Background

Tremendous progress has been made during the past 20 years in the identification of hearing loss in newborns. The National Institutes of Health issued a “Consensus Statement on Early Identification of Hearing Impairment in Infants and Young Children in 1993.”²⁹ The statement concluded that all infants admitted to the neonatal intensive care unit (NICU) should be screened for hearing loss before hospital discharge and that universal hearing screening should be implemented for all infants within the first 3 months of life. At the time of the National Institutes of Health consensus statement, only 11 hospitals in the United States were screening more than 90% of their newborn infants. In the United States, the percentage of infants screened for hearing loss increased significantly from 46% in 1999 to 97.8% in 2009.²⁹ The percentage of infants who fail the screening process is about 1.6%,¹ and rates of permanent hearing loss subsequently diagnosed by comprehensive audiology testing range from 1 to 3 per 1000, making congenital hearing loss the most common birth defect diagnosed as a result of the newborn screening process.²⁹ Undetected, hearing loss negatively affects communication development, academic achievement, literacy, and social and emotional development,^36,37 whereas early identification and intervention, particularly within the first 6 months of life, clearly provide benefit for communication development in infants.^{40,59–61,65} There is accumulating evidence that the brain may be optimally responsive to language input early in life.^{13,25,53–55}

Based on these findings, the Joint Committee on Infant Hearing 2000 and 2007 Position Statements2,5 published the 1-3-6 recommendation to maximize the outcomes of infants with all degrees of hearing loss: All infants should be screened for hearing loss no later than 1 month of age, and infants who do not pass the screen should have a comprehensive evaluation by an audiologist no later than 3 months of age for confirmation of hearing status. Infants with confirmed hearing loss should receive appropriate intervention no later than 6 months of age from professionals with expertise in hearing loss and deafness in infants and young children.

Normal Hearing and Hearing Loss

The ear consists of outer, middle, and inner components. The external ear includes the pinna and the outer ear canal. Sound waves travel through the air and are conducted through the outer ear canal to the tympanic membrane, where vibrations enter the middle ear and are amplified and transmitted through the ossicles to the fluid within the cochlea (inner ear). Sound waves in the inner ear are transmitted through the fluid and stimulate both the outer and inner hair cells of the cochlea. The outer hair cells respond to sound energy by producing an echo of sounds called otoacoustic emissions, and the inner hair cells act by converting mechanical energy into electrical energy transmitted to the cochlear branch of the eighth cranial nerve, the brainstem, and finally the auditory cortex for perception of the meaning of sounds. In normal hearing individuals, all components of the pathway are intact and functioning. Blockage of sound conduction in the outer or middle ear may result in either a transient (fluid or debris) or permanent (anatomic abnormality such as atresia or microtia) conductive hearing loss. Failure of sound transmission within the cochlea, outer and inner hair cells, and eighth cranial nerve are a manifestation of sensorineural hearing loss, whereas pathology of the inner hair cells and eighth cranial nerve with intact outer hair cells is characteristic of neural hearing loss, also referred to as auditory neuropathy or auditory dyssynchrony.12,49

Hearing loss can be classified as bilateral or unilateral and as mild, moderate, severe, or profound. The types of hearing loss that can be identified at birth are shown in Table 66-1. Types of permanent hearing loss that can be identified with newborn screening include sensorineural, neural, and conductive. Transient conductive hearing loss may also be present, especially in infants who have been hospitalized in a NICU. Mixed hearing loss is a combination of permanent hearing loss and transient conductive hearing loss.

Tests for Hearing Loss

Screening and hearing diagnostic tests are shown in Table 66-2. Physiologic tests include those that measure electrical activity or reflexes and include otoacoustic emissions (OAEs) and auditory brainstem response (ABR) testing. These tests do not require an active response from the infant and can be performed when asleep. Otoacoustic emission screen measurements are obtained using a sensitive microphone within a probe inserted into the ear canal that records the sound produced by the outer hair cells of a normal cochlea in response to a sound stimulus. Abnormal outer and middle ear function caused by blockage or background noise may interfere with recording OAEs. Automated auditory brainstem response (AABR) for screening and ABR for diagnostic testing are obtained from surface electrodes that record neural activity in the cochlea, outer and inner hair cells, auditory nerve, and brainstem in response to a click stimulus. In AABR, a predetermined algorithm provides an automated pass-or-fail response to the presence or absence of wave 5 on the ABR. Both OAE and ABR detect sensorineural and conductive hearing loss. A false-positive fail screen for permanent hearing loss may result from outer or middle ear dysfunction, including the presence of a transient conductive hearing loss (fluid or debris) or noise interference. Otoacoustic emissions cannot be used to screen for neural hearing loss because pathology in this disorder involves the inner hair cells, eighth cranial nerve, and brainstem with intact outer hair cells. Infants with neural hearing loss will therefore fail ABR but pass OAE. The Joint Committee on Infant Hearing 2007 states that infants cared for in the NICU for greater than 5 days are at highest risk for neural hearing loss and, therefore, should be screened only with AABR.⁴ Some hospitals use a two-step screen with both AABR and OAE.

Tympanometry (immittance) testing is used to assess the peripheral auditory system, including the function, intactness, and mobility of the tympanic membrane, the pressure in the middle ear, and the mobility of the middle ear ossicles. A probe is placed in the inner ear, and air pressure is changed to assess the movement of the tympanic membrane. The tympanogram shows the response of the tympanic membrane in response to the pressure stimulus: A type A curve is considered a normal response. A completely flat response may be reflective of fluid in the middle ear or perforation of the tympanic membrane. Tympanometry is not used for screening.

