Atlas of pathophysiology, 2 Edition

Part II - Disorders

Sensory Disorders

Hearing loss

Hearing loss, or deafness, results from a mechanical or nervous impediment to the transmission of sound waves and is the most common pathologic process associated with hearing alteration. Hearing loss is further defined as an inability to perceive the normal range of sounds audible to an individual with normal hearing. Types of hearing loss include congenital hearing loss, sudden deafness, noise-induced hearing loss, and presbycusis.

Causes

Congenital hearing loss

·   Dominant, autosomal dominant, autosomal recessive, or sex-linked recessive trait

·   Maternal exposure to rubella or syphilis during pregnancy

·   Use of ototoxic drugs during pregnancy

·   Trauma or prolonged fetal anoxia during delivery

·   Congenital abnormalities of ears, nose, or throat

·   Prematurity or low birth weight

·   Serum bilirubin levels above 20 mg/dl

Sudden deafness

·   Mumps—most common cause of unilateral sensorineural hearing loss in children

·   Other bacterial and viral infections—rubella, rubeola, influenza, herpes zoster, infectious mononucleosis, mycoplasma

·   Metabolic disorders—diabetes mellitus, hypothyroidism, hyperlipoproteinemia

·   Vascular disorders—hypertension, arteriosclerosis

·   Head trauma or brain tumors

·   Ototoxic drugs—tobramycin, streptomycin, quinine, gentamicin, furosemide, ethacrynic acid

·   Neurologic disorders—multiple sclerosis, neurosyphilis

·   Blood dyscrasias—leukemia, hypercoagulation

Noise-induced hearing loss

·   Prolonged exposure to loud noise (85 to 90 dB)

·   Brief exposure to extremely loud noise (greater than 90 dB)

Presbycusis

·   Loss of hair cells in the organ of Corti

Pathophysiology

The major forms of hearing loss are classified as conductive loss, interrupted passage of sound from the external ear to the junction of the stapes and oval window; sensorineural loss, impaired cochlea or acoustic (eighth cranial) nerve dysfunction, causing failure of transmission of sound impulses within the inner ear or brain; or mixed loss, combined dysfunction of conduction and sensorineural transmission.

Signs and symptoms

·   Deficient response to auditory stimuli

·   Impaired speech development

·   Loss of perception of certain frequencies (around 4,000 Hz)

·   Tinnitus

·   Inability to understand the spoken word

Age Alert

A deaf infant's behavior can appear normal and mislead the parents as well as the health care professional, especially if the infant has autosomal recessive deafness and is the first child of carrier parents.

Diagnostic test results

·   Computed tomography scan shows vestibular and auditory pathways.

·   Magnetic resonance imaging reveals acoustic tumors and brain lesions.

·   Auditory brain response shows activity in auditory nerve and brain stem.

·   Pure tone audiometry reveals presence and degree of hearing loss.

·   Electronystagmography shows vestibular function.

·   Otoscopic or microscopic examination reveals middle ear disorders and removes debris.

·   Rinne and Weber's tests show whether hearing loss is conductive or sensorineural.

Treatment

Congenital hearing loss

·   Surgery, if correctable

·   Sign language, speech reading, or other effective means of developing communication

·   Phototherapy and exchange transfusions for hyperbilirubinemia

·   Appropriate childhood immunizations

Sudden deafness

·   Prompt identification of underlying cause, such as acoustic neuroma or noise, and appropriate treatment

Noise-induced hearing loss

·   Normal hearing usually restored by overnight rest after several hours' exposure to noise levels greater than 90 dB

·   High-frequency hearing loss generally prevented by reducing exposure to loud noises

·   Speech and hearing rehabilitation possibly required after repeated exposure to such noise, because hearing aids are seldom helpful

Presbycusis

·   Amplifying sound, as with a hearing aid, helpful to some patients

·   Hearing aid no help for many patients who are intolerant of loud noise

Other

·   Antibiotics

·   Agents to dissolve cerumen

·   Decongestants

·   Analgesics

P.405

CAUSES OF CONDUCTIVE HEARING LOSS

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Clinical Tip

How Hearing Occurs

·   Sound vibrations strike the tympanic membrane (eardrum).

·   The auditory ossicles vibrate, and the footplate of the stapes moves at the oval window.

·   Movement of the oval window causes the fluid inside the scala vestibuli and scala tympani to move.

·   Fluid movement against the cochlear duct sets off nerve impulses, which are carried to the brain via the cochlear nerve.

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