49 HEARING DISORDERS
Harrison’s Manual of Medicine
Causes of Hearing Loss
Laboratory Assessment of Hearing
Hearing loss is one of the most common sensory disorders in humans. Nearly 10% of the adult population has some hearing loss, and up to 35 % of individuals over the age of 65 have a hearing loss of sufficient magnitude to require a hearing aid.
Causes of Hearing Loss
Hearing loss can result from disorders of the auricle, external auditory canal, middle ear, inner ear, or central auditory pathways. In general, lesions in the auricle, external auditory canal or middle ear cause conductive hearing losses, while lesions in the inner ear or eighth nerve cause sensorineural hearing losses.
CONDUCTIVE HEARING LOSS May result from obstruction of the external auditory canal by cerumen, debris, and foreign bodies; swelling of the lining of the canal; atresia of the ear canal; neoplasms of the canal; perforations of the tympanic membrane; disruption of the ossicular chain, as occurs with necrosis of the long process of the incus in trauma or infection; otosclerosis; and fluid, scarring, or neoplasms in the middle ear.
Cholesteatoma, i.e., stratified squamous epithelium in the middle ear or mastoid, is a benign, slowly growing lesion that destroys bone and normal ear tissue. A chronically draining ear that fails to respond to appropriate antibiotic therapy should raise the suspicion of a cholesteatoma; surgery is required.
Conductive hearing loss with a normal ear canal and intact tympanic membrane suggests ossicular pathology. Fixation of the stapes from otosclerosis is a common cause of low-frequency conductive hearing loss; onset is between the late teens to the forties. In women, the hearing loss is often first noticeable during pregnancy. A hearing aid or a surgical stapedectomy can provide auditory rehabilitation.
Eustachian tube dysfunction is common in adults and may predispose to acute otitis media (AOM) or serous otitis media (SOM). Trauma, AOM, or chronic otitis media are the usual factors responsible for tympanic membrane perforation. While small perforations often heal spontaneously, larger defects usually require surgical tympanoplasty (>90% effective). Otoscopy is usually sufficient to diagnose AOM, SOM, chronic otitis media, cerumen impaction, tympanic membrane perforation, and eustachian tube dysfunction.
SENSORINEURAL HEARING LOSS Damage to the hair cells of the organ of Corti may be caused by intense noise, viral infections, ototoxic drugs (e.g., salicylates, quinine and its analogues, aminoglycoside antibiotics, diuretics such as furosemide and ethacrynic acid, and cancer chemotherapeutic agents such as cisplatin), fractures of the temporal bone, meningitis, cochlear otosclerosis (see above), Ménière’s disease, and aging. Congenital malformations of the inner ear may cause hearing loss in some adults. Genetic predisposition alone or in concert with environmental influences may also be responsible.
Presbycusis (age-associated hearing loss) is the most common cause of sensorineural hearing loss in adults. In early stages, it is characterized by symmetric high frequency hearing loss; with progression, the hearing loss involves all frequencies. The hearing impairment is associated with significant loss in clarity. Hearing aids provide limited rehabilitation; cochlear implants are the treatment of choice for severe cases.
Ménière’s disease is characterized by episodic vertigo, fluctuating sensorineural hearing loss, tinnitus, and aural fullness. It is caused by an increase in endolymphatic fluid pressure due to endolymphatic sac dysfunction. Low-frequency, unilateral sensorineural hearing impairment is usually present. MRI should be obtained to exclude retrocochlear pathology such as cerebellopontine angle tumors or demyelinating disorders. Therapy is directed toward the control of vertigo; a low-salt diet, diuretics, a short course of glucocorticoids, and intratympanic gentamicin may be useful. For unresponsive cases, labyrinthectomy and vestibular nerve section abolish rotatory vertigo. There is no effective therapy for hearing loss, tinnitus, or aural fullness.
Sensorineural hearing loss may also result from any neoplastic, vascular, demyelinating, infectious (including HIV), or degenerative disease or trauma affecting the central auditory pathways.
TINNITUS Defined as the perception of a sound when there is no sound in the environment. It may have a buzzing, roaring, or ringing quality and may be pulsatile (synchronous with the heartbeat). Tinnitus is often associated with either a conductive or sensorineural hearing loss and may be the first symptom of a serious condition such as a vestibular schwannoma. Pulsatile tinnitus requires evaluation of the vascular system of the head to exclude vascular tumors such as glomus jugulare tumors, aneurysms, and stenotic arterial lesions; it may also occur with SOM.
Approach to the Patient
(See Figure 49-1) The history should define the duration of deafness, nature of onset (sudden vs. insidious), rate of progression (rapid vs. slow), and involvement of the ear (unilateral vs. bilateral). The presence or absence of tinnitus, vertigo, imbalance, aural fullness, otorrhea, headache, facial nerve dysfunction, and head and neck paresthesias should be ascertained. Information regarding head trauma, exposure to ototoxins, occupational or recreational noise exposure, and family history of hearing impairment may also be important. Sudden unilateral hearing loss may represent a viral infection of the inner ear or a vascular accident. Pts with unilateral hearing loss (sensory or conductive) usually complain of reduced hearing, poor sound localization, and difficulty hearing clearly with background noise. Gradual progression is common with otosclerosis, noise-induced hearing loss, vestibular schwannoma, or Ménière’s disease. Small vestibular schwannomas typically present with asymmetric hearing impairment, tinnitus, imbalance (rarely vertigo); cranial neuropathy (trigeminal or facial nerve) may accompany larger tumors. Hearing loss with otorrhea is most likely due to chronic otitis media or cholesteatoma.
