Impaired Hearing


Chapter 83

Impaired Hearing



Michael L. Norris



Definition and Epidemiology


Impaired hearing is a defect in the detection and/or processing of sound waves. Impaired hearing affects both communication ability and personal safety and can be a socially isolating experience. Hearing loss occurs at all ages, although its prevalence increases with advancing age. A complaint of hearing loss can reflect a wide variety of abnormalities and requires different considerations in children than in adults.


imageImmediate specialist referral to an otolaryngologist or neurologist is indicated for patients with sudden hearing loss.



Pathophysiology


The ear is the peripheral mechanism that converts sound waves into electrical impulses that are processed by the central auditory pathways. It is divided into three segments: the outer ear, middle ear, and inner ear. Each section must function properly for hearing to occur normally. The outer ear includes the auricle and ear canal. Its function is to collect sound waves and funnel them to the middle ear. The middle ear includes the tympanic membrane (TM) and the ossicles and the middle ear space that contains them. It transfers the sound waves to the inner ear, amplifying these vibrations as it does. Finally, the inner ear consists of the cochlea, the organ of hearing, and the semicircular canals, which function as a primary balance system. The cochlea converts the vibratory energy into electrical impulses that are then processed by the auditory nerve pathways in the brainstem, midbrain, and cerebrum.


Hearing losses are classified into three types. The first is conductive loss, which results from sound waves being attenuated at the external auditory canal or the middle ear. The second type is sensorineural loss, resulting from malfunction in the cochlea or central auditory pathways. Sensorineural losses can be subdivided into a peripheral (cochlear) loss or a central (nerve) loss. Finally, a mixed hearing loss has both conductive and sensorineural components. The vast majority of hearing losses are peripheral, and differential diagnosis is generally focused on the peripheral mechanism. It should never be forgotten, however, that the central auditory pathways are crucial to hearing and central losses will be encountered occasionally. A number of abnormalities may lead to hearing loss of each type.


In conductive hearing loss, any component of the anatomic structures of the outer or middle ear can be involved. In the outer ear, impacted cerumen, bacterial or fungal infection (swimmer’s ear), overgrowth of the bony wall (exostoses), tumors, congenital atresia, and fibrotic stenosis from recurrent infection may attenuate the sound reaching the middle ear and cochlea. In the middle ear, perforation of the TM, scar tissue, negative pressure from eustachian tube dysfunction, barotraumas, cholesteatoma, glomus tumor, otosclerosis, or any other condition that impairs the mobility of the TM or ossicles can reduce hearing sensitivity. Causes of conductive loss from middle ear disease include acute otitis media, serous otitis media, and chronic serous otitis. While otitis media is primarily associated with early childhood, it may occur at any age. Otosclerosis, which is fusion of the stapes over the oval window, is a common cause of hearing loss in adults, usually appearing between the ages of 20 and 40.1 Other conditions that interfere with the mechanical transmission of sound in the middle ear are trauma that damages the ossicles and congenital malformations.


Sensorineural hearing loss usually occurs from disorders of the cochlea. Less prevalent are disorders of the central auditory nervous system (CANS)—that is, cranial nerve VIII (acoustic), the internal auditory canal, or the brain. Congenital sensorineural hearing loss may result from noninherited factors such as maternal infections (toxoplasmosis, other, rubella, CMV, HSV [TORCH complex]) or medications or from inherited autosomal abnormalities. Adventitious sensorineural hearing loss can result from such factors as infections of the inner ear, Meniere disease, inner ear barotraumas, trauma, and tumors.


Presbycusis is a gradual degeneration within the cochlea that accompanies aging. Multiple factors may be at play in any given individual, including hair cell loss, metabolic changes, and circulatory insufficiency. Multiple factors influence the rate at which hearing loss occurs: genetics, medications, infections, and exposure to noise. High blood pressure, smoking, and diabetes may hasten presbycusis. There may also be degeneration of the mechanical structures and the central auditory connections.2


Noise trauma is a common cause of cochlear damage and may be a factor in presbycusis.2 Persistent or repeated exposure to excessive noise causes stress and mechanical damage to the delicate hair cells of the inner ear. High frequencies are affected initially, and over time the loss spreads to middle and lower frequencies. A loud, explosive noise may cause severe or profound damage to these structures and result in immediate hearing loss. In the United States the Occupational Safety and Health Administration (OSHA) has set standards and guidelines for noise exposure to protect workers. The OSHA standards limit the noise level exposure and require that hearing protection be worn at certain levels and that the hearing of those working in noise be monitored annually.3


Sensorineural hearing loss can also be caused by diseases that involve the endocrine or metabolic systems, autoimmune disorders, and neurogenic disorders. Ototoxic medications can also cause sensorineural hearing loss. The prime suspects in ototoxicity include antineoplastics, salicylates, aminoglycosides, furosemide, and quinine-related drugs. Audiologic evaluation before inception of treatment and regular monitoring of hearing during treatment is recommended for patients receiving a course of antineoplastics or aminoglycosides.


Central sensorineural hearing losses may be caused by acoustic tumors (vestibular schwannomas), stroke, and meningiomas.


