EAR DISORDERS HEARING LOSS

Hearing loss of some sort affects about V3 of all adults between 65 and 74 yr of age and about half of those between 75 and 79. In the USA, > 10 million elderly people have a hearing impairment.
Hearing loss may result from dysfunction of any component of the auditory system (see FIG. 103-1). In conductive hearing loss, the dysfunction affects the orderly transmission of sound from the external environment to the inner ear and may involve any of the structures lateral to the oval window (eg, the tympanic membrane or stapes). Sensorineural hearing loss involves dysfunction of the sensory elements (hair cells) or neural structures (cochlear nerve fibers). In cochlear sensorineural hearing loss, the structures within the cochlea are affected. In retrocochlear hearing loss, any element of the auditory system medial to the cochlea may be affected, although the term refers specifically to disorders of the cochlear nerve, particularly in the internal auditory canal or cerebellopontine angle. A mixed hearing loss combines both sensorineural and conductive elements. In central hearing loss, the dysfunction is localized to the brain’s higher auditory centers.
Besides being categorized according to the anatomy and physiology involved, hearing loss can be categorized by cause. Causes of hearing loss include diseases, noise exposure, ototoxicity, tumors, and injury to the cochlear nerve and brain. Yet, the most common type of hearing loss in elderly persons is presbycusis, a term that means “hearing loss of aging” and that describes not a cause but a type of hearing loss whose cause is unknown.
PRESBYCUSIS
A bilaterally symmetric, sensorineural hearing loss that is associated with aging.
The presence, progression, and severity of presbycusis depend upon a variety of factors. Typically, men are more severely impaired than women of the same age. Noise exposure, diet, hypertension, and metabolic and hereditary factors may play a role. Vascular lesions that result

Sensory presbycusis is marked by atrophy of the organ of Corti. The hearing loss generally begins in middle age and progresses slowly. A loss of cochlear neurons parallels the loss of the organ of Corti. The audiogram generally shows an abrupt, high-frequency hearing loss with good discrimination (see FIG. 103-2A).
Neural presbycusis, usually manifested later in life, is ascribed to a loss of cochlear neurons with a relative preservation of the organ of Corti. Neural degeneration is apparently related to genetic factors. Au-diometric evaluation shows a predominantly high-frequency hearing loss with very poor discrimination (see FIG. 103-2.B); a severe loss of speech discrimination (phonemic regression) occurs, but pure tone thresholds are maintained. Rapidly progressing neural presbycusis may be accompanied by other signs of CNS degeneration, including intellectual deterioration, memory loss, and motor incoordination.
Metabolic presbycusis has a familial tendency, onset in middle age, and slow progression. It appears to correlate with patchy atrophy of the stria vascularis, affecting the electrophysiologic function of the organ of Corti. Audiometric evaluation shows a flat threshold pattern, with normal speech discrimination scores until the thresholds exceed 50 dB (see FIG. 103-2C).
Cochlear conductive presbycusis usually begins in middle age and is thought to relate to alterations in the motion mechanics of the cochlear duct, such as that caused by stiffening of the basilar membrane, although this has not been proved. Audiometric evaluation shows bilaterally symmetric, linearly descending thresholds; speech discrimination inversely relates to the steepness of the slope of the pure tone curve (see FIG. 103-2£>).
OTOSCLEROSIS
A disease of the otic capsule and the most common cause of progressive conductive hearing loss in adults with normal tympanic membranes.
Otosclerosis, unique to the human otic capsule, is reported to occur in 10% of whites. About 10% of affected persons actually develop clinically significant conductive hearing loss. A family history of otosclerosis occurs in 50% to 60% of cases.
The disease is characterized by irregular areas of bone resorption and new bone formation, occurring predominantly at the oval window and eventually resulting in ankylosis of the stapes. Occasionally, when the cochlea is involved, otosclerosis also causes sensorineural hearing loss.
Otosclerosis generally is diagnosed before old age and is one of the types of hearing loss for which surgery is indicated. Surgical bypass of the stapes usually corrects the conductive component of the hearing loss. Although a hearing aid is an acceptable alternative for many persons, occasionally a patient with mixed hearing loss can benefit from stapedectomy. In some cases, closing the air-bone gap brings the patient’s hearing threshold to a level more amenable to hearing aid fitting.