eMedicine Specialties > Pediatrics: Surgery > Otolaryngology

Hearing Impairment

Author: Rahul K Shah, MD, FACS, FAAP, Assistant Professor of Otolaryngology and Pediatrics, George Washington University School of Medicine and Health Sciences; Attending Physician, Department of Otolaryngology, Children's National Medical Center
Coauthor(s): Michael Lotke, MD, Pediatric Residency Program Director, Mount Sinai Hospital; Assistant Professor, Department of Pediatrics, Rosalind Franklin University of Medicine and Science
Contributor Information and Disclosures

Updated: Jul 21, 2008

Introduction

Background

Hearing loss is more prevalent than diabetes mellitus, myelomeningocele, all pediatric cancers, and numerous other medical conditions. However, medical professionals learn little about hearing impairment, about how to advise parents of children who are deaf or hard of hearing, or about the special considerations needed in the care of children with hearing loss.

Recommendations for universal neonatal hearing screening have resulted in numerous articles regarding the tests, the efficacy of testing, the role of the audiologist in amplification, and the importance of early intervention programs. The importance of the otolaryngologist treating the patient along with the primary care physician cannot be overemphasized. In many instances, the otolaryngologist develops a long-term relationship with patients and their families, caring for the patients through their spectrum of development.

Pediatricians play a crucial role in providing referrals to audiologists, otolaryngologists, and special programs. To do so, they must understand the nature of hearing loss and the equipment that can improve auditory reception, the linguistic and social development of children who have hearing impairment, and the educational and linguistic options available to children who are deaf or hard of hearing.

The goals must always be to integrate the child into the family and into society and to enable the growth and development of a healthy, confident child who is deaf or hard of hearing. To meet these goals, clinicians should use any communication strategy and equipment that is best suited for the individual child and his or her family.

Pathophysiology

Relevant anatomy and physiology

Sound waves arrive to the auricle and are channeled through the external auditory canal to the tympanic membrane. When they strike the tympanic membrane, the waves cause it to vibrate, setting off a chain of vibrations along the ossicles (malleus, incus, and stapes) to the membrane of the oval window at the entrance to the cochlea. This process amplifies the environment sound by approximately 20-fold.

The cochlea is the end organ of hearing and is shaped like a snail shell with 2.5 turns. Inside, 2 membranes longitudinally divide the cochlea into 3 sections: the scala tympani, the scala vestibuli, and the scala media. All 3 are filled with fluid of various ion concentrations (similar to intracellular and extracellular constituents).

Along one of the membranes in the scala media, or cochlear duct, lie the internal and external hair cells. Movement of the stapes on the oval window creates a wave or vibration in the perilymph fluid of the cochlea. This fluid movement, which opens ion channels in the hair cells, displaces the hair cells, triggering an action potential and causing a nerve in the cochlea to fire to the brain.

Thousands of nerves representing more than 20,000 frequencies are located along the length of the cochlea; these nerves account for the hearing range. The microscopic nerves culminate in the cochlear portion of the eighth cranial nerve. The location of the vibration in the cochlea is correlated with the frequency of the original pitch. Low-frequency sounds are near the apex, and high-frequency sounds are near the base.

Types of hearing loss

Conductive hearing loss (CHL) results from anything that prevents the transmission of sound from the outside world to the cochlea. Causes range from impaction of cerumen to middle-ear effusions or dysfunction or fixation of the ossicular chain. Otosclerosis is one of the most common examples.

An important cause of CHL is a cholesteatoma. Another cause is a locally destructive but benign growth. Other neoplasms can affect the middle ear as well. Examples include glomus tympanicum or glomus jugulare, schwannomas of the facial nerve, and hemangiomas. Dehiscence of the roof of the middle ear (tegmen mastoideum), such as is caused by an encephalocele, can result in CHL. In CHL, sounds perceived by the brain are diminished but are generally not distorted.

Sensorineural hearing loss (SNHL) may result from disruptions in transmission after the cochlea. These disruptions may be a result of hair cell destruction in the cochlea or damage to the eighth cranial nerve. Sounds perceived by the brain are both diminished and distorted. The degree of distortion is independent of the degree of hearing loss (eg, mild hearing loss but very poor speech discrimination is possible).

Auditory dyssynchrony should be considered in the setting of no auditory brainstem response (ABR), no middle-ear muscle response, normal otoacoustic emissions, or normal cochlear microphonics.

Mixed hearing loss has components of both CHL and SNHL.

Categories of hearing loss

Regardless of the type, the American National Standards Institute defines hearing loss in terms of decibels (dB) lost, as follows:

• Slight hearing loss - 16-25 dB lost
• Mild hearing loss - 26-40 dB lost
• Moderate hearing loss - 41-55 dB lost
• Severe hearing loss - 71-90 dB lost
• Profound - More than 90 dB lost

Frequency

United States

Hearing loss occurs in 10 per 1000 children in the United States. Roughly 1 in 1000 has profound hearing loss, and 3-5 per 1000 have mild-to-moderate hearing loss that may affect language acquisition unless hearing, language, or both are aided. Acquired hearing loss may add 10-20% to these numbers.

