Porth's Essentials of Pathophysiology, 4e

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Nervous System

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Conductive Hearing Loss Conductive hearing loss occurs when auditory stimuli are not adequately transmitted through the auditory canal, tympanic membrane, middle ear, or ossicle chain to the inner ear. Temporary hearing loss can occur as the result of impacted cerumen in the outer ear or fluid in the middle ear. Foreign bodies, including pieces of cotton and insects, may impair hearing. More permanent causes of hearing loss are thickening or damage of the tympanic membrane or involvement of the bony structures (ossicles and oval window) of the middle ear caused by otosclerosis or Paget disease. Sensorineural Hearing Loss Sensorineural, or perceptive, hearing loss occurs with disorders that affect the inner ear, auditory nerve, or auditory pathways of the brain. 62,63 With this type of deafness, sound waves are conducted to the inner ear, but abnormalities of the cochlear apparatus or auditory nerve decrease or distort the transfer of information to the brain. Abnormal function resulting from damage or malformation of the central auditory pathways and circuitry is included in this category. Sensorineural hearing loss can vary with respect to onset and severity, and can affect one or both ears. It may have a genetic cause or result from an infection, other illness, trauma, or exposure to loud noise. Hearing loss that has its onset before speech-language acquisition is typically defined as prelingual, and that known to have developed after speech development as postlingual. Genetic hearing loss may result from mutation in a single gene (monogenetic) or from a combination of mutations in different genes and environmental factors (multifactorial). Genetic forms of hearing loss also can be classified as being part of a syndrome in which other abnormalities are present, or as nonsyndromic, in which deafness is the only abnormality. Environmentally induced deafness can occur through direct exposure to excessively intense sound, as in the workplace or at a concert. Sustained or repeated exposure to noise pollution at sound intensities greater than 100 to 120 dB can cause corresponding mechanical damage to the organ of Corti. Severe damage can result in permanent sensorineural deafness to the affected sound frequencies. A number of infections can cause sensorineural hearing loss. Deafness or some degree of hearing impairment is the most common serious complication of bacterial meningitis in infants and children. The mechanism causing hearing impairment is thought to be a suppurative labyrinthitis or neuritis resulting in the loss of hair cells and damage to the auditory nerve. Sudden sensorineural hearing loss represents an abrupt loss of hearing that occurs instantaneously or on awakening. It is most commonly caused by viral infections or circulatory disorders. Among the neoplasms that impair hearing are acoustic neuromas. Acoustic neuromas are benign Schwann cell tumors affecting CN VIII. These tumors usually are unilateral and cause hearing loss by compressing the cochlear nerve or interfering with blood supply to the

nerve and cochlea.Other neoplasms that can affect hearing include meningiomas and metastatic brain tumors. The temporal bone is a common site of metastases. Drugs that damage inner ear structures are labeled ototoxic. 65 Vestibular symptoms of ototoxicity include light-headedness, giddiness, and dizziness; if toxicity is severe, cochlear symptoms consisting of tinnitus or hearing loss occur. Hearing loss is sensorineural and may be bilateral or unilateral, or they may be transient, as often is the case with salicylate toxicity, or they may be permanent. Several classes of drugs have been identified as having ototoxic potential, including the aminoglycoside antimicrobials and some other basic antimicrobials, antimalarial drugs, some chemotherapeutic drugs, loop diuretics, and salicylates (aspirin). The risk of ototoxicity depends on the total dose of the drug and its concentration in the bloodstream. It is increased in persons with impaired kidney functioning and in those previously or currently treated with another potentially ototoxic drug. Diagnosis andTreatment of Hearing Loss Diagnosis of hearing loss is aided by careful history of associated otologic factors such as otalgia, otorrhea, tinnitus, and self-described hearing difficulties; physical examination to detect the presence of conditions such as otorrhea, impacted cerumen, or injury to the tympanic membrane; and hearing tests. Testing for hearing loss involves a number of methods, including a person’s reported ability to hear an observer’s voice, use of a tuning fork to test air and bone conduction, audioscopes, and auditory brain stem evoked responses (ABRs). 62 Tuning forks are used to differentiate conductive and sensorineural hearing loss. Audioscopes can be used to assess a person’s ability to hear pure tones at 1000 to 2000 Hz (usual speech frequencies). The ABR uses electroencephalographic (EEG) electrodes and high-gain amplifiers to produce a record of brain wave activity elicited during repeated acoustic stimulations of either or both ears. It involves subjecting the ear to loud clicks and using a computer to analyze nerve impulses as they are processed in the midbrain. Imaging studies such as computed tomography (CT) scans and magnetic resonance imaging (MRI) can be done to determine the site of a lesion and the extent of damage. 66 Untreated hearing loss can have many consequences. Social isolation and depressive disorders are common in the hearing-impaired. Hearing-impaired people may avoid social situations where background noise makes conversation difficult to hear. Safety issues, both in and out of the home, may become significant. Treatment measures for hearing loss range from simple removal of impacted cerumen in the external auditory canal to surgical procedures, such as those used to reconstruct the tympanic membrane. Hearing aids remain the mainstay of treatment for many persons with conductive and sensorineural hearing loss. With the advent of microcircuitry, hearing aids are now being designed with computer chips that allow multiple programs to be placed in a single hearing aid. The various programs allow the user to select a specific