They say that we wouldn’t be able to tolerate the smell of the Middle Ages, a pervasive stench produced by the dumping of all the raw sewage in the small, congested streets; citizens had the chance to cloister themselves in their homes with incense and spices. But those same citizens wouldn’t be able to function anywhere with our noise pollution.
In our modern hyper-connected age, there are many causes of hearing loss due to the constant bombardment of noise in the public media that is transmitted to people’s personal devices, inside and outside their homes. A significant hearing loss (more than 20 decibels) manifested in the elevation of the threshold for pure detection affects almost half of people over 65 years old and two thirds of those above 80. The loss of hearing does affect the mental capacity of patients, being an important dementia cause. If the hearing loss is greater than 25 decibels, the patient will age approximately 7 years more.
The sense of Hearing has two main neurological components, which are the following:
- Peripheral Hearing: it includes all the sensory elements of the outer and middle ear that receive and transmit the encoded sensations to the Central Nervous System.
- Central auditory processing (CAP); it includes all the CNS structures involved in the hearing pathway that ends up in the Temporal lobe.
From the paraphernalia of sounds that we are being exposed to on a daily basis, our nervous system must separate the important sounds and “classify” them before their distribution to the brain; our encoding system allows us to differentiate the pitch, rhythm and timbre of the incoming noises. As we age the central processing of sounds progressively deteriorate and affects a majority of people. A clinical study involving 120 participants with a mean age of 70 years old found that the cognitive decline was related to understanding of speech in noise and not the peripheral pick-up of sounds.
RK Gurgel et al. studied a large cohort of 4,545 individuals with a mean age of 75 years old and no clinical signs of any cognitive decline; the clinical examination showed that 836 individuals had baseline hearing loss. All the study participants were subsequently followed for almost 12 years. In the hearing loss group 16% eventually showed signs of cognitive decline and in the normal group 12% did; the mean time to show signs was 10.3 years in the first group and 11.9 in the other. An additional 10% of the patients with hearing loss that developed cognitive decline eventually showed signs of Alzheimer’s disease, compared to 8% of those that did not have any hearing loss.
Lin MY et al. found that study participants with hearing loss had a significant increase in cognitive decline—almost 40% in some cases—and almost a quarter of those with abnormal hearing did develop cognitive decline in the follow-up period of six years; those with hearing loss took on average 7.7 years to show intellectual deterioration compared to 11 years for those without it. It’s a self-sustaining vicious circle as another study showed that dementia worsens hearing loss.
Jonathan Peelle et al showed that impaired hearing capacity is associated with the decrease in the critical volume of gray matter in the auditory cortex bilaterally, as well as the one of the prefrontal cortex; as the study was not longitudinal, it is not clear whether these finding were before or after the loss. Profant et al., using the novel radiological techniques of MR morphometry and Diffuse tensor imaging (DTI), showed that hearing loss can produce deterioration of the white matter coming in and out of the auditory cortex, suggesting that age alone is not the only factor in cognitive decline; the hearing loss can lead to sensory deprivation, wrong information processing and anatomy changes.
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