Challenges of Congenital Hearing Loss

Wendy Osterling

Introduction

Congenital hearing loss or deafness, occurring in 1 to 3 per 1,000 infants, is the most common human birth defect in the United States (1). An average of 33 babies are born with a hearing loss each day, which totals approximately 12,000 children annually. These statistics do not include those who develop sensorineural hearing loss later in childhood.

Normal hearing is critical for language development. Hearing loss alone does not have direct behavioral or developmental consequences (2). Growing up with a lack of hearing in a heavily auditory environment does, however, have indirect consequences on the development of language, academic performance, and psychosocial interactions. Children with hearing loss face potential life-long communication barriers. Various physical abnormalities and mental retardation, often in distinct syndromes, may accompany hearing loss.

At various stages of development, affected children and their families cope, react, and adapt differently to this disability. Like children with other chronic illnesses or disabilities, children with hearing loss have an increased risk of developing comorbid psychiatric disorders.

The diagnosis of hearing loss or deafness is heterogeneous with respect to severity, age at onset, and cause. Amplification, communication mode, and language fluency also influence the child’s development and identity (3).

Anatomy of Hearing

Under normal circumstances, hearing begins when sound waves enter the external ear, travel down the ear canal, and strike the tympanic membrane. Sound waves striking the membrane trigger bone conduction through the three ossicles—stapes, incus, and malleus—in the middle ear cavity. The sound waves vibrate the ossicles, which amplify and convert the sound energy into mechanical, conduction energy that, in turn, resonates in the fluid-filled cavities of the inner ear cochlea.

Hair cells in the cochlea then convert the mechanical, conduction energy into electrical, sensorineural energy. Each hair cell connects with its own auditory nerve receptor. The mammalian inner ear contains 15,000 to 20,000 hair cells that make up the organ of Corti. Once these hair cells are damaged, they cannot recover or regenerate.

The sensorineural activity travels through the auditory division of the eighth cranial nerve to the superior olive, inferior colliculus, and medial geniculate nucleus of the thalamus. Pathways then bring it in the form of cortical auditory evoked potentials (CAEPs)
to the auditory cortices. The primary auditory cortex, Brodmann areas 40 and 41, is located in the temporal lobe near the lateral sulcus of the temporal gyrus (the Heschl gyri). This cortical region is also adjacent to Wernicke’s area, which is the site of word comprehension, and near the Broca’s area, the site of language production. Other auditory cortices lie in the frontal and parietal lobes.

Varieties of Hearing Loss

PERIPHERAL HEARING LOSS

Peripheral hearing loss can be divided into three varieties: conductive, sensorineural, and mixed. Conductive hearing loss (CHL), the most common variety, results from interrupted sound transmission in the external ear or impaired middle ear bone conduction. Fluid in the middle ear, often from otitis media causing an effusion, is the most common cause of CHL. An effusion in the middle ear can cause a temporary 25-decibel (dB) hearing loss, which is equivalent to wearing earplugs. Other causes of CHL include atresia, stenosis, otosclerosis, middle-ear cholesteatoma, and obvious mechanical factors, such as impacted cerumen, a foreign body in the ear canal, or tympanic membrane perforation.

Children with CHL can hear speech, but the resultant distorted sound quality leads to difficulties with early language discrimination. These children are at risk for speech, language, and learning problems. They require more intervention than just watchful waiting and follow-up auditory testing. For children with an effusion, the initial treatment is generally a 10- to 14-day trial of antibiotics to combat any underlying ear infection. If that strategy fails, a consultation with otolaryngology surgery is usually solicited for evaluation for tympanostomy or pressure equilibrating (PE) tubes. Persistent middle-ear effusion during a child’s first 3 years can result in decreased scores, at 7 years of age, in tests of speech, language, and cognitive abilities (4).

Sensorineural hearing loss (SNHL), which accounts for most cases of congenital deafness, results in severe hearing loss affecting high-frequency sounds. Maldevelopment of the cochlea, from neuronal changes, synaptic abnormalities, or altered connectivity, causes most cases of SNHL. In additional to children with congenital SNHL, in two to three per 1,000 others, SNHL develops. Causes of acquired SNHL include genetic mutations, infection, ototoxic medications, noise, and anatomic anomalies.

