Syndromes Primarily Involving Hearing

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Hearing loss is highly prevalent, especially in the older population, and three types are commonly encountered: conductive, sensorineural, and central hearing loss. In conductive hearing loss, sound is not transmitted into the inner ear. Diagnosis is ordinarily made via observation of an "air-bone gap" on audiometry, meaning that hearing is superior when sound is transmitted in such a way that it bypasses the middle ear ossicular chain. Causes include a buildup of ear wax, foreign body in the ear canal, otosclerosis, external or middle ear infections, allergy with serous otitis, and perforation of the tympanic membrane. Characteristically, hearing aids work well for this population.

Sensorineural hearing loss is the most common type of hearing loss, occurring in 23 percent of the population older than 65 years of age. The term sensorineural is used to indicate that there is either a cochlear or an eighth nerve lesion. The diagnosis of a sensorineural pattern hearing loss is made through audiometry, which shows a significant hearing loss without the air-bone gap characteristic of conductive hearing disturbances. Common causes include old age (when the hearing pattern is often somewhat confusingly called presbycusis), Meniere's disease, toxin exposures (such as to high-dose aspirin), and noise. Treatments are mainly aimed at preventing further damage. If the loss is minor, avoidance of noise and avoidance of ototoxic medications are appropriate "treatments." If the loss is significant, a hearing aid should be tried. Occasionally, persons with acquired deafness can be treated with a surgically implanted prosthesis that directly stimulates the spiral ganglion.

Central hearing loss is extremely rare compared with the

TABLE 12-7 -- SELECTED ETIOLOGIES ASSOCIATED WITH DISORDERS OF CRANIAL NERVE VIII

Etiological Category

Specific Etiologies

Chapter

STRUCTURAL DISORDERS

Developmental structural disorders

Arnold-Chiari malformation

28

Mondini malformation

Degenerative and compressive structural disorders

Paget's disease

HEREDITODEGENERATIVE DISORDERS

Storage diseases: Lipidoses, glycogen disorders and leukoencephalopathies

Hurler's, Hunter's, Sanfilippo's, Morquio's syndromes

29

Krabbe's disease

Chromosomal anomalies and the neurocutaneous disorders

Waardenburg's, Usher's, Down's, Pendred's syndromes

32

ACQUIRED METABOLIC DISORDERS AND NUTRITIONAL DISORDERS

Endogenous metabolic disorders

Kernicterus

38

Nutritional deficiencies and syndromes associated with alcoholism

Wernicke's (Central vertigo)

INFECTIOUS DISORDERS

Viral Infections

Vestibular neuritis

41

Herpes zoster oticus

Cytomegalovirus (infants)

Nonviral Infeetions

Chronic, acute otitis media (bacterial)

42

Meningitis (bacterial, fungal)

Syphilis

NEUROVASCULAR DISORDERS

AICA, PICA syndromes

45

Subarachnoid hemorrhage

NEOPLASTIC DISORDERS

Primary neurological tumors

Acoustic neuroma

46

Cholesteatoma

DEMYELINATING DISORDERS

Demyelinating disorders of the central nervous system

Multiple sclerosis

48

TRAUMATIC DISORDERS

Blunt head trauma

51

EPILEPSY

Central vertigo

52

HEADACHE AND FACIAL PAIN

Central vertigo (vertebrobasilar migraine)

53

DRUG-INDUCED AND IATROGENIC NEUROLOGICAL DISORDERS

Vestibular ototoxicity: aminoglycosides

55

AICA, anterior inferior cerebellar artery; PICA, posterior inferior cerebellar artery.

sensorineural or conductive types. The diagnosis is usually not made by the pure tone audiogram, which often yields a normal result. Rather, patients usually have an aphasia, which may be associated with disproportionally poor scores on the speech reception threshold tests or word recognition scores portions of the audiogram.

Pure word deafness is a rare subtype of central deafness. This disorder is defined as disturbed auditory comprehension without difficulties with visual comprehension. Patients characteristically have fluent verbal output, severe disturbance of spoken language comprehension and repetition, and no problems with reading or writing. [6 Nonverbal sounds are correctly identified. The lesion is classically postulated to be a disruption in connections between the dominant Heschl's transverse gyrus and the medial geniculate as well as callosal fibers from the opposite superior temporal region. Initially, it appears commonly as a Wernicke's aphasia. With recovery, difficulties in auditory comprehension persist. Although usually caused by a stroke, pure word deafness can arise from other causes of focal cortical lesions such as tumors.

Auditory agnosia, another rare subset of central deafness, is typified by relatively normal pure tone hearing on audiometry, but inability to interpret (recognize) nonverbal sounds such as the ringing of a telephone. Inability to interpret nonverbal sounds but preserved ability to interpret speech may be a result of a right hemisphere lesion alone.y Amusia is a particular type of auditory agnosia in which only the perception of music is impaired. Again, right-sided temporal lesions are thought to be the cause.

Cortical deafness is essentially the combination of word deafness and auditory agnosia. It is characterized by an inability to interpret either verbal or nonverbal sounds with preserved awareness of the occurrence of sound (for instance, by a startle reaction to a clap). In most instances, the cause is bilateral embolic strokes in the area of Heschl's gyri. Patients present with sudden deafness evolving later so that they can hear sounds but are unable to recognize their meaning. Relatively few cases of this disorder have been studied, and it is possible that bilateral lesions of the central auditory pathways, other than the speech cortex, can also result in these deficits.

Auditory hallucinations consist of an illusion of a complex sound such as music or speech. These hallucinations most commonly occur as a result of an injury to the superior temporal auditory association areas. Penfield discovered that stimulating this area induced an auditory sensation that seemed real to patients. Auditory hallucinations can also occur as a result of temporal lobe seizure. M

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