etiology of hearing impairment in children

7/31/2019 Etiology of Hearing Impairment in Children

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Official reprint from UpToDatewww.uptodate.com

2011 UpToDate

AuthorsBetsy Sanford, MCD, CCC-APeter C Weber, MD, FACS

Section EditorGlenn C Isaacson, MD, FAAP,FACS

Deputy EditorElizabeth TePas, MD, MS

Etiology of hearing impairment in children

Last literature review version 18.3: septiembre 2010 | This topic last updated:mayo 19, 2010

INTRODUCTION Hearing loss in the first years of life can cause delays in speech,

language, and cognitive development. Speech and language delays secondary to hearing

loss are often preventable [1,2]. Thus, early identification of hearing impairment,

whether it is permanent (usually sensorineural) or temporary (usually conductive), is thekey to a child's success with communication [2].

Significant hearing loss occurs in one to two per 1000 newborns; early childhood

deafness occurs in two per 1000 children. Nearly all children will develop transient

hearing loss related to ear infections during the period from birth to 11 years of age [3].

Knowledge of the etiology of the hearing loss, particularly if genetic, can affect family

planning and assist in the development of the optimal habilitation plan for the patient

[1,4-7].

The anatomy and physiology of the ear and the etiology of hearing impairment in

children are reviewed here. The evaluation and treatment of hearing impairment are

discussed separately. (See "Evaluation of hearing impairment in children" and

"Treatment of hearing impairment in children".)

ANATOMY AND PHYSIOLOGY The ear is divided into three segments (figure 1):

The outer ear, comprising the auricle and ear canal

The middle ear, comprising the tympanic membrane, ossicles, and the middle ear

space

The inner ear, comprising the cochlea, semicircular canals, and internal auditory

canals

Anatomically, the auricle has a unique structure. It "catches" incoming sound waves and

then funnels them down the external auditory canal. Sound waves vibrate the tympanic

membrane, causing motion of the ossicles that results in a piston-like effect of the

stapes. The piston motion of the stapes then pushes the inner ear fluid around the two

and one-half turns of the cochlea. Frequency-specific movement sets up fluid waves

within the cochlea, which in turn sets the organ of Corti into motion. Movement of the

organ of Corti bends the stereocilia, causing depolarization of the inner hair cells and

creating electrical impulses transmitted via the auditory nerve to the brain. The brain

organizes the information into what we perceive as complex sounds.

CLASSIFICATION Hearing loss may be classified as follows:

Conductive, involving any cause that in some way limits the amount of external

sound that gains access to the inner ear. Examples include cerumen impaction,

middle ear fluid, or ossicular chain fixation.

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Sensorineural, involving the inner ear, cochlea, or the auditory nerve.

Mixed, which is a combination of conductive and sensorineural hearing loss.

Central, involving higher brain centers. Advanced electrodiagnostic testing may

show absent or severely distorted auditory brainstem responses with preserved

otoacoustic emissions and cochlear microphonics [8].

Numerous abnormalities can lead to hearing loss of each type (table 1).

CONDUCTIVE Conductive losses imply a problem in the outer or middle ear: the

pinna, external canal, tympanic membrane, or ossicles.

Outer ear Congenital anomalies, infections, and trauma of the outer ear cause

conductive hearing loss.

Congenital The external auditory canal (EAC) develops from the 8th to the 28th

week of gestation; problems can occur anytime during this developmental phase.

Microtia, the absence or malformation of the auricle, may be associated with mild to

moderate conductive hearing loss, whereas atresia or significant stenosis of the EACcauses moderate to maximal (60 dB) conductive hearing loss.

The auricle itself forms from remnants of the first and second branchial arch during the

12th and 20th weeks; it is possible to have a normal auricle but an atretic canal.

Unilateral atresia or significant stenosis of the EAC is much more common than is

bilateral atresia.

Atresia malformations of the EAC occur in approximately 1 per 10,000 births and are

usually associated with other craniofacial abnormalities such as Treacher-Collins

syndrome, Robin sequence, or Crouzon syndrome (table 2) [9,10]. (See "Congenital

anomalies of the jaw, mouth, oral cavity, and pharynx" and "Craniosynostosissyndromes".)

