between the association of hearing loss and dementia there are certainly major challenges ahead. One major barrier is to bring to the forefront the significant clinical and socioeconomic impact of hearing loss, which is still greatly underappreciated. Assessment of hearing is only performed when loss is noted and it is rarely included in routine primary care screening assessments (5). In fact, communication difficulties associated with hearing loss is not commonly thought of as a medical condition that needs attention (5). On the opposite spectrum, there is often a misconception that hearing loss has been cured by the development and widespread acceptance and utilization of cochlear implants in the severely hearing-impaired. Although some of this is the result of the tremendous clinical impact of cochlear implants, they do not truly and completely represent the holy grail of auditory neuroscience, which is the complete regeneration of lost cellular populations such as auditory hair cells and neurons that comprise most patients with sensorineural hearing loss (14).
However, the positive clinical impact of hearing aids and cochlear implants also represent a potentially very effective treatment for patients with both hearing loss and dementia. The association between hearing loss and dementia is firmly established. The opportunity ahead is dependent on furthering our understanding of this association and elucidation of the inter-related pathophysiologic relationships. Although early research does appear to demonstrate a positive clinical impact with regards to dementia outcome measurements in patients with concurrent hearing loss that is treated, continued research is absolutely vital in exploring the efficacy of these already well established clinical treatments for dementia prevention and in dementia treatment. Early studies have demonstrated some stabilization and in some cases improvement of cognitive function as early as 6 months after cochlear implantation but larger and longer term studies are required (15).
Although we currently do not have any significant therapies that can prevent or delay the onset and progression of dementia, the strong association with hearing loss does present very unique opportunities. First, hearing loss or its diagnosis may help identify those either at risk of developing dementia or perhaps those already diagnosed who will progress more rapidly. Furthermore, as further studies are established, the most significant potential breakthrough is that perhaps the early and aggressive treatment of hearing loss may represent the first efficacious intervention to either stop or slow the progression of this insiduous and morbid condition. Individually, hearing loss and dementia represent major medical conditions with significant medical and socioeconomic impacts. However, together, treatment of one condition may represent a significantly impactful treatment for the other. The future in this emerging field at the intersection of hearing loss and dementia is indeed bright.
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