CENTRAL HEARING LOSS

Children who have impaired ability to perceive or process sounds in the auditory cortex are said to have “central hearing loss.” In practical terms, these children have normal hearing but difficulty in a variety of auditory tasks, particularly listening selectively in noisy environments, properly combining sound input from both ears, integrating auditory information, or processing speech.

Development of Hearing

Biobehavioral research has established a “critical period” of learning and development that ends at approximately 7 years of age. The central auditory pathways have maximal plasticity during the first 3.5 years of life (5). If stimulation is delivered within this critical period, CAEP latencies reach age-normal values within 3 to 6 months. In contrast, if stimulation is withheld for longer than 7 years, CAEP latencies and plasticity decrease, and the
auditory nerves undergo atrophy. Therefore the best time for intensive therapy with auxiliary aids (hearing aids or cochlear implant) is before or during the first 3.5 years.

Lack of hearing can seriously impair cognitive organization, language development, and speech perception. Restricted development of neural connections in animal models suggests that the primary auditory cortex may be functionally decoupled from the higher-order auditory cortex in situations of less sensory input (6). However, higher-order cortex for other sensory modalities, such as vision and sensory perception, may compensate for lack of auditory stimulation. For example, deaf people tend to have heightened peripheral visual acuity. Studies have shown that they are more easily distracted by peripheral field stimuli and less by central field stimuli compared with hearing people (7). This follows the theory of “cross-modal plasticity” (changes in auditory cortex responses to visual stimuli), in which the multimodal associative cortex combines information and is more sensitive to these functioning sensory modalities (8). Studies with functional magnetic resonance imaging (fMRI) and cognitive behavioral tests show evidence of neuroplasticity in the auditory area, the superior temporal sulcus, recruiting more visual, tactile, and signed input in deaf individuals (9). This cognitive reorganization does not appear to affect intellectual function: the deaf have a distribution similar to that of hearing individuals (10,11).

Neurobiologists are currently looking for biomarkers for CAEP to determine the developmental integrity of the central auditory pathways as a way to evaluate the acoustic amplification and stimulation for normal development (5). Monitoring CAEP biomarkers should allow a study of the maturation of the central auditory pathways and help guide the decision regarding hearing aids or cochlear implant.

“Auditory-verbal advocates” support wearing hearing aids or undergoing cochlear implantation or both as soon as hearing loss is diagnosed to maximize auditory stimulation. Even marginal stimulation, these advocates state, helps preserve nerve functioning and prevents their atrophy even in prelingual deafness (hearing loss occurring before the development of language). Continued auditory stimulation is equally important to maintain the neural integrity in postlingual deafness (hearing loss occurring after the development of language).

Etiologies

Approximately 50% of cases of deafness are inherited, with 80% in an autosomal-recessive, 18% in an autosomal-dominant, and 2% in an X-linked recessive pattern. More than 100 genes are associated with hearing loss. Genetic causes of hearing loss are sometime identifiable by their accompanying distinctive physical features, such as low-set ears, light-colored hair, white forelock, hypertelorism, thin lips, and broad nasal bridge (12). When hearing loss is regularly associated with these features, it is termed a syndromic hearing loss. About 30% of the genetic causes occur as one of the 350 forms of syndromic hearing loss (Table 17.1).

Conversely, 50% to 70% of cases of genetic hearing loss occur as isolated deficits. Because these cases lack associated distinctive clinical features, they are termed nonsyndromic hearing loss. Physicians can often identify them by their pattern of inheritance, age at the onset of the hearing loss, progression, audiologic characteristics, and accompanying otologic findings, such as vestibular dysfunction.

The most common hereditary cause of hearing loss is a mutation in the gap-junction gene (GJB). More than one half of infants with nonsyndromic hearing loss will have identifiable mutations in one of two gap-junction genes, GJB2 (connexin 26) (30% to 50%) and GJB6 (connexin 30). The gap junctions are responsible for recycling potassium ions from the hair cells to the stria vascularis (one of the three fluid-filled compartments of the cochlea) and actively pumping them back into the cochlear endolymph. Sound perception
depends on the maintenance of high endocochlear potential within the cochlea, which fails in malformed gap junctions (13). Many children and adults with this hearing loss have residual hearing that can be amplified with hearing aids, which indicates the presence of a few remaining normal hair cells among the impaired gap junctions.

