The sense of hearing is unequalled by our other sensory modalities in terms of its sensitivity, dynamic range, and discrimination of the finest nuances in stimuli. It does serve us well through a part of our lifetime, but beginning in our forties (slightly earlier for men and later for women), our inner ears suffer the influence of aging in a very subtle yet progressive manner. Age-related hearing impairment (ARHI) affects most people aged 65 years and older and represents the predominant neurodegenerative disease of aging. Hippocrates had already noted deafness to be more prevalent among his elderly patients and in The Comedy of Errors, Shakespeare's elderly merchant, Aegeon, complains of his own “dull deaf ears.” Thus, ARHI or presbycusis is not a disease of modern societies but has been accepted for centuries as one of Lord Byron's inevitable “woes that wait on age” that it still appears to be.
It was the New York otologist, St. John Roosa, who first drew the attention of his colleagues to hearing loss of the elderly as a medical condition. In 1885 he proposed the name presbycusis that he had coined from the Greek
, old man, and
, to hear. Systematic studies of the anatomical pathology began in the late nineteenth century, leading by the 1930s to the realization that the decreased auditory acuity could be attributed to deterioration of the auditory sensory cells and the auditory nerve. These changes frequently affect the perception of the upper frequencies first, resulting in high-frequency hearing loss as a hallmark of presbycusis. However, age-related hearing loss is not a uniform condition, but a multifactorial one combining genetic predispositions with a plethora of lifetime insults to the hearing organ because, in all its versatility and efficacy, our sense of hearing is also uniquely vulnerable to environmental influences. These may include noise, chemicals, and solvents at the workplace, lifestyle (e.g., smoking), and leisure activities (from iPods to rock concerts and target shooting), diseases (e.g., diabetes, respiratory disorders), viral or bacterial infections, and even the adverse effects of the very medications designed to cure the diseases and infections. This spectrum of potential abuse of our auditory organ yields an exceedingly intricate etiology and pathology of hearing loss in the elderly. Age-related pathology of the central auditory system adds to the complexity of the problem. Presbycusis has been defined as “hearing impairment associated with various types of auditory dysfunction, peripheral or central, that accompany aging and that cannot be accounted for by extraordinary ototraumatic, genetic, or pathological conditions” but it is almost impossible in practice to separate the confounding factors from a “true” effect of aging. It has therefore often been debated whether presbycusis indeed exists in populations that are isolated from adverse environmental influences.
Animal models provide hope to resolve some of the basic mechanisms that underlie the deterioration of hearing and point to ways and means to delay or prevent presbycusis. By virtue of the availability of molecular ...