Infection Infections can lead to blockage of the EAC caused by the accumulation

of debris, edema, or inflammation. Otitis externa usually develops after local trauma

coupled with contamination by bacteria (or occasionally fungi) after swimming,

showering, or exposure to hot and humid conditions. Significant edema of the EAC

occurs; the canal also is often filled with squamous and purulent debris. The most

common symptoms are otalgia, pruritus, discharge, and hearing loss. (See "External

otitis".)

Trauma Penetrating trauma to the EAC or meatus caused by a bullet, knife, or

fracture may cause mild or profound conductive hearing loss, depending upon the degreeof EAC occlusion.

Middle ear Conductive hearing loss associated with the middle ear may be caused by

congenital anomalies, infection, tympanic membrane perforation, or tumors.

Congenital Congenital conditions leading to atresia, malformation, fracture, or

dislocation of the ossicular chain cause conductive hearing loss. The most common

ossicular abnormality is absence or malalignment of the cura of the stapes. However, the

abnormal incus or malleoincudal joint seems to cause the most consistent conductive

hearing loss. Other malformations of the ossicular chain include fixation of the

malleoincudal joint or one of the ossicles to the scutal or attic ridge.Osteogenesis imperfecta is an autosomal dominant condition associated with fragile

bones that fracture easily. Children with this disorder may develop ossicular dislocation,




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stapes fixation, or fracture of the ossicles, resulting in a conductive hearing loss. (See

"Clinical features and diagnosis of osteogenesis imperfecta".)

Infection Otitis media (OM) is a common childhood disorder associated with

conductive hearing loss. By the age of three years, 83 percent of children will experience

at least one episode of OM, and 46 percent will have at least three episodes [11]. About

3 to 20 percent of children have six episodes of OM in a year [12].

Acute OM is defined as fluid in the middle ear accompanied by signs or symptoms of ear

infection: pain, fever, ear fullness, and/or decreased hearing. Conductive hearing loss

occurs because fluid filling the middle ear space prevents the tympanic membrane from

vibrating adequately, thereby diminishing movement of the ossicular chain. The hearing

loss persists as long as fluid fills the middle ear space. The median loss is 25 dB which is

similar to putting plugs in the child's ears. (See "Acute otitis media in children:

Epidemiology, pathogenesis, clinical manifestations, and complications".)

Despite adequate therapy, middle ear effusion persists in many patients. The

approximate incidence of asymptomatic effusion is 40 percent at one month, 20 percent

at two months, and 10 percent at three months [13,14]. In the 10 to 15 percent ofchildren in whom the fluid does not clear, it may need to be removed to promote healing

and resolution of conductive hearing loss [15]. Small, retrospective, case-control, and

longitudinal cohort studies have noted that children who have prolonged middle ear

effusion have lower scores on tests of speech, language, and cognitive abilities [16-18].

However, a study in which children with persistent middle ear effusion were randomly

assigned to prompt or delayed insertion of tympanostomy tubes found that

developmental outcome at 3, 4, 6, and 9 to 11 years of age did not differ between the

groups [19-23]. (See "Otitis media with effusion (serous otitis media) in children".)

TM perforation Conductive hearing loss caused by tympanic membrane

perforation is common. The degree of conductive hearing loss depends upon the size andlocation of the perforation. Small perforations and those located in the anterior/inferior

quadrant cause a lesser degree of conductive hearing loss; near total or

posterior/superior quadrant perforations are more likely to cause significant hearing loss.

Tympanic membrane perforations can be caused by many events, including blast injury,

barotrauma, foreign body trauma, temporal bone fractures, ear infections, self-inflicted

trauma from a Q-tip or other object (picture 1), or the hole may persist after

myringotomy and tubes fall out. After an acute perforation, the ear needs to be

examined under the microscope to ensure that skin is not trapped on the undersurface of

the tympanic membrane, because trapped skin could lead to cholesteatoma formation.

Documentation of a patient's auditory status also is mandatory for any newly diagnosedperforation. (See "Cholesteatoma in children".)

Trauma Blunt trauma can lead to longitudinal or transverse temporal bone

fracture. The longitudinal type is most common (80 percent). It is usually caused by a

blow to the temporal parietal region (picture 2) [24]. Hearing loss is typically conductive

and associated with tympanic membrane perforations and blood in the middle ear space.

