TITLE: Sensorineural Hearing Loss in Adults
SOURCE: Department of Otolaryngology, UTMB, Grand Rounds
DATE: May 27, 1998
RESIDENT PHYSICIAN: Gregory Young, M.D.
FACULTY PHYSICIAN: Jeffrey Vrabec, M.D.
SERIES EDITOR: Francis B. Quinn, Jr., M.D.
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Sensorineural hearing is dependent upon the integrity of cochlear hair cells, cochlear neurons, and neural pathways to the cerebral cortex. Insult either to any of these components, or to their blood supply, may lead to sensorineural hearing loss. The known causes of sensorineural hearing loss in adults include noise exposure, normal aging process, infections, ototoxic medications or chemicals, trauma, neoplasms, and idiopathic or systemic conditions such as Meniereís disease, autoimmune disease, idiopathic sudden sensorineural hearing loss, neurologic disorders, vascular or hematologic disorders, bony abnormalities, and endocrine disorders.
Evaluation of the adult with sensorineural hearing loss should begin with a complete history and physical examination. The severity, rapidity, symmetry, and progression of the hearing loss are important, as are any associated symptoms. Any tinnitus, vertigo, aural fullnes, pain, or synchronous neurological deficit should be documented. Past history of trauma, noise exposure, ototoxic medication or chemical exposure, or systemic disease(diabetes mellitis, autoimmune, cardiovascular) should be assessed. Any family history of hearing loss is also important to note. The physical examination begins with a thorough otologic and tuning fork examination. A neurologic examination, facial feature examination, neck ausculation, or fistula test are sometimes performed, depending on the history. An audiogram is performed, including pure-tone, speech discrimination, word recognition, tympanogram, and acoustic reflexes. The audiogram results are combined with those of the tuning fork exam in order to better define the hearing loss. An ABR or MRI is often indicated if retrocochlear disease is suspected or the hearing loss is asymmetric. A CT of the temporal bone is obtained if bony anomalies, fractures, or infections involving the temporal bone are suspected. In cases of unexplained SNHL, routine laboratory tests are limited to CBC, ESR, and MHA-TP. Western blotting may be used to confirm active syphilis infection in an FTA-ABS or MHA-TP positive patient, and can also be used to identify anti-heat shock protein 70(Hsp-70) antibodies in patients suspected of having idiopathic , progressive, bilateral sensorineural hearing loss(IPBSNHL).
Noise-induced hearing loss and presbycusis are the leading causes of adult sensorineural hearing loss in The United States. The usual presentation of noise-induced hearing loss is high frequency sloping sensorineural hearing loss with high-pitched tinnitus and loss of speech discrimination. For broad-spectrum industrial noise, hearing loss is greatest at 3-6 KHz. The mechanism of noise-induced hearing loss appears to be a softening, fusion and eventual loss of stereocilia in the outer hair cells of frequency specific regions of the cochlea. Acoustic trauma in the form of intense short-duration sound may cause a permanent hearing loss by tearing cochlea membranes, thereby allowing endolymph and perilymph to mix. Federally mandated regulations on noise exposure require employers to prevent exposures exceeding 90dB for 8 hours, 95dB for 4 hours, 100dB for 2 hours, 105dB for 1 hour, and 130dB for 2 minutes. No noise greater than 140dB is allowed. Hearing conservation programs must be used for all workers exposed to noise greater that 85dB. Hearing aids are the treatment for noise-induced hearing loss, but the best management is prevention.
Presbycusis is a bilateral symmetric loss of hearing associated with the aging process. It is unclear at this time how much of the hearing loss we call presbycusis is genetically predetermined, and how much is determined by a lifetime of environmental insults to the cochlea and neural pathways. Presbycusis may be a result of hair cell loss, neuronal degeneration or atrophy of the stria vascularis. The treatment for presbycusis is usually amplification. The success of amplification increases with better speech discrimination scores and higher patient motivation.
Infectious causes of hearing loss in adults include viruses, syphilis, chronic and acute otitis media, lyme disease, and bacterial meningitis.
Viral infections are common causes of sensorineural hearing loss in children. In adults, herpes zoster oticus is a well known cause sensorineural hearing loss. Other viruses, such as human spumaretrovirus and adenovirus have been implicated in selected cases of sudden sensorineural hearing loss.