FIGURE 49-1. An algorithm for the approach to hearing loss. HL, hearing loss; SNHL, sensorineural hearing loss; TM, tympanic membrane; SOM, serous otitis media; AOM, acute otitis media; *, CT scan of temporal bone; †, MRI scan.
The exam should include the auricle, external ear canal, and tympanic membrane. The external ear canal of the elderly is often dry and fragile; it is preferable to clean cerumen with wall-mounted suction and cerumen loops and to avoid irrigation. Careful inspection of the nose, nasopharynx, cranial nerves, and upper respiratory tract is indicated. Unilateral serous effusion in the adult should prompt a fiberoptic exam of the nasopharynx to exclude neoplasms.
The Weber and Rinne tuning fork tests help to differentiate conductive from sensorineural hearing losses. The Rinne test compares hearing by air and bone conduction. The tines of a vibrating tuning fork are held near the opening of the external auditory canal, and then the stem is placed on the mastoid process. Normally, and with sensorineural hearing loss, a tone is heard louder by air conduction than bone conduction; however, with conductive hearing losses the bone-conduction stimulus is perceived as louder. The Weber test uses the stem of a vibrating tuning fork placed on the head in the midline. With a unilateral conductive hearing loss, the tone is perceived in the affected ear; with a unilateral sensorineural hearing loss, the tone is perceived in the unaffected ear.
Laboratory Assessment of Hearing
AUDIOLOGIC ASSESSMENT Pure tone audiometry assesses hearing acuity for pure tones. Responses are measured in decibels. Pure tone audiometry establishes the presence and severity of hearing impairment, unilateral vs. bilateral involvement, and the type of hearing loss. Speech recognition requires greater synchronous neural firing than is necessary for appreciation of pure tones; the clarity with which one hears is tested in speech audiometry. Tympanometry measures the impedance of the middle ear to sound and is particularly useful in the identification and diagnosis of middle-ear effusions. Otoacoustic emissions (OAE) can be measured with sensitive microphones inserted into the external auditory canal; the presence of OAE indicates that the outer hair cells of the organ of Corti are intact and can be used to assess auditory thresholds and to distinguish sensory from neural hearing losses. Electrocochleography measures the earliest evoked potentials generated in the cochlea and the auditory nerve; useful in the diagnosis of Ménière’s disease. Brainstem auditory evoked responses (BAER) can distinguish the site of sensorineural hearing loss (Chap. 181).
IMAGING STUDIES Axial and coronal CT of the temporal bone with fine 1-mm cuts can define the caliber of the external auditory canal, integrity of the ossicular chain, presence of middle ear or mastoid disease, inner ear malformations, and bone erosion often seen in the presence of chronic otitis media and cholesteatoma. MRI is superior to CT for imaging of retrocochlear pathology such as vestibular schwannoma, meningioma, other lesions of the cerebellopontine angle, demyelinating lesions of the brainstem, and brain tumors.
In general, conductive hearing losses are amenable to surgical intervention and correction, while sensorineural hearing losses are permanent. Atresia of the ear canal can be surgically repaired. Tympanic membrane perforations due to chronic otitis media or trauma can be repaired with tympanoplasty. Conductive hearing loss associated with otosclerosis can be repaired with stapedectomy. Tympanostomy tubes allow the prompt return of hearing to normal in individuals with middle-ear effusions. Hearing aids are effective in conductive hearing losses.
Patients with mild, moderate, and severe sensorineural hearing losses are regularly rehabilitated with hearing aids of varying configuration and strength. Hearing aids have been improved to provide greater fidelity and miniaturized so that they can be placed entirely within the ear canal, thus reducing the stigma associated with their use. Digital hearing aids lend themselves to individual programming, and multiple and directional microphones at the ear level may be helpful in noisy surroundings. If the hearing aid provides inadequate rehabilitation, cochlear implants are appropriate.
Treatment of tinnitus is problematic. Relief of the tinnitus may be obtained by masking it with background music. Hearing aids are also helpful in tinnitus suppression, as are tinnitus maskers, devices that present a sound to the affected ear that is more pleasant to listen to than the tinnitus. Antidepressants have also shown some benefit.
Conductive hearing losses may be prevented by prompt antibiotic therapy for acute otitis media and by ventilation of the middle ear with tympanostomy tubes in middle-ear effusions lasting ³12 weeks. Loss of vestibular function and deafness due to aminoglycoside antibiotics can largely be prevented by careful monitoring of serum peak and trough levels. Ten million Americans have noise- induced hearing loss, and 20 million are exposed to hazardous noise in their employment. Noise-induced hearing loss can be prevented by avoidance of exposure to loud noise or by regular use of ear plugs or fluid-filled muffs to attenuate intense sound.
For a more detailed discussion, see Lalwani AK, Snow JB Jr., Disorders of Smell, Taste, and Hearing, Chap. 29, p. 178 in HPIM-15.