Hearing Loss Assessment in Children

Anatomy and Physiology

The experience of hearing requires structural and neural processes to work in coordination with each other to transmit a mechanical sound wave into an electrical stimulus that the brain can interpret. Due to the differences in the surface area of the tympanic membrane and the stapes, and through the processes of ossicular coupling and acoustic coupling, sound pressure through the middle ear space is amplified such that the mechanical transmission of sound at the stapes is 82.5 times greater than at the tympanic membrane.[12]

The inner ear portion responsible for hearing is the cochlea, which is divided into the scala vestibuli superiorly, the scala tympani inferiorly, and the scala media between the two. The scala vestibuli and scala media are separated by the Reissner membrane, and the scala media is separated from the scala tympani by the basilar membrane. The scala vestibuli and scala tympani contain perilymph, which closely resembles extracellular fluid, while the scala media contains endolymph, which resembles intracellular fluid. Within the scala media is the organ of Corti, which contains inner and outer hair cells as well as the tectorial membrane.

Transmission of mechanical energy into the middle ear via movement of the stapes footplate causes fluid pressure shifts to occur, which causes the tectorial membrane to contact and depolarize tonotopically-arranged hair cells. Low frequency sounds preferentially stimulate the apex of the cochlea, while high frequency sounds stimulate the base. The hair cells then generate an electrochemical signal that is transmitted to the brain with the pathway progressing to the auditory nerve, cochlear nucleus, lateral lemniscus, inferior colliculus, superior olivary complex, medial geniculate body, and terminating at the primary auditory cortex on the superior surface of the temporal lobe.

The two primary testing methods used in the newborn hearing screening are otoacoustic emissions (OAE) and automated auditory brainstem response (AABR). Otoacoustic emissions result from detecting cochlear outer hair cells moving in response to a sound stimulus from the environment, which is recorded by a probe placed in the patient’s ear. While there are multiple types of OAEs, transient evoked OAE (TEOAE) and the distortion product OAE (DPOAE) are the most common tests used in newborn screening. TEOAE can generally be detected by 30 weeks gestation and involve using a broadband signal to assess cochlear function. At the same time, DPOAE is generally more frequency-specific but does not perform as well as TEOAE below 4 kHz. 

The presence of an OAE is commonly used to reflect an entire auditory system and is designated as a “PASS” in newborn hearing screening. In contrast, the absence of an OAE is designated as a “REFER.”[13] An absent OAE generally corresponds to either middle ear pathology or a sensorineural hearing loss (SNHL) of at least 40 decibels (dB). The presence of OAEs does not rule out hearing loss, as the OAE is simply testing the mechanical function of the cochlea.

AABR is used to test the electrical function of the auditory pathway. This test utilizes electrodes placed on the scalp to detect electrical activity along the transmission pathway, with results seen through five waves that indicate a different portion of the auditory pathway. Wave I corresponds to the auditory nerve (cranial nerve VIII), wave II to the cochlear nucleus, wave III to the superior olivary complex, wave IV to the lateral lemniscus, and wave V to the inferior colliculus. The decreased amplitude or increased latency in a specific wave can indicate the severity of hearing loss and the location of an abnormality when one is present. Up to 10% of patients with a dyssynchronous ABR or an ABR indicating severe to profound hearing loss have been diagnosed with auditory neuropathy spectrum disorder (ANSD). 

In ANSD, the sound is appropriately transmitted to the inner ear, and a neural signal is generated, but there is abnormal conduction of the neural signal to the brain. Hence, diagnostic criteria for ANSD include normal OAEs, but absent or severely abnormal ABR with maximal stimulus.[14] While ANSD can be unilateral, it is bilateral in the vast majority of cases.[15] Acoustic reflexes will be absent bilaterally in most cases of ANSD, although this is not a universal finding and is not part of the diagnostic criteria.