TABLE 17.1 FREQUENTLY OCCURRING SYNDROMES INVOLVING ASSOCIATED HEARING LOSS AND THE RELATIVE FREQUENCY

Alport syndrome: 1%
	(progressive renal failure, progressive late-onset high-frequency HL)
Alström syndrome: common in Acadians of Nova Scotia and Louisiana
	(childhood truncal obesity, progressive retinal dystrophy, progressive SNHL)
Bartter syndrome (type 4): most common in consanguineous Middle Easterners
	(polyhydramnios, metabolic acidosis)
Biotinidase deficiency: 1/60,000
	(seizures, hypotonia, ataxia, organic acidemia, SNHL)
Branchio-oto-renal (BOR) syndrome: 2%
	(HL, preauricular pits, malformed pinnae)
Fabry disease: 1/40,000
	(vascular skin lesions, abdominal pain, nephropathy, renal failure)
Jervell and Lange-Nielson syndrome: 0.25% to 0.5%
	(SNHL, prolongation of QT interval, syncope)
Nance deafness: >1%
	(congenital fixation of stapes footplate with mixed HL)
Pendred syndrome: 4% to 10%
	(SNHL, goiter, cochlear malformation)
Treacher-Collins syndrome: 1%
	(CHL, malformed ossicles, microtia, cleft palate, micrognathia)
Usher syndrome: 4% to 6%
	(SNHL, vestibular symptoms, retinitis pigmentosa)
Waardenburg syndrome: 1% to 4%
	(patches of eye, skin, hair hypopigmentation)
From Morton CC, Nance WE. Newborn hearing screening: a silent revolution. N Engl J Med 2006;354:20, 2151-2164, with permission.

With this genetic information, medical professionals have shifted from the simple detection of hearing loss to the identification of its cause (14). Genetic information offers patients and families guidelines regarding disease prevention, therapy, and interpretation of the results of early intervention. Understanding the etiology of the child’s disease offers psychological benefits and helps prepare for the child’s future.

About 50% of the infants born with SNHL have one of a group of known risk factors for neonatal hearing loss (Table 17.2), but the other 50% have none of them. Thus newborn hearing screening is as important for infants who have no risk factors as for those who do.

TABLE 17.2 RISK FACTORS FOR NEONATAL HEARING LOSS

Family history of sensorineural hearing loss

Congenital infection

Presence of craniofacial anomalies

Birth weight <1,500 g

Neonatal jaundice necessitating an exchange transfusion

Ototoxic medications (e.g., furosemide, aminoglycosides)

Bacterial meningitis

Apgar scores at 5 min of ≤3

Syndrome associated with hearing loss

From Joint Committee on Infant Hearing. Year 2000 Position Statement: principles & guidelines for early hearing detection & intervention programs. Pediatrics 2000;106:798-817, with permission.

TABLE 17.3 AMERICAN ACADEMY OF PEDIATRICS RECOMMENDS INFANTS AND CHILDREN SHOULD BE SCREENED IN THE FOLLOWING SITUATIONS

Parental concerns regarding hearing/language development

History of bacterial meningitis

Confirmed neonatal infections associated with hearing loss (e.g., CMV)

History of significant head trauma, especially with temporal bone fracture

Presence of syndrome associated with hearing loss

Exposure to ototoxic medications (e.g., gentamicin)

Presence of neurodegenerative disorder

Confirmed incidence of infectious diseases (e.g., mumps, measles)

From Cunningham M, Cox EO, the Committee on Practice and Ambulatory Medicine and the Section on Otolaryngology and Bronchoesophagology. Hearing assessment in infants and children: recommendations beyond neonatal screening. Pediatrics 2003;111:2, 436-440, with permission.

Other important environmental causes of hearing loss are prematurity, prenatal and postnatal infections, anoxia, head trauma, subarachnoid hemorrhage, and pharmacologic ototoxicity. Ototoxic drugs, aminoglycoside antibiotics and chemotherapeutic agents, particularly cisplatin and carboplatin, can cause permanent hearing loss. Ten percent of patients have mutations in mitochondrial genes that make them susceptible to aminoglycoside ototoxicity due to impaired metabolism (15).

Commonly used antidepressants and antipsychotics are not known to affect hearing, except for one report that valproate induced hearing loss and tinnitus (16). Teratogens, such as antineoplastic and antiepileptic drugs, may result in multiple fetal anomalies, including atresia of the ear canal, lobes, low-set ears, and malformation of the inner ear. The Food and Drug Administration (FDA) strongly advises pregnant women not to use category C and D medications.

