Hearing Loss in the Newborn Infant

Background

Tremendous progress has been made during the past 25 years in the identification of hearing loss (HL) in newborns. The National Institutes of Health issued a “Consensus Statement on Early Identification of Hearing Impairment in Infants and Young Children” in 1993 that concluded that all infants admitted to the neonatal intensive care unit (NICU) should be screened for HL before hospital discharge and that universal hearing screening should be implemented for all infants within the first 3 months of life. The percentage of infants screened for HL in the United States has increased from 46% in 1999 to 98.4% in 2014 ( https://www.cdc.gov/ncbddd/hearingloss/2014-data/2014_ehdi_hsfs_summary_h.pdf ). The percentage of infants who fail the screening process is about 1.6%, and rates of permanent HL subsequently diagnosed by comprehensive audiology testing range from 1-3 per 1000, making congenital HL the most common birth defect diagnosed as a result of the newborn screening process. However, a report to assess the impact of universal hearing screening in a large cohort of infants identified a prevalence of deafness by school age of 3.65/1000 compared to a neonatal rate of 1.79/1000. It is important to recognize that the neonatal screen does not identify all HL and is least sensitive to mild impairments. Undetected, HL in young infants and children negatively affects communication development, academic achievement, literacy, and social and emotional development, whereas early identification and intervention, particularly within the first 6 months of life, clearly provide benefit for communication development in infants. There is accumulating evidence that the brain may be optimally responsive to language input early in life.

Based on these findings, the Joint Committee on Infant Hearing 2007 Position Statement published the 1-3-6 recommendation to maximize the outcomes of infants with all degrees of HL : 1) All infants in the NICU and well-baby nursery should be screened for HL no later than 1 month of age. 2) Infants who do not pass the screen should have a comprehensive evaluation by an audiologist skilled in assessing infants and children, no later than 3 months of age for confirmation of hearing status. 3) Infants with confirmed HL should receive appropriate intervention no later than 6 months of age from professionals with expertise in HL 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 HL. Failure of sound transmission within the cochlea, outer and inner hair cells, and eighth cranial nerve are a manifestation of sensorineural HL, whereas pathology of the inner hair cells and eighth cranial nerve with intact outer hair cells is characteristic of neural HL, also referred to as auditory neuropathy or auditory dyssynchrony .

Hearing loss can be classified as bilateral or unilateral and as slight, mild, moderate, severe, or profound. Hearing loss severity is defined by measuring the hearing threshold in decibels (dB) across frequencies ( Box 59.1 ). Normal hearing has a threshold of 10-15 dB. It is important to recognize that the presence of even slight and mild HL can impact on language development in young children and may be progressive. For children with bilateral HL, the severity of loss is based on the better-functioning ear.

The types of transient and permanent HL that can be identified at birth with newborn screening include sensorineural, neural, and conductive ( Table 59.1 ). Transient conductive HL may also be present, especially in infants who have been hospitalized in a NICU. Mixed HL is a combination of permanent HL and transient conductive HL.

Neonatal hearing screening programs in the United States are conducted under the guidance of an audiologist. An audiologist, experienced in assessing infants and young children, is responsible for completing the comprehensive diagnostic assessment needed to confirm the diagnosis of HL. Increasing numbers of NICUs are completing the diagnostic assessment prior to discharge to avoid a delay in the diagnosis for the most medically high-risk infants.

Tests for Hearing Loss

Screening and diagnostic hearing tests are shown in Table 59.2 . 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 the infant is asleep or quiet awake. 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 HL. A false-positive fail screen for permanent HL may result from outer or middle ear dysfunction, including the presence of a transient conductive HL (fluid or debris) or noise interference. Otoacoustic emissions cannot be used to screen for neural HL, because pathology in this disorder involves the inner hair cells, eighth cranial nerve, and brainstem with intact outer hair cells. Infants with neural HL 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 HL and, therefore, should be screened only with AABR. Some hospitals use a two-step screen with both AABR and OAE. Screen time with OAE is quicker and more cost effective, and OAE is, therefore, considered an acceptable screen in the well-baby nursery. OAE will not identify auditory neuropathy and, therefore, is not recommended for screening in the NICU.

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 2.5 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. 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 surveillance and 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. However, NICU infants who fail the newborn screen should never be screened in the medical home but should be referred to an audiologist. In addition, children who do not pass the OAE screen in the medical home need to be referred to audiology for further diagnostic testing.

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 HL. Before universal hearing screening, children with severe to profound HL were identified at 24-30 months of age and subsequently demonstrated significant delays in communication, language, and literacy. The Colorado study first reported that children with HL 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. A second report demonstrated that at 12-16 months, children with HL 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. The Joint Committee on Infant Hearing 2007 recommends that infants with all degrees of unilateral or bilateral HL 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 HL, including educators of the deaf, speech-language pathologists, and audiologists. The 2013 Supplement to JCIH 2007 Position statement provides comprehensive guidelines for early intervention after confirmation that the child is deaf or hard of hearing. The 12 best practice goals and guidelines recommended provide an evidence-based framework for family-centered culturally competent, individualized early intervention services to meet the diverse needs of children and families regardless of type and degree of HL and modality of communication ( Table 59.3 ).

Etiology of Hearing Loss

It is estimated that at least 50% of congenital HL is hereditary. Nearly 400 syndromes and hundreds of genes associated with HL have been identified. Genetic HL is about 30% syndromic and 70% nonsyndromic. Among children with nonsyndromic HL, 75%-85% of cases are autosomal recessive ( DFNB, deafness , neurosensory , autosomal recessive ), 15%-24% are autosomal dominant ( DFNA , deafness , neurosensory , autosomal dominant ), and 1%-2% are X-linked ( DFN ). Therefore, most infants with HL have nonsyndromic autosomal recessive HL 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 HL. 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 HL will be reviewed relative to the risk factors for HL published by the Joint Committee on Infant Hearing 2007 ( Box 59.2 ).

Several mitochondrial DNA mutations of the 12S rRNA gene are associated with aminoglycoside-induced nonsyndromic HL. This is potentially important for NICU infants, because aminoglycosides are one of the most common medications administered in the NICU. Two studies have examined the frequency of these genes in NICU populations and identified a rate of ≈1%-1.8%. Neither study, however, identified an association in neonates between the presence of the genetic marker in conjunction with aminoglycoside administration and HL. Neonatal genetic screening with rapid turnaround is not available. A concern with the current reports, however, is that mitochondrial HL has variable age of onset and may not be identified in the newborn period. In addition, it has been suggested that there may be a modifier gene effect that is protective.