Syphilis cases have risen rapidly in the past few years. Congenital infections may not appear until after the 3rd decade of life, so both acquired and congenital cases may arise in adulthood. The usual audiometric presentation is a bilateral flat sensorineural hearing loss, with poorer than expected word discrimination scores, pronounced recruitment, and weak or absent calorics. The hearing loss may be sudden in onset, or slowly progressive. Hennebertís sign(positive fistula test) and/or Tullioís phenomenon(loud sound induced dysequilibrium) may be present. Syphilis is diagnosed with treponema-specific tests such as the fluorescent treponema antibody absorption test(FTA-ABS), or the micro-hemagglutination assay for T. pallidum(MHA-TP), both of which are highly specific for syphilis(>97%). A western blot assay can differentiate active from treated infections. Active infections are associated with both antitreponemal IgG and IgM, whereas treated infections have only IgG. Treatment is with penicillin for extended periods. Oral steroids have been used successfully to stabilize or improve hearing in these patients, but serious complications are occasionally reported, and so their use is not universally recommended. Histopathologically, otosyphilis is associated with obliterative endarteritis, mononuclear cell infiltrates, osteitis of the otic capsule, and tissue necrosis.
Chronic otitis media may lead to progressive sensorineural hearing loss, either by tympanogenic supporative labyrinthitis, or by labyrinthine fistula. Acute otitis media may also lead to sensorineural hearing loss, but the etiology is unclear. The tick-borne infections Rocky Mountain Spotted Fever and Lyme disease may cause sensorineural hearing loss. Lyme disease may also cause vertigo and facial palsy.
Bacterial meningitis causes hearing loss in about 20% of cases, most commonly bilateral, permanent, severe to profound sensorineural loss. Haemophilis influenza, Neisseria meningitidis, and Streptococcus pneumoniae are the most common in children. The hearing loss associated with bacterial meningitis occurs early in the course of the disease, and appears to be due to the penetration of bacteria and toxins along the cochlear aqueduct or internal auditory canal leading to a suppurative labyrinthitis, perineuritis, or neuritis of the eighth nerve. Thrombophlebitis or emboli of the small labyrinthine vessels and hypoxia of the neural pathways may also contribute to the hearing loss. Antibiotics may contribute to hearing loss indirectly by causing a rapid accumulation of bacterial degradation products such as endotoxins and cell wall antigens. The evoked host inflammatory response is thought to worsen destruction of normal tissue. Support for this theory comes from corticosteroid use during bacterial meningitis, which dampens the inflammatory response, and has been shown to improve post-meningitis hearing. Because of potential complications that may arise from steroid use in this setting, other down-regulators of the inflammatory response are being studied. A normal ABR after a few days of hospitalization and antibiotic therapy is a good prognostic sign for hearing in this setting.
Ototoxic medications include antibiotics, loop diuretics, chemotherapeutic agents, and antiinflammatory medications. The cochleotoxic activity of aminoglycosides is in part due to their accumulation and prolonged half-life in perilymph, especially in patients with renal compromise. Energy-dependent incorporation of the antibiotics into hair cells occurs, and results in damage in the outer hair cells, beginning in the basilar turn. In the vestibular system, Type I hair cells are more susceptible to aminoglycosides. Gentamicin, tobramycin, and streptomycin are primarily vestibulotoxic, while kanamycin and amikacin are primarily cochleotoxic. Ototoxicity is increased with treatment greater than 10 days, preexisting hearing loss, concomitant exposure to noise, or use of other ototoxic agents. Hearing loss can be reduced by serial high frequency(8-14 KHz) audiometric screening and vestibular evaluation, and by careful monitoring of peak and trough serum levels.
Loop diuretics such as furosimide and ethacrynic acid, as well as erythromycin and vancomycin may cause permanent ototoxicity, especially in combination with other ototoxic agents. Salicylates in high levels may cause reversible hearing loss and tinnitus. The mechanism is likely a salicylate-mediated increase in the membrane conductance of the outer hair cells. Cisplatin causes a permanent, dose-related, bilateral high frequency sensorineural hearing loss. As with aminoglycoside usage, high frequency audiologic testing(8-14K) may identify early ototoxicity when using cisplatin.