Transverse fractures are usually associated with sensorineural hearing loss; however,

conductive hearing loss can occur with ossicular injury, typically due to separation of the

incudal stapedial joint and/or incus dislocation. (See 'Sensorineural' below.)

Tumors Malignant tumors such as squamous cell carcinoma and proliferative

disorders, including Langerhans cell histiocytosis can cause conductive hearing loss.However, these entities are relatively rare when compared with benign cholesteatoma or

otosclerosis.




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Cholesteatoma Cholesteatoma is a benign growth composed of a live matrix of

stratified, squamous epithelium filled with desquamated, cells and keratin (picture

3). As keratin desquamates from the epithelial lining of the sac, it gradually

enlarges, with eventual erosion of the ossicular chain, mastoid bowl, and EAC.

Formation of a cholesteatoma typically occurs after a retraction pocket has formed

in the posterior/superior quadrant, usually the result of poor eustachian tube

function. It may also occur after tympanic membrane trauma, such as a traumatic,inflammatory, or iatrogenic perforation. (See "Cholesteatoma in children".)

Otosclerosis Otosclerosis is abnormality of bone remodeling that produces

overgrowth of sclerotic, hypervascular bone. It has a predilection for the footplate

of the stapes. As the overgrowth develops, the stapes can no longer function as a

piston, but rocks back and forth and eventually becomes fixed. Conduction

gradually becomes worse until a maximal conductive hearing loss of 60 dB is

reached.

SENSORINEURAL Disorders of the inner ear cause sensorineural hearing loss

(SNHL). Problems may occur at the level of the cochlea, eighth nerve, internal auditorycanal, or brain.

Congenital Congenital hearing loss is any hearing loss that occurs at or shortly after

birth; it may be hereditary or nonhereditary. Nonhereditary etiologies involve an insult to

the developing cochlea, such as intrauterine infection, medications, or toxins that have a

teratogenic effect on the developing ear of the fetus (eg, recreational drugs, alcohol,

methyl mercury, quinine, trimethadione, retinoic acid, and maternal thyroid peroxidase

autoantibodies) [7,25].

Infection Congenital infection caused by cytomegalovirus (CMV), toxoplasmosis,

rubella, or syphilis is associated with SNHL. The hearing loss in these conditions

frequently is progressive or delayed in onset, emphasizing the need for universal

newborn hearing screening and continued monitoring of children with known congenital

infections. The USPSTF clinical practice guideline for newborn hearing screening, as well

as other USPSTF guidelines, can be accessed through the website for the Agency for

Healthcare Research and Quality at www.ahrq.gov/clinic/uspstfix.htm. (See "Screening

tests in children and adolescents" and "Screening the newborn for hearing loss".)

Congenital cytomegalovirus infection Congenital CMV is the most common

intrauterine infection in humans as well as the leading cause of sensorineural

deafness [26,27]. Forty thousand infants in the United States are born each year

with congenital CMV; ninety percent are asymptomatic. Among the 10 percent whoare symptomatic, approximately 60 percent develop SNHL [28]. Even among

asymptomatic children with documented CMV infection in one study, 7 percent

developed SNHL [29]. The hearing loss was progressive in 50 percent and delayed

in onset (median age 27 months) in 18 percent. (See "Overview of TORCH

infections", section on 'Congenital cytomegalovirus' and "Cytomegalovirus infection

and disease in newborns, infants, children and adolescents".)

Congenital toxoplasmosis Congenital toxoplasmosis occurs in one in 1000 to

3000 births. Ninety percent of the cases are asymptomatic at birth. SNHL occurs in

10 to 15 percent of infected children [30] and is usually delayed and progressive

[31]. The hearing loss may be prevented by treatment with pyrimethamine andsulfonamide preparations at birth [30]. (See "Overview of TORCH infections",

section on 'Toxoplasmosis'.)




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Congenital rubella Although the incidence of congenital rubella has declined by

more than 99 percent since the introduction of the rubella vaccine in 1969 [32],

approximately 5 to 25 percent of women of child-bearing age remain susceptible to

rubella infection [33]. Hearing loss affects 68 to 93 percent of children with

congenital rubella; it is usually profound and bilateral and sometimes progressive

[7]. (See "Congenital rubella syndrome: Clinical features and diagnosis".)