Indications

All newborns should be screened for hearing loss before being discharged from the hospital. Those who pass initial screening should follow up as needed in their primary care medical home. Those who do not pass should be re-tested before discharge, with repeat testing occurring at least several hours after the initial screening.[7]

In the well-baby nursery, this testing is often performed using OAEs. Newborns with risk factors for hearing loss should receive screening with AABR testing due to the higher risk of ANSD in these patients. Significant risk factors for hearing loss in children include care in the neonatal intensive care unit (NICU) for five days or longer, hyperbilirubinemia requiring exchange transfusion, culture-proven sepsis, and administration of ototoxic medications.[3]

Newborns cared for in the NICU have been shown to have 6.9 times higher risk of hearing loss than newborns who do not require NICU care. Additional risk factors include in utero infection, neonatal infection, and craniofacial abnormalities or syndromic conditions.[11] The risk of ANSD in healthy newborns is low enough that ABR is not a requirement for those who pass initial OAE screening.

In addition to newborn testing, children should be referred for further audiologic testing when a parent/caregiver, teacher, or other medical professional has concerns regarding the hearing. Research has demonstrated that parents often underestimate hearing loss in their children, though a 61% detection rate has been reported for parents or guardians who suspect hearing loss.[16] Detection rates when speech pathologists or school teachers suspected hearing loss are reported as 36% and 18%, respectively.

Contraindications

There are no contraindications to hearing screening in newborns and children. Patients with congenital aural atresia or other visible ear deformities may not need to undergo screening while in the newborn nursery, as it would be expected to be abnormal, but should always have a referral placed immediately for comprehensive audiology evaluation to guide the next steps.[7]

Equipment

Standard audiometry equipment should be available for OAE, ABR, tympanometry, and comprehensive age-appropriate audiometric equipment. Examples of age-appropriate testing will be described below in further detail. Visual reinforcement audiometry (VRA) is most appropriate for children under age two who can sit upright unassisted and demonstrate good head and neck control to turn the head toward a sound stimulus. This most often occurs at approximately six months of corrected age.

From age two until age 5, or until the child can more meaningfully participate in an evaluation, conditioned play audiometry (CPA) is the recommended evaluation approach. Standard pure-tone audiometry (PTA) is utilized after age 5, or sooner if the child can participate. Acoustic reflex testing is often employed in evaluating adults and older children, although it is not frequently used to evaluate newborns and young children.

Personnel

The assessment and care of children with hearing loss are best accomplished through a multi-disciplinary team. They should include an audiologist skilled in the audiometric techniques used in children, an otolaryngologist, and a pediatrician. A geneticist should be consulted in cases where ANSD or a hereditary etiology is suspected, as 50% of cases of ANSD have been shown to have a genetic component.[14] 

Additionally, patients with known genetic abnormalities such as sickle cell disease should undergo comprehensive audiometric testing, as these patients are known to have an elevated risk of hearing loss.[17] Patients who present with non-syndromic SNHL have a 2 to 3 times higher risk of having ocular abnormalities, and hence ophthalmology should also be consulted in these patients.[18]

Preparation

Children must be able to participate in the planned testing method, or results will not be valid. Cooperation can be optimized by ensuring that older children are well-fed, well-rested and that hygiene such as diaper changes are performed just before testing where applicable. Newborns should not be fed or rested before arriving for evaluation, especially for a non-sedated ABR; at the time of assessment, monitors will be connected to the patient, after which point the patient will be fed and allowed to sleep during the examination. This maximizes the chance of appropriate behavior and accurate results from testing.

Technique or Treatment

Several techniques are available to screen for hearing loss. Newborn screening is typically via OAE and/or ABR, with further diagnostic testing including tympanometry, behavioral, and pure-tone audiometry as needed. The role of imaging and genetic testing in the evaluation of pediatric hearing loss will also be briefly discussed below.

Otoacoustic Emissions

A probe is placed within the child's ear and serves as both a stimulus emitter and recorder. Standard testing utilizes multiple frequencies ranging from 2000 Hz to 5000 Hz. Many practices use TEOAE for screening purposes, although both TEOAE and DPOAE are acceptable. Results are binary and qualitative with either a "pass" or "refer" result based on the presence or absence, respectively, of cochlear microphonics.

Auditory Brainstem Response

Earphones are placed in the child's ear, and electrodes are placed on the scalp. If the patient is a newborn or young child, feeding and performing diaper hygiene may facilitate sleep. The earphones emit a sound signal ranging in frequency and intensity that are detectable by electrical changes seen on a monitor. The results can be viewed at waves I to V, representing the anatomic landmarks described in the Anatomy & Physiology section.