Data from the United States Census show that almost 3% of the population in the workforce reports having some hearing loss, including CHL, SNHL, or mixed loss.

The prevalence of hearing loss requiring intervention among graduates from neonatal intensive care unit (NICU) is 1-4%.

International

SNHL occurs in 9-27 per 1000 children worldwide.

Sex

No sex predilection is known. Some hereditary causes of deafness or acquired deafness may occur more frequently in one sex than the other. However, the overall prevalence of deafness is equal in male and female individuals.

Age

Most hearing loss in children is congenital or acquired perinatally. However, hearing loss may occur at any age. Approximately 10-20% of all cases of deafness are acquired postnatally, although some genetic causes of deafness result in hearing loss that begins during childhood or adolescence.

Clinical

History

General presentation and age at detection

The presentation of patients with hearing loss depends on its degree, the patient's age when the hearing loss begins, the threshold of suspicion by parents' and healthcare providers, and the presence of other identifiable risk factors.

Before neonatal hearing screening was routine, deafness was diagnosed at the mean age of 2.5 years. This age recently improved to a mean of 14 months. The goal is universal screening, in accordance with the Newborn and Infant Hearing Loss: Detection and Intervention policy statement from the American Academy of Pediatrics (AAP).¹

In almost two thirds of patients, parents are the first to suspect hearing loss. Pediatricians detect roughly 10% of cases, and other healthcare providers suspect it first in approximately 15% of patients. The mean time between the first suspicion of hearing loss and its diagnosis is 9 months.

High-risk criteria for hearing loss in neonates and infants

The dissemination of high-risk criteria for neonates and infants in 1990 did not notably alter the mean age at diagnosis. About 50% of children with SNHL do not meet any of the criteria listed, and only 10% of neonates have 1 or more of the high-risk criteria that prompt an evaluation. These rates are among the reasons cited for the need for universal neonatal hearing screening. The goals of such screening are to identify children who are deaf or hard of hearing and to start intervention by age 6 months.

High-risk criteria for neonates (birth to 28 d) are as follows:

• Family history of congenital or early SNHL
• Congenital infection known to be associated with SNHL
• Craniofacial anomalies
• Birth weight of more than 1500 g (<3.3 lb)
• Hyperbilirubinemia over the exchange level
• Exposure to ototoxic medications
• Bacterial meningitis
• Low Apgar scores at birth
• Prolonged mechanical ventilation
• Findings of a syndrome associated with SNHL

High-risk criteria for infants (29 d to 2 y) are as follows:

• Concern about hearing, speech, language, and/or developmental delay
• Bacterial meningitis
• Neonatal risk factors associated with SNHL
• Head trauma, especially with fracture of the temporal bone
• Findings of a syndrome associated with SNHL
• Exposure to ototoxic medications
• Neurodegenerative disorders
• Infectious diseases associated with SNHL

Indications for a hearing evaluation and findings

Concern about a child's hearing loss expressed by a parent or caregiver should prompt immediate evaluation. Hearing loss can now be diagnosed as soon as it is suspected. Delaying the diagnosis in the belief that a child is too young to be tested is unjustified. Little justification supports delaying the diagnosis while language delay or behavioral problems are ruled out, especially because hearing loss may be the cause of these problems.

Children with congenital or perinatally acquired profound SNHL (>90 dB) may present with loss of cooing by the age of 6-9 months and with frank language delay. Those with hearing loss less severe than this may present with minor speech impediments, language delay, behavioral problems, or school failure. The degree of hearing loss or loss of speech discrimination is correlated with the patient's speech and language problems.

Behavioral problems may be major or minor, and they are probably best correlated with the child's personality and with how the parents deal with what they may consider a stubborn child. Before speech impediments, mental retardation, autism, attention deficit, or adjustment disorders are presumed, many children could benefit from a hearing evaluation. 

Upon evaluation, a child who cannot normally hear speech can reproduce only what he or she hears. A child who cannot hear the teacher cannot learn. In addition, a child who is bored fidgets. A child who cannot understand and who is berated by caregivers for lack of cooperation may act out.

Healthcare providers must remember that normal speech volumes are 30-50 dB, whereas typical street traffic is about 60 dB. Standard phones ring and shouts register at approximately 80 dB, and lawnmowers are approximately 90 dB. Therefore, children with a 60-dB hearing loss hear their mother when she yells at them and startle when the telephone rings. However, they do not hear most conversational speech, and they may not always hear the teacher in a classroom.

If a child has poor speech discrimination, the diagnosis is usually made relatively early. If a child has good speech discrimination, hearing loss may go undetected.

Children with mild hearing losses that begin prelingually or postlingually may present with difficulties late in childhood.

In general, children who lose their hearing postlingually have a decline in language skills they previously achieved. In children of this age group, clinically significant hearing loss is usually the result of an obvious medical event. Those who lose their hearing postlingually may be able to describe their loss. However, if the decline is gradual, they may not recognize the deficit.