Behavioral tests include vision reinforcement audiometry (VRA), which is appropriate for rested alert infants with a developmental age of at least 6 months. The infant must have the functional capability of turning to sounds. For administration of VRA, the infant sits on the mother’s lap in a sound booth, earphones are inserted, and the infant is conditioned to turn to sounds that are paired to animated toys that appear either to the right or left side. Traditional behavioral testing is used for toddlers at least years of age. Children respond by placing a block in a box each time they hear a sound.

For confirmation of an infant’s hearing status, a test battery is required to cross-check results of both the physiologic measures and the behavioral measures.³⁰ The purposes of the audiologic test battery are to assess the integrity of the auditory system, estimate hearing sensitivity across the frequency range, and determine the type of loss. Infants who fail a newborn screen should have a diagnostic assessment as soon as possible after the newborn screen and not later than 3 months of age.

Primary care physicians and health centers are beginning to implement routine hearing screening of children with OAE and tympanometry during well-child visits. This would appear to be an important adjunct to newborn screening because of the known rate of late onset by school age, which is equivalent to the rate identified in newborn screening. In one report⁸ of infant/toddler screening in health centers, 1965 OAE screens were completed. Among those, 205 (10%) failed the OAE and 102 (50%) of those had a tympanogram indicating middle ear fluid. There were 45 who raised concerns because of the history and findings regarding hearing loss and 5 were confirmed with a sensorineural hearing loss. With appropriate training, protocols, and equipment, effective screening can be conducted in a primary care setting.

Early Intervention Services

There is a body of evidence supporting the importance of early enrollment in early intervention services to improve the outcomes of children with hearing loss. Before universal hearing screening, children with severe to profound hearing loss were identified at 24 to 30 months of age and subsequently demonstrated significant delays in communication, language, and literacy.36,37 The Colorado study first reported that children with hearing loss who received intervention services before 6 months of age had speaking, sign, or total communication language scores comparable with hearing children at 3 years of age.⁶⁴ A second report demonstrated that at 12 months, children with hearing loss who were enrolled in early intervention at 3 months or younger had significantly higher scores for number of words understood, words produced, early gestures, later gestures, and total gestures compared with children enrolled after 3 months of age.⁵⁸ The Joint Committee on Infant Hearing 2007 recommends that infants with all degrees of unilateral or bilateral hearing loss need to be referred to Early Intervention Services at the time of diagnosis and receive services no later than 6 months of age. These services should be provided by professionals who have expertise in hearing loss, including educators of the deaf, speech-language pathologists, and audiologists.

Etiology of Hearing Loss

It is estimated that at least 50% of congenital hearing loss is hereditary. Nearly 400 syndromes and hundreds of genes associated with hearing loss have been identified.27,42,43,51 Genetic hearing loss is about 30% syndromic and 70% nonsyndromic. Among children with nonsyndromic hearing loss, 75% to 85% of cases are autosomal recessive (DFNB, deafness, neurosensory, autosomal recessive), 15% to 24% are autosomal dominant (DFNA, deafness, neurosensory, autosomal dominant), and 1% to 2% are X-linked (DFN). Therefore, most infants with hearing loss have nonsyndromic autosomal recessive hearing loss and are born to hearing parents. A single gene, GJB2, which encodes connexin 26, a gap-junction protein expressed in the connective tissues of the cochlea, accounts for up to 50% of all cases of profound nonsyndromic hearing loss. More than 100 mutations of GJB2 have been identified. A single GJB2 mutation, 35delG, accounts for up to 70% of the mutations. The etiology of hearing loss will be reviewed relative to the risk factors for hearing loss published by the Joint Committee on Infant Hearing 2007 (Box 66-1).

Box 66-1   Risk Factors Associated with Permanent Congenital, Delayed Onset, or Progressive Hearing Loss

1. Caregiver concerns* regarding hearing, speech, language, or developmental delay

2. Family history of permanent hearing loss*

3. Neonatal intensive care for greater than 5 days, including any of the following (regardless of length of stay):

• Extracorporeal membrane oxygenation*

• Assisted ventilation

• Exposure to ototoxic medications (gentamicin and tobramycin) or loop diuretics (furosemide, lasix)

• Hyperbilirubinemia requiring exchange transfusion

4. In utero infections, such as cytomegalovirus,* herpesvirus, rubella, syphilis, and toxoplasmosis

5. Craniofacial anomalies, including those that involve the pinna, ear canal, ear tags, ear pits, and temporal bone anomalies

6. Physical findings, such as white forelock, that are associated with syndromes known to include a sensorineural or permanent conductive hearing loss

7. Syndromes associated with hearing loss or progressive or late-onset hearing loss,* such as neurofibromatosis, osteopetrosis, and Usher syndrome. Other frequently identified syndromes include Waardenburg, Alport, Pendred, and Jervell and Lange-Nielsen syndromes.

8. Neurodegenerative disorders* such as Hunter syndrome or sensory motor neuropathies such as Friedreich ataxia or Charcot-Marie-Tooth disease

9. Culture-positive postnatal infections* associated with sensorineural hearing loss, including confirmed bacterial and viral (especially herpesviruses and varicella) meningitis

10. Head trauma, especially basal skull or temporal bone fracture that requires hospitalization

11. Chemotherapy*

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