Tympanometry

An ear probe with a soft tip (immittance probe) is placed within the ear canal of the test ear, with the tip sealing the ear to maintain pressure. The pressure within the external ear is changed while the child hears low-frequency noise stimuli. Recordings of the movement of the tympanic membrane are obtained. The standard probe tone is a 226 Hz signal, which is used for most purposes in children over six months of age. Some research suggests that a 1000 Hz probe-tone may be more accurate at diagnosing middle ear effusions in children under six months old due to improved stiffening of the tympanic membrane with a higher frequency probe tone.

A typical tympanogram indicates the ear canal volume in cubic centimeters, the maximum pressure in decapascals (daPa), and the peak compliance in milliliters (ml). "Type A" tympanograms suggest normal middle ear function with peak compliance between -100 and +100 daPa and compliance from 0.3 to 1.5ml. "Type Ad" and "Type As" tympanograms demonstrate highly compliant and less compliant middle ear systems, respectively. "Type B" or flat tympanograms occur when there is no identifiable peak compliance, usually due to the presence of middle ear fluid or a perforated eardrum. An ear canal volume (ECV) larger than 1.5 cm suggests a perforation or the presence of a tympanostomy tube in the tympanic membrane. "Type C" tympanograms demonstrate peak compliance below -100 daPa and suggest negative middle ear pressure and eustachian tube dysfunction.

Acoustic Reflex Testing

While not commonly performed in newborns or children, reflex testing may be employed in certain circumstances. An immittance probe is inserted into the test ear, with a second probe placed in the other ear. A 1000 Hz tone is typically used in neonates, while a 226 Hz tone is more commonly used after the neonatal period. A sound stimulus is initiated at 70-80 dB sound pressure level (SPL) with 5 dB SPL increases until an acoustic reflex is detected due to stapedial contraction. Stimulus intensity should not exceed 105 dB SPL unless a conductive hearing loss is suspected. Results can be confirmed by repeating the test or increasing the intensity by 5 dB SPL and ensuring a reflex is present. Reflex testing is not performed if a patient is found to have a flat or type B tympanogram.

Behavioral Audiometry

Between approximately 6 to 24 months of age, children should undergo VRA testing. A patient must be able to sit unassisted and have enough head and neck control to be able to turn the entire head in the direction of a sound stimulus. Earphones are placed in the child's ear, and visual reinforcers are placed at 90 degrees to the patient's left and right sides. Examples of visual reinforcers may include a toy behind tinted glass or a video-based reinforcer. Sound stimuli are presented to both ears at different frequencies and intensities. When the child correctly turns to the direction of the ear in which the sound was presented (turning right when stimulus presented to right ear), they are rewarded with either a video clip or a view of the toy.

From ages 2-5 years, children undergo audiometric testing via CPA. Sound stimuli are presented to both ears via headphones or earphones with varying frequencies and intensities. The child participates via play, such as putting a peg into a board or placing a ball into a basket in response to a sound. Testing can occur at different frequencies until all frequencies have been tested in both ears.

Pure-Tone Audiometry

Pure tone audiometry is often considered the standard form of audiometry. It can generally be performed on patients older than age 5, although some patients may be able to participate earlier than five, and some may be slightly delayed. Headphones are placed on the patient's head, and sounds of varying frequencies and intensities are presented to each ear. The patient is typically given a button to depress each sound heard. An alternative set-up for children may include a verbal response, raising the hand, or clapping every time a sound stimulus is heard. This continues until all frequencies have been tested bilaterally, and threshold levels can be calculated based on the procedure results.

Imaging

Both CT and MRI may be used to evaluate pediatric hearing loss. However, research and consensus expert opinion favors not ordering routine imaging for children with bilateral hearing loss.[14] Imaging may be beneficial in children with ANSD or unilateral hearing loss; however, the diagnostic yield of relevant findings in imaging for unilateral hearing loss is reported as only 37% for CT and 35% for MRI.[19] Children who may be considered candidates for cochlear implants should undergo MRI to rule out cochlear dysplasia or cochlear nerve aplasia, which are contraindications to implantation.

Genetic and Laboratory Testing

Patients with bilateral sensorineural hearing loss or ANSD may benefit from a genetic workup. Approximately 50% of congenital hearing loss is reportedly genetic, with the vast majority of genetic cases being non-syndromic in nature. Syndromic cases can be divided into autosomal dominant and autosomal recessive cases.