In utero rubella infection was historically the most common infectious cause of congenital deafness, but, since the introduction of the rubella vaccine, the incidence of rubella deafness has decreased significantly. Currently, the majority of infectious causes of congenital deafness are in utero cytomegalovirus (CMV) and toxoplasmosis infections. In utero CMV infection causes about 10% of cases of congenital hearing loss and 35% of those with moderate-to-severe late-onset loss (17). However, because 84% of newborns with congenital CMV infection lack distinctive clinical findings, this virus infection is usually not recognized as the cause of congenital hearing loss in many children. Although the development of Haemophilus and pneumococcal vaccinations has also reduced the incidence of bacterial meningitis leading to hearing loss, bacterial meningitis still occurs and causes deafness in about 10% of cases.

In contrast to congenital hearing loss, progressive early-onset hearing loss, which is not present at birth, can result from infection, ototoxic drugs, autoimmune disease of the inner ear, enlargement of the vestibular aqueduct, injury, or other genetic mutations. The Joint Committee of Infant Hearing has identified 10 risk indicators that should prompt monitoring of hearing status, even if the results of newborn screening are normal (18) (Table 17.3).

Newborn Hearing Screening and Diagnosis

Prior to the 1990s, hearing loss was diagnosed in infants around 18 months of age. During the first few months of life, deaf babies with babbling and gestural behaviors can develop similarly to hearing babies. Their hearing loss can be subtle and difficult to diagnose in reciprocal parent-child interactions based on smiling, cooing, and visual tracking
before 6 months of age. From 6 months onward, parents may suspect a hearing problem if the child fails to respond to his or her name, imitate sounds, or produce words by 12 months of age. A hearing loss should not affect fine or gross motor development, but primarily language development, which can contribute to other psychosocial consequences. The presence of other disorders such as seizures, intellectual disability, or cerebral palsy has been associated with increased prevalence of language delay, psychosocial challenges, and adjustment problems (19).

The Colorado Newborn Screening program showed that infants who were diagnosed and received intervention within the first 6 months of life were more likely to have better language, social, and emotional development than were those diagnosed later. This observation was consistent, regardless of the mode of communication—speaking or signing (20). Early identification and early intervention influence prognosis, development, language acquisition, and educational outcome (21). Interestingly, deaf children born to deaf parents acquire native fluency in American Sign Language (ASL) and achieve the same parallel language milestones in ASL as do children with no hearing loss (22).

Hearing loss can have detrimental effects on brain development. Once the diagnosis of deafness is confirmed, physicians should look for comorbidities, including language deficits, cognitive dysfunction, learning disabilities, speech disability, mental retardation, and psychiatric disturbances. If the child has a syndromic hearing loss, neurological components may be present, such as epilepsy, tics, attention-deficit disorder, mental retardation, ataxia, and balance problems. A deaf child can appear autistic or mentally retarded simply because he or she cannot interact with others. This can result in a misdiagnosis of the deaf child as having learning disability or attention deficit disorder. If an in utero infection or toxicity caused the hearing loss, depending on the timing of embryologic development, the insult may also result in blindness, mental retardation, and delayed gross motor skills. Similarly, perinatal trauma and sepsis may cause lifelong, static motor impairment (cerebral palsy) and mental retardation, as well as deafness.

As a result of research data and the increasing pressure for early diagnosis, the National Institutes of Health Consensus Development Conference in 1993 endorsed a universal newborn hearing screening program. The American Academy of Pediatrics (AAP) now recommends universal screening of infants with the goal of screening of all infants by 3 months of age. Every state has established Early Hearing Detection and Intervention (EHDI) programs to provide audiologic screening (23,24). Today, nearly 93% of all newborn infants in the United States complete hearing screening before discharge. As result, the average age that hearing loss is confirmed has dropped to 2 to 3 months (25).

In the past, newborn hearing screening received criticism because of the high rate of false positives, which resulted in unwarranted parental anxiety. Recently, the testing performance has improved, and now the average failure rate is less than 0.5%. About half of these infants whose results are false positives actually have normal hearing. To minimize anxiety, the euphemism “refer” was adopted to characterize failed screening tests (26). Counseling the parent about abnormal screening results should avoid creating anxiety. At the same time, the approach should not belittle the screening results to ensure that the parents bring their child for repeated testing in a few weeks.