Congenital syphilis The incidence of congenital syphilis, which is approximately

0.005 percent of live births in the United States, has been increasing steadily since

1985. One-third to two-thirds of infected infants are asymptomatic at birth [7,33].

SNHL, which occurs in 30 to 40 percent of these infants, is a late manifestation of

congenital syphilis and appears after 2 years of age. It is the result of scarring and

inflammation from untreated disease and can be prevented by appropriate therapy

before 3 months of age [7,33]. (See "Overview of TORCH infections", section on

'Congenital syphilis'.)

Malformations Congenital malformations (dysplasias) of the inner ear cause

SNHL. Four types are:

Michel complete lack of inner ear

Mondini incomplete development and malformation of the inner ear

Scheibe membranous cochleosaccular degeneration of the inner ear

Alexander malformation of the cochlear membranous system

Knowledge of these inner ear abnormalities is important for proper diagnosis and

treatment. Genetic counseling is recommended for the parents, as well as for the child.

Diagnostic considerations should include complete audiometric evaluation, complete

otolaryngologic evaluation, and proper diagnostic imaging studies.

Perilymph fistula Perilymph fistula (PLF) is a leak of inner ear fluid through a holein either the round window membrane or the oval window annular ligament. This hole

permits an open communication between the middle ear and the inner ear. It can be

caused by trauma, or in certain cases by a genetic defect associated with the stapes

footplate.

PLF is difficult to diagnose, but typical symptoms include fluctuating severe SNHL,

disequilibrium, and aural fullness. Exploratory surgery performed by an otolaryngologist

may be necessary to provide a definite diagnosis and to repair the leak.

Hereditary Hereditary bilateral SNHL occurs from 1 in 2000 to 1 in 6000 births [3].

Hereditary causes account for up to 50 percent of cases of SNHL and include SNHL with

(one-third) and without (two-thirds) associated abnormalities (syndromic and

nonsyndromic, respectively) (table 2) [34,35].

The hearing deficit in hereditary hearing impairment (HHI) may be present at birth

(congenital), progressive from birth, or it may develop when the child is older. One clue

to the presence of hereditary SNHL is that no set pattern exists; it may fluctuate,

progress, or remain stable.

Approximately 80 percent of cases of HHI are inherited in an autosomal recessive

pattern, 18 percent autosomal dominant, and 2 percent X-linked recessive [36]. In rare

patients, HHI reflects a defect in the mitochondrial rather than nuclear DNA [34].

Autosomal recessive Because HHI is most often inherited as an autosomal

recessive trait, many children with HHI will not have affected relatives. There are both

syndromic and nonsyndromic forms of HHI. Among the syndromic autosomal recessive




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disorders are Alport syndrome, Usher syndrome, Pendred syndrome, Jervell-Lange-

Nielsen syndrome, albinism, and Hurler syndrome.

Advances are being made in the understanding of the molecular biology of hearing [37-

41]. As examples:

Autosomal recessive Alport syndrome is associated with mutations in the alpha-3

or alpha-4 chains of type IV collagen [42]. The defect in hearing, as well as theassociated renal disease, is thought to be caused by impaired formation of a

unique type of collagen. (See "Genetics, pathogenesis, and pathology of hereditary

nephritis (Alport syndrome)".)

At least thirty distinct loci have been linked with nonsyndromic recessive hearing

loss [43,44]. Mutations in the connexin 26 gene (CX26, also called gap junction

protein beta 2 [GJB2]) on chromosome 13 are responsible for approximately one-

half of all cases of childhood nonsyndromic hearing loss [35,39,45,46]. More than

60 mutations have been described in the connexin 26 gene, six of which are

associated with dominant or syndromic sensorineural hearing loss [47,48].

These advances promise new information about the genetics of hearing loss and will

certainly impact clinical management of hearing impaired children.

Autosomal dominant Waardenburg syndrome types I and II, neurofibromatosis I

and II, branchio-oto-renal syndrome, and Jervell-Lange-Nielsen syndrome (long QT

syndrome with deafness) are examples of autosomal dominant disorders that cause

syndromic SNHL. (See "Clinical features of congenital long QT syndrome", section on

'Congenital sensorineural deafness'.)