Common causes of autosomal dominant congenital hearing loss include Waardenburg syndrome, Crouzon syndrome, branchio-oto-renal syndrome, and neurofibromatosis. The most common causes of autosomal recessive congenital hearing loss include Usher, Pendred, and Jervell and Lange-Nielsen syndromes. After audiometry, genetic testing has been shown to have the second-highest diagnostic rate of any single test for bilateral sensorineural hearing loss. Asymmetric hearing loss is unlikely due to genetic causes and should not routinely prompt a genetics evaluation.

While 50% of congenital hearing loss is genetic, 50% is acquired. The most common cause of acquired hearing loss is congenital cytomegalovirus (CMV) infection, responsible for approximately 25% of all cases of congenital hearing loss. Within the first three weeks of life, CMV testing can be performed via saliva or urine samples; however, after three weeks, these samples become less accurate, and testing is performed using blot spot testing. Previous guidelines have suggested that no treatment for congenital CMV is necessary unless the patient is experiencing other symptoms of CMV infection. Limited research has shown a potential role for oral valganciclovir to improve hearing outcomes in patients with asymptomatic congenital CMV infection; however, this treatment has not been FDA approved.[20]

Complications

While there are no significant complications from assessing for hearing loss in children, the primary concern is the risk of false results, both false positives and, more importantly, false negatives. A false positive, or a falsely abnormal test, may result in additional testing for the patient and added stress or anxiety for parents and caregivers, but ultimately will not result in long-term deficits for the patient. On the other hand, false negative, or falsely normal testing in a patient who truly does have hearing loss, may result in delayed diagnosis of hearing loss.

The associated social, intellectual, and developmental consequences of false-negative assessments may negatively affect the child's developmental potential. Complications of the hearing assessment, such as perforation of the tympanic membrane or injury to the ear canal, are rare and typically not long-term. These risks are present whenever the ear is instrumented. The presence of behavioral concerns during evaluation may increase the risk of any of the above complications and make the testing more challenging to perform. Behavioral issues may decrease the accuracy of results.

Clinical Significance

As further research is conducted, significant harmful effects of hearing loss have been revealed. When diagnosed early, interventions can be put into place to optimize one’s chance for normal hearing and development. Primary interventions include conventional hearing aids, bone-anchored hearing aids (BAHA) with softband for children under age 5, contralateral routing of signals (CROS) hearing aids, and remote microphone systems. When hearing loss is severe enough, the patient should be evaluated for cochlear implantation.[11]

Hearing aids have been shown to reduce hearing thresholds by up to 25 dB hearing loss (HL); however, in the setting of severe to profound hearing loss, hearing aids perform much worse than cochlear implantation.[21] Cochlear implantation has been shown to lead to similar hearing thresholds as control patients, with word reception scores of 85%.[22] Decreased developmental delays are also noted in patients with cochlear implantation, with 95% of implanted patients not requiring learning adaptations or full-time educational support.[23]

If screening is not performed or if careful attention is not paid to following up on abnormal results, patients may not have hearing loss diagnosed until a later age when the brain is less plastic. The ability to integrate a hearing intervention is made more difficult. Data from the 2018 Early Hearing Detection and Intervention (EHDI) Hearing Screening and Follow-Up Survey indicate loss of follow-up in greater than 1 in 4 patients nationwide.[24]

These patients are at much higher risk for social, developmental, and intellectual delays. While significant progress has been accomplished since the initiation of newborn hearing screening in 2001, continued efforts must be made to close the gap between diagnostic completion and initiation of therapy. Genetic testing and imaging may play a role in evaluating some patients with hearing loss; however, these should not be ordered routinely for every patient. With proper implementation of the 1-3-6 month guidelines at a minimum and 1-2-3 month guidelines where possible, children with hearing loss can most often usually develop and attain a high quality of life.

Enhancing Healthcare Team Outcomes

The assessment and management of hearing loss in children involve a multidisciplinary approach. Healthcare providers should work together as part of a coordinated team to ensure the rapid diagnosis and initiation of appropriate interventions. Communication between specialists is crucial to ensure that appropriate care is being provided to each patient and patients do not become lost to follow-up. Without a patient-centered team approach, efforts to improve the developmental potential for patients with hearing loss will falter.