Excerpt from Inner Ear, Presbycusis

Please click here to view the full topic text: Inner Ear, Presbycusis

Presbycusis is an important problem in society. It occurs in an elderly population that relies on special senses to compensate for other age-associated disabilities. Elderly individuals may rely on their hearing to overcome limitations of impaired vision and slowed reaction time. Arthritis and impaired dexterity may limit their ability to take advantage of the amplification devices used in rehabilitation of age-associated hearing loss. Finally, hearing loss may contribute to the isolation of some elderly people by restricting their use of the phone, causing them to forfeit social opportunities such as concerts and social gatherings, and amplifying their sense of disability.

Pathophysiology: Histologic changes associated with aging occur throughout the auditory system from the hair cells of the cochlea to the auditory cortex in the temporal lobe of the brain. These changes may correlate with different clinical findings and auditory test results depending on the severity of the changes and the anatomic level at which they occur. Over the past 50 years some brilliant research in the field of presbycusis has been witnessed, but elucidation of the pathophysiology of presbycusis is still incomplete. Many researchers have investigated the causes of this disease. Crowe and associates, Saxen, and Gacek and Schuknecht have studied histologic changes in the cochleae of human ears with presbycusis. Gacek and Schuknecht identified 4 sites of aging in the cochlea and divided presbycusis into 4 types based on these sites. The histologic changes are correlated approximately with symptoms and auditory test results.

• Sensory presbycusis refers to epithelial atrophy with loss of sensory hair cells and supporting cells in the organ of Corti. This process originates in the basal turn of the cochlea and slowly progresses toward the apex. These changes correlate with a precipitous drop in the high-frequency thresholds, which begins after middle age. The abrupt downward slope of the audiogram begins above the speech frequencies; therefore, speech discrimination is often preserved. Histologically, the atrophy may be limited to only the first few millimeters of the basal end of the cochlea. The process is slowly progressive over time. One theory proposes that these changes are due to the accumulation of lipofuscin pigment granules.

• Neural presbycusis refers to atrophy of nerve cells in the cochlea and central neural pathways. Schuknecht estimated that 2100 neurons are lost every decade (of 35,000 total). This loss begins early in life and may be genetically predetermined. Effects are not noticeable until old age, because pure-tone average is not affected until 90% of neurons are gone. Atrophy occurs throughout the cochlea, with the basilar region only slightly more predisposed than the remainder of the cochlea. Therefore, no precipitous drop in the high-frequency thresholds on audio is observed. A disproportionately severe decrease in speech discrimination is a clinical correlate of neural presbycusis and may be observed before hearing loss is noted because fewer neurons are required to maintain speech thresholds than speech discrimination.

• Metabolic (ie, strial) presbycusis results from atrophy of the stria vascularis. The stria vascularis normally maintains the chemical and bioelectric balance and metaboli .....

  Please click here to view the full topic text: Inner Ear, Presbycusis