The AAP recommends using one of two screenings: the auditory brainstem response (ABR) or the otoacoustic emissions (OAE) test. The ABR test measures average neural response to repetition of a large number of sound signals of the same pitch and intensity. The OAE test detects movements of outer hair cells of the cochlea with evoked sounds. Most hospital nurseries use both methods as part of a two-stage screening protocol. The OAE is most commonly used first for screening, as it is simpler to use in the nursery, but
this test more frequently produces false-positive results. If the baby fails, then testing is repeated with the more-sensitive ABR test.

If newborn hearing screening is not done or if the child has a progressive hearing loss after initially passing the newborn screen, parents may be unaware that their infant has a hearing loss until the infant fails to reach language milestones, shows communication challenges, or fails school hearing tests. Therefore the AAP additionally recommends formal hearing screening for all children at 3, 4, and 5 years of age and then every 2 to 3 years until adolescence to identify children in whom a hearing loss may develop. In addition, children who have persistent middle ear effusions for more than two months should undergo screening. Usually the more severe or rapid onset of the hearing loss, the earlier its identification is possible.

Diagnosis of Hearing Loss

Audiologists perform a series of tests and construct an audiogram that shows the severity, sensitivity, thresholds, and frequencies of the hearing loss. These data will also allow the audiologist to recommend appropriate auxiliary aids, such as hearing aids and cochlear implants, and make the referral to early-intervention and support groups. The spectrum of hearing loss ranges through mild (25- to 45-dB loss); moderate (45- to 65-dB loss); severe (65- to 85-dB loss); profound (85- to 130-dB loss); and total (no residual hearing).

With increasing severity of hearing loss, distinguishing between the elements of spoken language becomes more difficult. Even a hearing loss of 15 dB during early childhood can impair speech perception, delay speech acquisition, and impair school performance. Children with borderline hearing loss (16 to 25 dB) may miss 10% of speech in a noisy environment, such as in a classroom, and have impaired interaction with peers. Children with a mild hearing loss may miss up to 50% of speech in a noisy classroom and, because of school and social difficulties, may lose self-esteem. Moderate hearing loss can cause significant problems with communication.

Although deaf children must undergo special testing for learning disabilities, proper assessment and methods for correction are thought to be lacking (27,28). Certain combinations of tests would be useful for education and vocational, as well as psychiatric counseling (29). Neuropsychiatric testing should be done by a trained professional who has experience and understands the deaf culture and language to avoid misunderstandings and misdiagnosis.

All options should be considered for the hearing-impaired child: hearing aids, cochlear implants, speech therapy, sign language, special education, mainstream education, and one-on-one instruction. Once diagnosed, children with hearing loss require close monitoring by an audiologist, yearly audiologic evaluations to monitor the level of hearing loss, and evaluations of auxiliary aids. Many schools require yearly evaluations for the Individual Education Program (IEP) for special education support. Although hearing will not improve with growth, deaf and hard-of-hearing children often learn to adapt. For optimal auditory-verbal development, comprehensive collaboration among the medical specialists, educators, and the family is vital.

Initial Parental Reaction

More than 95% of deaf children are born to hearing parents who have little or no knowledge of or experience with deafness (30). Most of these parents may be initially shocked
and need to grieve the loss of their “normal” child. They may feel overwhelmed by the voluminous and sometimes conflicting information about deafness, early intervention, education, communication methods, and cochlear implants. Some parents never overcome these emotional hurdles or do so later in their child’s life. Their feeling of loss may recur during their deaf child’s life, especially if they wonder about schooling, graduation, higher education, work, and marriage. To lessen the impact, parents reach out to understand, accept, and advocate for their deaf child. The needs of siblings should not be overlooked. The deaf child must receive significant attention, including speech therapy, audiology appointments, and language tutoring. This extensive and time-consuming support often has an effect on the siblings and wider family. Families with access to adequate resources and support during early intervention experience decreased stress and, therefore, lower risk for socioeconomic problems. Nevertheless, challenging situations—family, home, school, work, community, and social— will persist throughout life. These stresses do not become easier with age; however, they become more manageable as the deaf child develops strategies and learns to advocate for himself.

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