Porth's Essentials of Pathophysiology, 4e

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

C h a p t e r 3 8

tinnitus. Intermittent periods of mild, high-pitched tinnitus lasting for several minutes are common in normal-hearing persons. Impacted cerumen is a benign cause of tinnitus, which resolves after the earwax is removed. Medications such as aspirin and stimulants such as nicotine and caffeine can cause transient tinnitus. Conditions associated with more persistent tinnitus include noise-induced hearing loss, presbycusis (sensorineural hearing loss that occurs with aging), hypertension, atherosclerosis, head injury, and cochlear or labyrinthine infection or inflammation. The physiologic mechanism underlying subjective tinnitus is largely unknown. It seems likely that there are several mechanisms, including abnormal firing of auditory receptors, dysfunction of cochlear neurotransmitter function or ionic balance, and alterations in central processing of the signal. Treatment measures for tinnitus are designed to reduce the symptoms, rather than effect a cure. 58,59 They include elimination of drugs or other substances, such as caffeine, some cheeses, red wine, and foods containing monosodium glutamate, that are suspected of causing tinnitus. Tinnitus retraining therapy, which includes directive counseling and extended use of low- noise generators to facilitate auditory adaptation to the tinnitus, has met with considerable success. Central Auditory Pathway Disorders The auditory pathways in the brain involve communi- cation between the two sides of the brain at many lev- els. As a result, strokes, tumors, abscesses, and other focal abnormalities seldom produce more than a mild reduction in auditory acuity on the side opposite the lesion. For intelligibility of auditory language, lateral dominance becomes important. On the dominant side, usually the left side, the more medial and dorsal portion of the auditory association cortex is of crucial impor- tance. This area is called the Wernicke area, and dam- age to it is associated with auditory receptive aphasia. Persons with damage to this area of the brain can speak intelligibly and read normally but are unable to under- stand the meaning of major aspects of audible speech. Irritative foci that affect the auditory radiation or the primary auditory cortex can produce roaring or clicking sounds, which appear to come from the auditory environment of the opposite side (i.e., auditory hallucinations). Focal seizures that originate in or near the auditory cortex often are immediately preceded by the perception of ringing or other sounds preceded by a prodrome (i.e., aura). Damage to the auditory association cortex, especially if bilateral, results in deficiencies of sound recognition and memory. If the damage is in the dominant hemisphere, speech recognition can be affected (i.e., sensory or receptive aphasia). Hearing Loss Nearly 36 million Americans have some degree of hearing loss. 61 It affects persons of all age groups. Two to 3 out of every 1000 children in the United States are born deaf or hard of hearing. 61 Approximately 30% of adults between the ages of 65 and 74 years and

approximately 47% of those age 75 and older have hearing loss. 61 The level of hearing is measured in decibels (dB), where 0 dB is the threshold for perception of sound at a given frequency in persons with normal hearing. A 10-fold increase in sound pressure level from 0 dB is measured as 20 dB. Hearing loss is classified as mild (26 to 40 dB), moderate (41 to 55 dB), severe (71 to 90 dB), or profound (91 dB or greater). 61–63 “Hard of hearing” is defined as hearing loss between 20 and 25 dB in adults and greater than 15 dB in children. Profound deafness is defined as hearing loss greater than 90 dB in adults 62 or 70 dB in children. 64 There are many causes of hearing loss or deafness. Most fit into the categories of conductive, sensorineural, or mixed deficiencies that involve a combination of conductive and sensorineural function deficiencies of the same ear. 63 Chart 38-1 summarizes common causes of hearing loss. Hearing loss may be genetic or acquired, sudden or progressive, unilateral or bilateral, partial or complete, reversible or irreversible.

CHART 38-1

Common Causes of Conductive and Sensorineural Hearing Loss

Conductive Hearing Loss

■■ External ear conditions

■■ Impacted earwax or foreign body

■■ Otitis externa

■■ Middle ear conditions

■■ Trauma

■■ Otitis media (acute and with effusion)

■■ Otosclerosis

■■ Tumors Sensorineural Hearing Loss

■■ Trauma

■■ Head injury

Noise

■■

■■ Central nervous system infections (e.g., meningitis)

■■ Degenerative conditions

■■ Presbycusis

■■ Vascular

■■ Atherosclerosis ■■ Sudden deafness

■■ Ototoxic drugs (e.g., aminoglycosides, salicylates, loop diuretics)

■■ Tumors

■■ Vestibular schwannoma (acoustic neuroma)

■■ Meningioma

■■ Metastatic tumors

■■ Idiopathic

■■ Ménière disease Mixed Conductive and Sensorineural Hearing Loss

■■ Middle ear conditions

■■ Barotrauma

■■ Cholesteatoma

■■ Otosclerosis

■■ Temporal bone fractures