Mixed hearing loss is a combination of both conductive and sensorineural loss. Usually, the presence of a mixed hearing loss is the result of two unrelated disease processes. Occasionally, however, injury to the ear, infection, and congenital disorders may affect the outer and/or middle ear and the inner ear.



Clinical Presentation and Physical Examination


Hearing loss may be sudden, progressive, or fluctuating in nature. It is important to determine whether the problem is unilateral or bilateral. Associated symptoms of otalgia, ear fullness, vertigo, tinnitus, or cranial neuropathies should be docu­mented. The medical history should incorporate current and past treatments with oral and intravenous medications or nonprescription drugs, particularly aminoglycosides, diuretics, antineoplastics, or large doses of aspirin. Chronic illnesses, hospitalizations, head injuries, and surgeries should be included in the history. A family history of hearing loss, neoplasms, renal disease, and balance disorders should be investigated. Finally, exposures to trauma and noise should also be noted.


A complete examination of the head, neck, and throat and an evaluation of cranial nerves and the auditory and vestibular system are essential. The pinna and external auditory canal should be inspected for malformations, lesions, exudates, and obstruction. Examination of the TM should assess for mobility (via pneumoscopy) and determine whether effusion, infection, perforation, or cholesteatoma is present.



Diagnostics



• Weber and Rinne tests, performed in conjunction, are used to differentiate conductive and sensorineural hearing loss.


• The Weber test is performed by placing a vibrating tuning fork at the midline of the forehead. With normal hearing or symmetric sensorineural hearing loss, the sound is heard equally in both ears. With asymmetric sensorineural loss, the sound is heard in the better ear. With an asymmetric conductive loss, the sound is heard in the poorer (greater conductive loss) ear.


• The Rinne test compares air conduction (AC) and bone conduction (BC). A vibrating tuning fork is held next to the ear and the patient reports when he or she can no longer hear the sound. The still-vibrating fork is then placed on the mastoid process behind the ear. In normal hearing, AC is better than BC (AC > BC)—that is, the patient does not hear the tuning fork when it is placed on the mastoid. With a conductive hearing loss, BC is better than AC—that is, the patient hears the tuning fork when it is placed on the mastoid. In the presence of a sensorineural hearing loss, AC remains better than BC but the patient does not hear it at as soft a level as a normal hearing person.4


• A screening audiogram is optimal.


• A recent noise exposure history should be taken before administering a hearing test for more accurate results. Exposure to loud noises over the weekend, for example, may decrease hearing if tested on a Monday morning. If the individual works in a noisy environment, a test in the morning, before the patient has been to work, is preferred.


• Tympanometric screening is used to determine middle ear function.


• A formal audiogram, performed by an audiologist in a sound-treated environment, is recommended if hearing is impaired on clinical examination. Formal testing should include the following:


• Pure tone tests by AC—to determine hearing thresholds for selected frequencies through earphones, and hence for the entire auditory system.


• Pure tone tests by BC—to establish the thresholds for the same frequencies with a bone oscillator placed on the mastoid. This bypasses the outer and middle ear and determines the sensorineural component of the loss. Analogous to the Rinne tuning fork test, better response on this test than on the AC test indicates a conductive component to the loss.


• Speech reception test under phones—to determine the softest level at which the patient can identify two-syllable words chosen from a closed set and to provide confirming evidence that the pure tone AC results are accurate.


• Speech recognition testing—to evaluate the patient’s ability to understand single-syllable words at a given presentation level.


• Impedance audiometry—to evaluate middle ear function. The tympanogram determines whether the TM is intact, how well it moves, and what the air pressure is in the middle ear. Acoustic reflexes evaluate the movement of the stapedius muscle and are particularly sensitive to the presence of otosclerosis.


• Additional specialized tests—to evaluate outer hair cell function (otoacoustic emissions [OAEs]) in the inner ear and integrity of the auditory nerve and brainstem auditory pathways (auditory brainstem response [ABR]). The tests are done when indicated by the results of conventional audiometry.


• Pediatric audiologists can evaluate hearing at any age. Behavioral tests provide good estimates of hearing thresholds, and objective tests such as OAEs and threshold ABR can provide reliable information about hearing in those patients too young to respond reliably to behavioral testing.


• Laboratory tests should also be done to evaluate the patient for systemic or metabolic causes for the hearing loss. For example, CBC/differential if anemia or infection is suspected; VDRL or RPR to exclude syphilis; ESR, antinuclear antibodies, rheumatoid factor, if autoimmune disorder is a consideration; TSH to exclude thyroid disorder.


• Magnetic resonance imaging (MRI) or computed tomography (CT) scans are useful in ruling out tumors; gauging the extent of chronic inflammatory middle ear disease and cholesteatomas; evaluating otosclerosis, erosion, or displacement of the ossicles; and identifying cochlear atresias or enlarged vestibular aqueducts,5 which may cause sensorineural hearing loss.6

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Oct 12, 2016 | Posted by in CRITICAL CARE | Comments Off on Impaired Hearing

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