Alzheimer's disease, a progressive neurodegenerative disorder, has long been associated with cognitive decline and memory impairment. However, emerging research has unveiled a compelling connection between AD and hearing loss, as well as other forms of dementia and cognitive dysfunction. According to a 2011 study by Dr. Frank Lin and his team at Johns Hopkins, people with hearing loss are five times more likely to develop dementia than people without hearing impairments. These findings ignited curiosity among researchers, prompting them to explore the underlying mechanisms linking these two conditions.

Amyloid plaques, abnormal protein aggregates, are known to play a crucial role in the development and progression of Alzheimer's disease. Initially, these plaques form in the brain regions responsible for memory and cognitive functions. However, recent research conducted at the University of Rochester Medical Centre indicates that amyloid plaques can also have a detrimental effect on hearing.

The study, led by neuroscience graduate student Daxiang Na and his colleagues, utilised mouse models to investigate the relationship between Alzheimer's pathology and central auditory processing disorder (CAPD). CAPD, often associated with "hidden hearing loss," refers to a condition where individuals struggle with speech understanding despite normal hearing test results. Na's team made a serendipitous discovery when examining the amyloid plaques in older mice. They found that the plaques were more frequently located in the auditory processing regions, suggesting a potential contribution to hearing loss.

In their meticulous analysis of the two different mouse models of AD, Na and his team observed intriguing differences in hearing changes. The older 5xFAD mice, with plaques primarily located in the auditory processing regions, exhibited hearing changes similar to those observed in people with AD. In contrast, the other mouse model (APP/PS1) and younger mice in the 5xFAD group did not demonstrate the same degree of hearing changes.

These findings shed light on the significance of plaque location in hearing decline. It is believed that the presence of plaques in the auditory brainstem disrupts its ability to coordinate responses to sound, leading to the auditory symptoms observed in Alzheimer's patients. Moreover, the researchers speculate that the location of plaques could serve as a potential biomarker to track disease progression and aid in the diagnosis of AD.

To further explore the auditory dysfunction associated with AD, the researchers employed auditory brainstem response (ABR) waveforms. The ABR patterns observed in the mouse models provided valuable insights into the distinct characteristics of each model. The 5xFAD mice exhibited increased central gain, indicating central hyperactivity, while the APP/PS1 mice showed reduced central gain. Notably, the increase in central gain in the 5xFAD mice occurred before other hearing-related changes, suggesting it could serve as an early indicator of auditory dysfunction in AD. In terms of which parts of the brain were affected, the researchers saw a lot of plaque buildup in the 5xFAD mice's auditory cortical regions, auditory thalamus (MGB), and superior auditory brainstem regions. However, these increases were not observed in the APP/PS1 mice. Additionally, the study revealed a deficit in hearing in noise, a core symptom of CAPD, in the 5xFAD mice for the first time.

While this study provides significant insights into the intricate connection between AD and hearing loss, there is still much to be explored and understood. The severity and progression of hearing loss were found to vary between the different mouse models, suggesting that different rates of plaque accumulation influence the extent of hearing loss. Longitudinal studies are crucial to validating these findings and investigating their clinical applications. Monitoring ABRs over time could potentially assist in identifying different patient populations and guiding future treatment approaches. By deepening our understanding of the link between auditory dysfunction and AD pathology, we can enhance patient care and improve the quality of life for individuals affected by AD.

The groundbreaking research conducted at the University of Rochester Medical Centre has unveiled a new dimension in our understanding of Alzheimer's disease and hearing loss. The study highlights the potential impact of amyloid plaques on hearing decline and emphasises the importance of plaque location as a potential biomarker for disease progression and diagnosis. By exploring the intricate connection between AD and hearing loss, we take significant strides towards advancing treatment methods and improving the lives of individuals affected by these conditions.

Reference: Na D, Zhang J, Beaulac HJ, Pikna-Przybylska D, Nicklas PR, Kiernan AE, White PM. Increased central auditory gain in 5xFAD Alzheimer's disease mice as an early biomarker candidate for Alzheimer's disease diagnosis. Front Neurosci. 2023;17. Published online May 26, 2023. doi.org/10.3389/fnins.2023.1106570