Nonsyndromic autosomal dominant disorders that cause progressive SNHL may present

with early onset (during the first decade of life) or delayed onset (during the second or

third decade) [34]. Two genes causing dominant progressive hearing impairment have

been located: one on chromosome 1p and one on chromosome 5q [43]. The latter

disorder is associated with a mutation in the gene encoding the transcription factor

POU4F3 [49], whereas the former involves a potassium channel gene impairing the

function of outer hair cells [50]. Interestingly, the defect in the Jervell-Lange-Nielsen

syndrome also involves a potassium conductance but impairs endolymph production

[51]. (See "Genetics of congenital and acquired long QT syndrome".)

X-linked Among the X-linked disorders associated with SNHL are Hunter syndrome

(mucopolysaccharidosis 2), Alport syndrome, X-linked congenital SNHL, and early onset

progressive sensorineural hearing loss [3,43,52].

Mitochondrial Mitochondrial inheritance accounts for less than 1 percent of all

HHI. Transmission occurs from affected females. Hearing loss may be isolated or

associated with other features of mitochondrial disorders: lactic acidosis,

encephalopathy, myopathy, seizures, ophthalmoplegia, diabetes mellitus,

cardiomyopathy, stroke-like episodes, ataxia, and optic atrophy [43]. (See "Approach to

the metabolic myopathies".)

Acquired SNHL Acquired SNHL is not present at birth and is not inherited. Major

causes in children include prematurity, infection, hyperbilirubinemia, ototoxins, noise,

and tumors.

Prematurity The prevalence of congenital SNHL among babies with low birth

weight (less than 1500 g) is 51 per 10,000 [53]. The high rate of hearing loss among

these children has been linked to several factors, including administration of ototoxic




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drugs such as aminoglycoside antibiotics, ambient noise produced by the incubator, and

perinatal complications (eg, hypoxia and acidosis) [54,55]. Continued improvements in

obstetrical and newborn care have led to decreased mortality in premature infants. As

further advances are made in this area, decreased mortality will pose an increased risk of

developmental disabilities, including hearing loss for premature and low-birth-weight

babies [56,57].

Although the majority of neonates admitted to the neonatal intensive care unit (NICU)

meet the high-risk requirement for hearing screening, universal newborn hearing

screening is necessary to detect hearing loss among those without obvious risk factors.

The USPSTF clinical practice guideline for newborn hearing screening, as well as other

USPSTF guidelines, can be accessed through the website for the Agency for Healthcare

Research and Quality at www.ahrq.gov/clinic/uspstfix.htm. (See "Screening the newborn

for hearing loss".)

Hyperbilirubinemia Hyperbilirubinemia in infants is frequently caused by

hemolysis. Causes of hemolysis include Rh or ABO blood group incompatibility and

glucose-6-phosphate-dehydrogenase deficiency. Hyperbilirubinemia, which can also

result from other newborn complications, leads to sensorineural hearing loss becausebilirubin is toxic to the cochlear nuclei and the central auditory pathways (auditory

neuropathy). (See "Clinical manifestations of unconjugated hyperbilirubinemia in term

and late preterm infants", section on 'Kernicterus'.)

Hyperbilirubinemia may need to be treated with phototherapy or exchange transfusion

depending upon a number of factors, including the level of and rate of rise of bilirubin,

the gestational age of the infant, and others. The risk of hearing loss is increased in

children with prolonged hyperbilirubinemia and adverse perinatal factors, such as

acidosis and low birth weight [58].

Infection Bacterial meningitis is the most common cause of acquired deafness inchildhood [59,60]. The reported frequency of persistent hearing impairment has varied

from 2.5 to 18 percent in survivors [59-62]. Another 10 percent of children have

transient hearing impairment [62]. (See "Neurologic complications of bacterial meningitis

in children".)

Sensorineural hearing loss occurs early in the course of bacterial meningitis (within the

first 48 hours), with possible recovery or worsening during the first two weeks of illness

[63,64]. Permanent hearing loss may be caused by damage to the cochlea, labyrinth, or

eighth cranial nerve from direct bacterial invasion or the inflammatory response elicited

by the infection [61,65].

Treatment with dexamethasone early in the course of meningitis decreases the incidenceof hearing loss and neurologic sequelae. (See "Treatment and prognosis of acute

bacterial meningitis in children", section on 'Use of dexamethasone'.)