Worsening speech or school performance may herald long-standing or progressive mild-to-moderate hearing loss.

Physical

Carefully evaluate children with congenital hearing loss for any evidence of other physical stigmata or for possible associated syndromes (ie, a white forelock to suggest Waardenburg syndrome). For a partial list of syndromes related to deafness, see Causes. Because almost every organ system can potentially provide evidence of an associated syndrome, detailed physical examination is necessary.

For patients with acquired hearing loss an otolaryngologist should evaluate their ears by inspecting them for external defects or for obstructions that block sound conduction down the ear canal (eg, cerumen, foreign bodies). In addition, pneumatic otoscopy should be performed to detect any evidence of current or chronic infections, such as perforation or scarring of the tympanic membrane, cholesteatoma, abnormal landmarks, and fluid behind the tympanic membrane.

The remainder of the otolaryngology examination should be focused on the head and neck to carefully rule out other abnormalities that may lead to a diagnosis.

If children are old enough to cooperate, they should undergo tests of balance because dysfunction of the inner ear or vestibular nerve may also be present.

Causes

Genetic causes

Genetic causes account for 30-50% of hearing losses and can be divided into syndromic and nonsyndromic types. As with all genetic syndromes, genetic causes of hearing loss may be autosomal dominant (AD), autosomal recessive (AR), X-linked, or sporadic.

Nonsyndromic deafness accounts for slightly more than half of all cases of genetic deafness. It probably accounts for most cases classified as unknown. Children with nonsyndromic deafness are deaf or hard of hearing; however, they have no other physical abnormalities, no particular risk to other organ systems, and no increased risk of mental deficiency. Some children have a history of deafness in a close or distant family member. Others have new mutations or an AR gene with no known proband. The histories of subsequent siblings and progeny may help to distinguish a genetic cause from developmental arrest or a prenatal insult.

Exciting developments in genetic mapping have revealed approximately 2 dozen abnormal genes that lead to deafness. In some, the molecular or structural defect has been identified (eg, collagen in the basilar membrane, structural defect in a membrane-gating protein). These genes have been classified according to their mode of inheritance: ADs (DFNA1 through DFNA11), ARs(DFNB1 through DFNB12), X-linked recessives (DFN1 through DFN6), or mitochondrial (12Sr RNA and tRNA-Ser UCN). Some of the genes exhibit variable penetrance.

Syndromic deafness accounts for the other cases of genetic deafness. Some syndromes have a particular inheritance pattern (eg, AD for Gernet syndrome, AR for Winter syndrome, X linked for Rosenberg syndrome). Others are sporadic (eg, cat-eye syndrome, Turner syndrome, or Klinefelter syndrome). Physical findings usually indicate the presence of  a syndrome; however, children with some syndromes develop the associated physical findings late in childhood. Other children present with either deafness or the sequelae of a biochemical or metabolic derangement. As noted above, syndromes may affect any single organ or several organ systems.

Syndromic associations

A small sample of syndromes associated with deafness is summarized below. A few may be familiar, although many are not; most are fairly uncommon. For many of these syndromes, good data about their actual prevalences are difficult to find.

The first few syndromes listed for each each organ or system are most commonly known. They may be most widely recognized because their associated findings or illnesses may result in high morbidity or mortality rates, because the physical stigmata are classic and therefore make the syndrome easily identifiable, or because they are overrepresented in test questions on pediatric examinations.

Some Syndromes Associated with Deafness

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Prenatal causes

Prenatal causes lead to 5-10% of hearing losses. Congenital infections (eg, cytomegaloviral [CMV] infections, herpes, rubella, syphilis, toxoplasmosis, varicella) can result in SNHL.² Fetal exposure to teratogens (eg, alcohol, cocaine, methyl mercury, thalidomide) may also result in SNHL. All of these perinatal insults result in physical abnormalities, which should prompt the clinician to recognize the diagnosis and perform a confirmatory evaluation. Even if these children pass the neonatal screen, careful follow-up of their hearing is necessary

Perinatal causes

Perinatal causes are responsible for 5-15% of hearing losses. A history of prematurity, low birth weight, anoxia and/or low Apgar scores, hyperbilirubinemia, or sepsis should prompt an evaluation of hearing because these conditions may also result in SNHL.

Postnatal causes

About 10-20% of hearing loses are due to postnatal causes. Childhood infections, such as meningitis or mumps, may result in SNHL. Treatment with ototoxic medications, such as aminoglycosides or furosemide, can lead to SNHL. Otitis media or major head injury may cause SNHL or CHL.

Unknown causes

About 20-30% of deaf children have no certain etiology. Their hearing losses likely result from a maldevelopment of the ear or neurologic system. Such an event may have been a developmental accident or the result of an undiagnosed infectious or exposure to a teratogenic agent. However, many are likely due to previously undiagnosed genetic defects that may represent new mutations or a genetic recessive trait.

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