Porth's Essentials of Pathophysiology, 4e

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Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function

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setting for different listening situations. The development of microcircuitry has also made it possible for hearing aids to be miniaturized to the point that, in many cases, they can be placed deep in the ear where they take advantage of the normal shape of the external ear and ear canal. Although modern hearing aids have improved greatly, they cannot replicate the person’s ability to hear both soft and loud noises. They also fail consistently to filter out distorted or background noise. 57 Surgically implantable cochlear prostheses for the pro- foundly deaf have been developed and are available for use in adults and children. 67 These prostheses are inserted into the cochlea and work by providing direct stimu- lation to the auditory nerve. For the implant to work, the auditory nerve must be functional. Although early implants used a single electrode, current implants use multielectrode placement, enhancing speech perception. Much of the progress in implant performance has been achieved through improvements in the speech proces- sors that convert sound into electrical stimuli. Advances in the development of the multichannel implant have improved performance such that cochlear implants have been established as an effective option for adults and children with profound hearing impairment. 67,68 Hearing Loss in Infants and Children Even mild or unilateral hearing loss can have a detrimental effect on the language development and hearing-associated learning of the young child. 69,70 In the United States, the average incidence of hearing loss is 1.1 per 1000 infants. 70 The cause of hearing impairment in children depends on whether the hearing loss is conductive or sensorineural. Most conductive hearing loss is caused by middle ear infec- tions. Causes of sensorineural hearing impairment include genetic, infectious, traumatic, and ototoxic factors. Genetic causes are probably responsible for as much as 50% of sensorineural hearing loss in children. The most common infectious cause of congenital sensorineural hearing loss is cytomegalovirus (CMV), which infects 1 out of every 100 newborns in the United States each year; of these, about 1200 to 2000 have sensorineural hearing loss. 70 Of particular concern is the fact that congenital CMV infection can cause both symptomatic and asymptomatic hearing loss in the newborn. Some children with congenital CMV infection, who were asymptomatic as newborns, have suddenly lost residual hearing at 4 to 5 years of age. Postnatal causes of sensorineural hearing loss include β -hemolytic streptococcal sepsis in the newborn, and although less frequent with the routine administration of the conjugate pneumococcal vaccine, S. pneumoniae bacterial meningitis is the most frequent cause after the neonatal period. Other causes of sensorineural hearing loss are toxins and trauma. Hearing impairment can have a major impact on the development of a child; therefore, early identification through screening programs is strongly advocated. 70–72 The currently recommended screening techniques are either the transient evoked otoacoustic emissions (OAE) or the ABR. 70 Both methodologies are noninvasive, relatively quick (<5 minutes), and easy to perform. The

OAE measures sound waves generated in the inner ear (cochlea) in response to clicks or tone bursts emitted and recorded by a minute microphone placed in the external ear canals of the infant. The ABR uses three electrodes pasted to the infant’s scalp to measure the EEG waves generated by clicks. Because many children become hearing impaired after the neonatal period and are not identified by neonatal screening programs, it is recommended that all infants with risk factors for delayed onset of progressive hearing loss receive ongoing audiologic and medical monitoring for 3 years and at appropriate intervals thereafter. 72 Once hearing loss has been identified, a full developmental and speech and language evaluation is needed. Parental involvement and counseling are essential. Children with sensorineural hearing loss should be evaluated for possible hearing aid use by a pediatric audiologist. Hearing aids may be fitted for infants as young as 2 months of age. 70 Infants and young children with profound congenital or prelingual deafness have benefited frommultichannel cochlear implants. 70 Because of the increased risk of pneumococcal meningitis, children who receive implants should receive age-appropriate immunization against pneumococcal disease.  Hearing Loss in the Elderly The term presbycusis is used to describe degenerative hearing loss that occurs with advancing age. 73,74 The degenerative changes that impair hearing may begin in the fifth decade of life and may not be clinically apparent until later. The hearing loss is typically gradual, bilateral, and characterized by high-frequency hearing loss. It is further characterized by reduced hearing sensitivity and speech understanding in noisy environments, slowed central processing of acoustic information, and impaired localization of sound sources. The disorder first reduces the ability to understand speech and, later, the ability to detect, identify, and localize sounds. The most common complaint of persons with presbycusis is not that they cannot hear, but rather that they cannot understand what is being said. High-frequency warning sounds, such as beepers, turn signals, and escaping steam, are not heard and localized, with potentially dangerous results. Given the high prevalence of hearing loss in people of retirement age and its adverse effects on well-being, screening for hearing loss should be performed at annual health care visits. Clinical measures for hearing loss such as whispered voice tests and finger friction tests are reportedly imprecise and are not reliable methods for screening. Screening audiometry administered by someone trained in its use is a practical and cost-effective method for detecting significant hearing loss. The majority of hearing loss in the elderly is sensorineural. In mild to severe loss, the most effective treatment is hearing amplification with hearing aids, lip reading, and assistive listening devices (e.g., hearing aids with the telephone, captioning on televised programs, flashing alarms). Cochlear implants are indicated at any age for people with bilateral hearing losses not materially helped by hearing aids.