Temporal bone trauma may lead to sensorineural hearing loss, especially transverse temporal bone fractures. Head injuries without temporal bone fractures may lead to high-frequency sensorineural hearing loss, due to labyrinthine or cochlear concussions. Up to 25% of adults with post-traumatic sensorineural loss will get some spontaneous improvement over time. No treatment has been shown to be useful in this setting.
Cholesteatomas are usually associated with conductive hearing loss. If sensorineural hearing loss is present, a labyrinthine fistula should be suspected. Up to 10% of patients with long-standing or recurrent cholesteatomas have a labyrinthine fistula. Complete hearing loss raises suspicion that the fistula has led to suppurative labyrinthitis. Modified radical mastoidectomy is the usual treatment of choice for cholesteatoma with labyrinthine fistula.
Cerebellopontine angle tumors typically cause a high frequency sensorineural hearing loss with poor speech discrimination. Rollover and tone decay are usually present, and stapedial reflexes are usually absent. ABR typically shows an absent or prolonged latency of wave V, and can usually detect larger CPA tumors. The diagnostic procedure of choice in most cases is an MRI with gadolinium, since these studies can detect tumors as small as 2mm with very high specificity. Surgical excision is the standard treatment, although observation is sometimes appropriate.
Meniereís disease causes fluctuating, sensorineural hearing loss, with associated episodes of vertigo, tinnitus, and aural fullness. The hearing loss may be unilateral or bilateral. Histopathologically, endolymphatic hydrops is seen. Treatment consists of diuretics and dietary salt restriction.
Sensorineural hearing loss caused by autoimmune disease may be associated with vertigo and facial palsy. The following autoimmune disorders may cause hearing loss: Coganís disease, systemic lupus erythematosus, Wegenerís granulomatosis, polyarteritis nodosa, relapsing polychondritis, temporal arteritis, and Takayasuís disease. Histopathologic findings are quite variable. Some cases of hearing loss from autoimmune disease improve with steroids, with or without antineoplastic agents.
Idiopathic, progressive, bilateral sensorineural hearing loss(IRBSNHL) is a rare immune-mediated type of bilateral sensorineural hearing loss that progresses over days to months, and may respond to steroids. The steroid regimen for these patients is Prednisone 1 mg/kg for up to 6 months. The prednisone is tapered to a maintenance dose of 10-20mg every other day for responders, and tapered off completely after 1 month for the non-responders. Risks of long term steroid use include intestinal ulcers, weight gain, blood pressure changes, worsening of diabetes, psychiatric problems, insomnia, and cataract formation. Those patients unable to take steroids may benefit from methotrexate or cyclophosphamide.
Idiopathic Sudden Sensorineural Hearing Loss
Unilateral hearing loss not attributable to known causes of hearing loss that occurs in less than 3 days is called idiopathic sudden sensorineural hearing loss. Two-thirds of these patients recover to near-normal hearing levels spontaneously. Those with high frequency loss, and loss over 40dB are less likely to recover. The reported incidence is 1 in 10,000. The main-stay of treatment is prednisone 1mg/kg/day for 10-14 days, followed by a tapering dose. The steroid should be started within 2 weeks of the onset of symptoms, preferable sooner. The use of antiviral agents such as acylclovir in the treatment of idiopathic sudden sensorineural hearing loss is investigational.
Neurological conditions such as multiple sclerosis may be associated with sensorineural hearing loss. Fewer than 5% of patients with multiple sclerosis develop sensorineural hearing loss. The sensorineural loss may be unilateral or bilateral, may or may not be associated with tinnitus or vertigo, and usually improves with time.
Hematologic disorders such as sickle cell anemia and blood viscosity disorders may lead to sensorineural hearing loss. Sickle cell disease likely causes thrombosis and infarction of the end vessels of the cochlea of some affected patients. Some 20% of sickle cell patients have sensorineural hearing loss. Similarly, blood viscosity disorders and megaloblastic anemias may induce cochlea end vessel disease.
Both otosclerosis and Pagetís disease have been associated with sensorineural hearing loss. However, the etiology of the hearing loss is unclear. Histopathologic specimens have failed to show consistent injury to the cochlea or neural pathways.
Endocrine disorders such as diabetes mellitus, hypothyroidism, and hypoparathyroidism may be associated with hearing loss in adults. However, the etiologies of hearing loss associated with endocrine disorders are unclear, and the causal relationships have yet to be established.
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