Children who have had bacterial meningitis should have a complete hearing evaluation

initially and regular follow-up if the evaluation is abnormal. (See "Evaluation of hearing

impairment in children".)

Ototoxic drugs Ototoxic drugs such as aminoglycosides, high dose intravenous

loop diuretics, and chemotherapeutic agents such as cisplatin can cause significant

hearing loss [66,67]. The hearing loss caused by antibiotic or chemotherapeutic agents

usually begins at high frequencies; with continued use, the hearing loss will becomemore pronounced and may even worsen for a time after the drug is discontinued. Any

sensorineural hearing loss associated with these drugs is permanent.




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Aminoglycosides All aminoglycosides are ototoxic. Some are more vestibulotoxic

than cochleotoxic. The relative order of cochleotoxicity is

gentamicin>tobramycin>amikacin>neomycin. There also appears to be a genetic

predisposition to the development of ototoxicity with aminoglycosides [68,69].

(See "Pathogenesis and prevention of aminoglycoside nephrotoxicity and

ototoxicity".)

Otoacoustic emissions, which reflect the functional status of the outer hair cells in

the cochlea, are a sensitive measure of the early effects of aminoglycoside-induced

injury to the peripheral auditory system [70] and may be useful in monitoring

cochlear function during ototoxic treatment.

Other antibiotics Other antibiotics that can cause ototoxicity include

erythromycin, vancomycin, and tetracycline. These drugs have a more pronounced

ototoxic effect in patients with impaired renal function.

Chemotherapeutic agents Numerous chemotherapeutic agents can cause

hearing loss. The most common ones are cisplatin, 5-fluorouracil (5-FU),bleomycin, and nitrogen mustard. The worst ototoxicity occurs with cisplatin, which

causes hearing impairment in 10 to 26 percent of children [71,72]. (See

"Neurologic complications of platinum-based chemotherapy", section on

'Cisplatin'.)

Salicylates Aspirin and other nonsteroidal antiinflammatory drugs can cause

hearing loss that is reversible with discontinuation of the drug. The etiology is

believed to be enzymatic inhibition, which occurs only at very high doses (eg, 6 to

8 g/day ofaspirin).

Other medications Antimalarial medications such as quinine and chloroquine can

cause sensorineural hearing loss and tinnitus but, as with salicylates, these effects

are usually reversible. High-dose intravenous-loop diuretics are an additional cause

of temporary hearing loss and tinnitus [73]. The risk is increased if the patient is

also receiving other ototoxins such as an aminoglycoside [67].

Noise exposure Constant exposure to loud noises, as may occur with personal

listening devices [74], can cause high-frequency SNHL. The estimated prevalence of

noise-induced hearing loss in one or both ears in United States children aged 6 to 19

years is 12.5 percent [75]. Figure 2 represents a typical audiogram of noise-induced

SNHL (figure 2). The worst hearing loss is at 4000 Hz; it improves at 8000 Hz. However,

this shape will decrease over time as more and more frequencies are involved.The mechanism by which excessive noise induces hearing loss includes direct mechanical

damage of cochlear structures and metabolic overload caused by overstimulation [76].

Some potential metabolic effects are excess nitric oxide release, which can damage hair

cells; generation of oxygen free radicals that become toxic to membranes; and low

magnesium concentrations that weaken hair cells by reducing the concentration of

intracellular calcium.

The Occupational Safety and Health Administration (OSHA) and National Institute for

Occupational Safety and Health (NIOSH) have set standards and guidelines for noise

exposure in order to protect workers. OSHA requires all employees who are exposed to a

greater than 85 dB time-weighted average (85 dBA) to be enrolled in a hearingconservation program and provided hearing protection. OSHA also limits employee's

exposure to noise as follows: eight hours of exposure to 90 dBA (which is equivalent to




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the noise made by a power lawn mower); four hours of exposure to 95 dBA; two hours

of exposure to 100 dBA; and so on [77]. NIOSH limits noise exposure as follows: eight

hours to 85 dBA; four hours to 88 dBA; two hours to 91 dBA; and so on [78].

The OSHA and NIOSH standards are not necessarily applicable to portable music-

listening devices, since music differs from industrial noise spectrally and temporally [74].

