Presbycusis

Presbycusis

Presbycusis impacts about 35-40% of subjects aged 60 to 70 years, and over 50% of those aged 75 and above. It is considered to be the second most frequent disorder of the elderly, even if the selective loss of high ranged frequencies begins being of clinical importance already for those aged between 40 and 50. Some correlation with depression, cognitive decline and dementia has been shown.

It is characterised by a multisectoral senescence of the auditory organ:

• Tympanosclerosis
• Degeneration / demineralization of the auditory ossicles
• Degeneration of the nerve fibres of the acoustic pathways leading to the neuronal degeneration of the pertaining brain areas
• Degeneration / reduction of cochlear hair cells starting from the cells responsible for the perception of acute frequencies, involving initially the base of the cochlea.

Precisely the latter factor is the predominant cause.

Its pathogenesis appears to be linked to oxidative stress inducing mutations in the mitochondrial DNA with consequent dysfunction of these organelles. This may result in an accelerated metabolic senescence of the cochlear hair cells which, for this reason, would go into apoptosis.

Presbycusis has a higher prevalence in males than females. Correct lifestyles and the cultural component (mnemonic commitment, reading…) are of relevance.

Hearing loss has an important impact on interpersonal communication leading to social isolation that can be truly severe, consequently, also worsening one’s well-being and quality of life. The cognitive difficulty of language and verbal discrimination are added to the impairing effect of hearing loss.

Said difficulties become particularly accentuated in environments where there is an overlap of voices or background noise, hence, the difficulties in participating in conversations and daily life. Understanding radio and television also becomes difficult, with a consequent aggravation of isolation.

The slow progress of the disorder is insidious because the patient is led to neglect the condition. It is detected by means of audiometric examination characterized, at least initially, by a downward bend of the audiometric curve on bilateral and symmetrical high frequencies.

There are accelerators of the condition, truly favouring factors:

• Chronic noise damage / acoustic trauma.
• Ototoxic substances / ototoxic drugs (aspirin, chemotherapy, diuretics, anti-inflammatories, antibiotics…).
• Concomitant systemic pathologies (arterial hypertension, atherosclerosis, hypercholesterolemia, hyperhomocysteinemia, hypertriglyceridemia, liver disease, heart disease, thyroid disease, diabetes, and endocrine diseases in general).
• Cigarette smoking.
• Genetic / family predisposition.

Complications often associated with Presbycusis are:

• Difficulty in sound perception in terms of hearing loss, but also in identifying the location of the sound source.
• Impaired cognition and understanding of language.
• Social isolation.
• Mnemonic deficits.
• Cognitive impairment.
• Balance deficit characterized above all by chronic and worsening dizziness (chronic instability)
• Tinnitus, often pulsating, with frequent association of ear muffling accentuated with the perception of more intense sounds.

In summary, presbycusis is associated with a general decline in the quality of life which can be really severe. It is essential to diagnose it as early as possible.

Schuknecht’s classification

1. Sensory Presbycusis – Degeneration / reduction of sensory cells of the cochlear base and, initially, of the outer hair cells, with early onset.
2. Neural Presbycusis – Reduction of the nerve fibres of the acoustic pathways, with a pantonal hearing deficit, alteration of sound perception and of audiometric speech discrimination, a characteristic phenomenon known as verb-tonal dissociation.
3. Metabolic Presbycusis – Degeneration of the vascular stria, with metabolic-oxidative stress and consequent impairment of the outer hair cells. The loss of the tonal threshold is generally of a low entity and pantonal.
4. Mechanical Presbycusis– A purely histological form. The Corti organ, vascular stria, and neurons are free from degeneration. A clinical hypothesis is the thickening of the basilar membrane, an extremely fragile structure. The audiometric curve shows a reduction in the high frequencies involving at least five frequencies. Verbal discrimination remains good. Calcification of the base of the cochlea and / or the presence of lipid deposits seem to play an important role.
5. Central Presbycusis – Damage to the central acoustic pathways starting from the reduction of the nerve fibres of the VIII cranial nerve up to the portion of cerebral cortex responsible for auditory perception. In such clinical condition, verbal discrimination is strongly affected, representing those patients who gain poor satisfaction and results from hearing aids. There is a frequent finding in diagnostic imaging, especially in MRI, of cortical atrophy.

Decidedly rarer and less defined are vascular presbycusis (reduced cochlear vascular supply) and hyperostotic presbycusis (neurogenic compression due to hyperostosis of the external auditory canal).

Mixed Presbycusis, where no criteria seem to respond to those previously indicated, and Indeterminate Presbycusis, characterized by a lack of audiometric and histological correlation, have a wide field of discussion that is not completely universally accepted.

As previously mentioned, presbycusis seems to have a decisive impact on the patient’s cognitive decline, starting from the social isolation to which it can lead, up to dementia.

• At the same age, presbycusics as compared to normal hearing patients show accelerated rates of decline on non-verbal cognitive tests (Lin & coll., Neuropsycol. 2011,25).
• Compared to people with normal hearing, those with mild, moderate and severe hearing loss have shown a two-, three- and five-fold increased risk of developing dementia (Lin & coll., Arch.Neurol. 2011,68)
• In subjects with hearing problems, the incidence of new cases of dementia is higher and the latency time shorter (10.3 vs 11.9 years) (Gurgel RK et al., Otol Neurotol 2014).
• The hypothesis is that common causes such as metabolic and vascular endocrine disorders, smoking, prolonged exposure to noise, induce a neurodegenerative pathology, as the basis of both the onset of auditory sensorineural pathology, and cognitive decline (Fortunato S et al., ACTA otorhinolaryngologica italica 2016) .
• Of considerable interest is the “cascade hypothesis” in which the reduction of long-term auditory input can have an impact on cognitive functions, both directly with the reduction of information reaching the brain, and indirectly, due to the consequent social isolation and depressive state resulting from hearing loss, which are known progression factors of cognitive decline (Lin FR et al., Aging Ment. Health, 2014).
• The vicious circle that arises between hearing loss, reduction of participation and understanding, threshold of attention and memory capacity, aggravates cognitive decline. (Fortunato S et al., ACTA otorhinolaryngologica italica 2016, Lin FR et al., Aging Ment. Health, 2014)

The increase in the average life of human beings has led to an increase in the diagnosis of Presbycusis in the current population.

A fundamental message for a condition which is extremely widespread, underdiagnosed and rapidly increasing, is to carry out diagnosis and treatment, be it rehabilitation or prosthetic or other, as soon as possible.