Age-Dependent Changes in the Cochlea.

Age-related hearing loss is a progressive, bilateral, and symmetrical sensorineural hearing loss due to degeneration of inner ear structures and it is considered a multifactorial complex disorder. Changes in tissue performance act from the cellular level to the molecular level. Histological visible damage in different structures of the inner ear signifies that the high frequency region is more sensitive and earlier affected. For many years it has been known that, with advancing age, there is an increasing loss of outer and inner hair cells and peripheral and central neurons. Furthermore, it comes to an atrophy of the stria vascularis. In recent years, investigations of the inner ear have shown that a loss of cochlear nerve synapses also plays an important role in age-related hearing loss. Degeneration of cochlear nerve synapses at the inner hair cell leads to a worsening of complex auditory tasks, such as understanding speech in difficult listening environments with minimal effects on pure-tone thresholds. This phenomenon has been called "hidden hearing loss." Recent research has shown that senescence of the immune system plays an important role in age-related degeneration of the inner ear, termed inflammaging. A mild form of chronic systemic inflammatory response is thought to activate cochlear macrophages.