However, studies documenting the output levels of various portable listening devices

(compact disc [CD] players and digital music players) have been used to develop safety

guidelines based upon NIOSH standards [79,80].

For the typical listener, listening to a portable CD player with supra-aural

headphones (earphones that sit on top of the ear) at 60 percent maximum volume

should be limited to less than one hour per day in order to reduce the risk of noise-

induced hearing loss (NIHL) [79].

For the typical listener, listening to a digital music player with the earphones that

came with the device at 70 percent of maximum volume should be limited to less

than 4.6 hours per day and at 80 percent of maximum volume should be limited to

less than 1.2 hours per day in order to reduce the risk of NIHL [80].

The listening times should be adjusted according to presence of other risk factors for

hearing loss (eg, ototoxic drugs) [79].

A short blast of loud noise also can cause severe to profound SNHL, pain, or hyperacusis

(loudness intolerance). This reaction usually involves exposure to noise greater than 120

to 155 dB (eg, an ambulance siren or a firecracker). Thus, hearing protection in the form

of muffs or plugs is highly recommended any time a person is exposed to loud noises.

Trauma Trauma to the temporal bone usually causes sensorineural or mixed

hearing loss. Both penetrating and blunt trauma can result in transverse temporal bone

fractures. Penetrating trauma is typically due to gunshot wounds and blunt trauma to

blows to the occipital or frontal regions. Transverse temporal bone fractures usually run

through the inner ear with the result of a "dead" ear (picture 2). In addition, blunt

trauma causes sensorineural hearing loss when concussive forces are exerted on the

inner ear fluids, causing a shearing effect on the cochlear organ of Corti.

Tumors The most common tumor that causes SNHL is an vestibular schwannoma

(acoustic neuroma), which is rare in children without neurofibromatosis. They are benign

tumors that usually originate from the vestibular portion of the eighth cranial nerve. The

acoustic neuromas associated with neurofibromatosis are typically bilateral and

asymmetric [81]. Other symptoms and signs include tinnitus, disequilibrium, dizziness,

headaches, and facial hyperesthesia or facial muscular twitching. (See "Vestibular

schwannoma (acoustic neuroma)".)

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GRAPHICS

External ear anatomy

The external auditory canal is a cylinder measuring approximately 2.5 cm inlength and 7 to 9 mm in width, extending from the conchal cartilage of the

auricle to the tympanic membrane. It is divided into a lateral (outer)cartilaginous portion that occupies approximately two-thirds of the canal,and a medial (inner) bony portion that occupies the other one-third. Theirjunction is termed the isthmus and is the most narrow region of the earcanal. The outer cartilaginous portion is lined by thicker skin with numerousadnexal structures, including cerumen glands (a modified apocrine typegland), sebaceous glands, and hair follicles. Cerumen is formed here. Theinner osseous portion of the canal contains thin skin without subcutaneoustissue. The inferior tympanic recess is a small depression in the inferiormedial aspect of the ear canal, adjacent to the tympanic membrane. Debriscan collect in this area and cause or perpetuate infection.Adapted withpermission from Cantor, RM, Emerg Med 1999; 31:40. Copyright Quadrant HealthCom,

Inc, 1999.




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Causes of hearing loss

Conductive

Outer-ear causes

Congenital microtia or atresia

External otitis

Trauma

Squamous cell carcinoma

Exostosis

Osteoma

Psoriasis

Cerumen

Middle-ear causes

Congenital atresia or ossicular chainmalformation

Otitis media

Cholesteatoma

Otosclerosis

Tympanic membrane perforation

Temporal bone trauma

Glomus tumors

Sensorineural

Inner-ear causes

Hereditary hearing loss

Congenital viralinfections

Congenital malformations

Presbycusis

Meningitis

Thyrotoxicosis

Viral cochleitis

Ototoxic drugs

Otologic surgery

Meniere's disease

Noise exposure

Barotrauma

Penetrating trauma

Acoustic neuroma

Meningioma

Autoimmune disease

Multiple sclerosis

Cerebrovascular ischemia




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Selected syndromes associated with hearing loss

Syndrome FeaturesCaused

bymutation

Sensorineural Conductiv

Waardenburg

syndrome (AD)

White forelock,

heterochromicirides, broadmandible, deafness

X X

Usher syndrome (AR) Retinitispigmentosa,ataxia, deafness

X X

Pendred syndrome(AR)

Familial goiter,dysfunctionaliodideorganization,deafness

X X

Alport syndrome (XL,AR, AD)

Nephritis,deafness, lens,defects, retinitis

X X

Craniofacial anomalies(eg, Apert syndrome,Pfeiffer syndrome,Crouzon syndrome)

Craniosynostosis,micrognathia,syndactyly

X X

CHARGE (AD, isolatedcases)

Choanal atresia,colobomas, heartdefect, MR, genitalhypoplasia, earanomalies,deafness

X X

Hemifacialmicrosomia (oculo-auriculo-vertebralspectrum, Goldenharsyndrome)(sporadic,AD)

Facial hypoplasia,ear anomalies,hemivertibrae,parotid glanddysfunction

X X

MucopolysaccharidosisHurler (AR), Hunter(XL), Maroteaux-Lamy(AR)

Coarse facies, stiffjoints, MR, cloudycorneas

X

Treacher-Collinssyndrome (AD)

Facialmalformation, cleftpalate, deafness

X X

Otopalatodigitalsyndrome (XL)

Deafness, cleftpalate, broad digits

X

Stickler syndrome(AD)

Cleft palate,micrognathia,myopia, cataracts,spondyloepiphysealdysplasia, deafness

X X

LEOPARD syndrome(AD) Multiple lentigenes,pulmonic stenosis,hypertelorism,

X X




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deafness, genitalanomalies

Kartagener syndrome(AR)

Situs inversus,immobile cilia,heart defects,splenic anomalies,deafness

X

Cockayne syndrome(AR)

Retinaldegeneration,senile-likechanges, growthretardation,photosensitivity,deafness

X X

Achondroplasia (AD) Short limbs,hydrocephalus

X

Branchiootorenalsyndrome (AD)

Branchialanomalies, ear

malformations,renal anomalies

X X X

Klippel-Feil syndrome(sporadic, AD, AR)

Fused cervicalvertebrae, webbedneck, deafness,CHD

X X

Duane syndrome(sporadic, AD)

Ocular strabismus,ear anomalies,skeletal anomalies,cranial nervepalsies, deafness

X X

Marfan syndrome(AD)

Lens subluxation,arachnodactyly,aortic aneurysm,hyperextensibility,deafness

X X X

Mobius syndrome(sporadic, AD, AR)

Cranial nervepalsies, limbanomalies,hypoglossia,micrognathia,deafness

X

Muckle-Wells

syndrome (AD)

Amyloid

nephropathy,urticaria, deafness

X X

Pierre-Robinsyndrome (AR, XL)

Micrognathia, cleftpalate,glossoptosis,deafness

X

Jervell and Lange-Nielsen syndrome(AR)

Long QT, deafness X X

Neurofibromatosistype I (AD)

Neurofibromas,cafe-au-lait spots,

optic glioma

X




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Osteogenesisimperfecta (AD, AR)

Fragile bones, bluesclera

X

Ehlers-Danlos (AD,AR)

Jointhyperextensibility,fragile skin

X

AD: autosomal dominant inheritance; AR: autosomal recessive inheritance; XL: X-linked

inheritance.




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Traumatic tympanic membrane perforation

The anterior canal wall skin is on the left side; the malleus is onthe right side. The perforation appears dark and is surroundedby mildly hyperemic smooth edges. Courtesy of Adele Karen Evans,MD, FAAP and Steven D Handler MD, MBE.




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Transverse temporal bone fracture

A line of fracture is demonstrated through the cochlea (arrow).Courtesy of Peter C Weber, MD.




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Cholesteatoma

Multiple white pearls on the lateral surface of the right tympanicmembrane. Reproduced with permission from: Kim, SA, Haupert, MS.Clinical photograph: Congenital cholesteatoma of the tympanic membrane.

Otolaryngol Head Neck Surg 2002; 127:359. Copyright 2002 Elsevier

Science.




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A typical audiogram of noise-induced sensorineural hearing loss

The worst hearing level is at 4000 hz, and then it improves at 8000 hz.However, over time this shape will lessen as more and more frequencies areinvolved.




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