Tuli Masa Kecil

01Pathophysiology

Congenital SNHL arises from dysfunction at two possible sites along the auditory pathway: the cochlea (sensory) or the auditory nerve and brainstem pathways (neural).

In the cochlea, normal hearing depends on the mechanoelectric transduction performed by inner hair cells, which convert sound-wave vibrations into electrical impulses. Outer hair cells serve as cochlear amplifiers that sharpen frequency tuning and enhance sensitivity. When outer hair cells are damaged or absent, the cochlea loses its ability to amplify low-intensity sounds, producing a sensorineural threshold shift. This is the basis of most congenital SNHL cases.

The single most common genetic cause is a mutation in GJB2 (Connexin-26), which encodes a gap junction protein critical for potassium recycling in the cochlea. During transduction, potassium ions flow into the hair cells from the endolymph. After stimulation, these ions must be recycled back into the endolymph via supporting cells connected by gap junctions. A defective Connexin-26 disrupts this recycling pathway, leading to progressive potassium accumulation and hair cell death. This mutation follows autosomal recessive inheritance, which explains why most affected children have no family history of deafness, a point that test-writers exploit.

In nongenetic causes, congenital CMV is the leading culprit. The virus infects the stria vascularis and the organ of Corti, causing inflammatory damage to both the endolymphatic ion transport machinery and the hair cells themselves. CMV-related hearing loss can be progressive or fluctuating, meaning a child who passes the initial newborn screen may still develop hearing loss later, an important concept for follow-up surveillance.

A distinct subtype is Auditory Neuropathy Spectrum Disorder (ANSD), where the outer hair cells are intact (OAE is present/normal) but neural transmission is abnormal (AABR is refer/absent). ANSD is strongly associated with hyperbilirubinemia requiring exchange transfusion and aminoglycoside exposure in the NICU population. The pathology lies at the synapse between inner hair cells and the auditory nerve, or within the nerve itself.

Understanding these mechanisms is critical because they directly determine which screening test will catch which pathology. OAE tests outer hair cells only, so it will miss ANSD. AABR tests the full pathway from cochlea to brainstem, so it can detect ANSD but may miss mild cochlear loss below its threshold of 40-45 dB HL.

02Classification and Clinical Manifestation

WHO Classification of Hearing Loss Severity

Etiologic Classification

Laterality and Configuration

Auditory Neuropathy Spectrum Disorder (ANSD)

03Diagnostic Workup

Summary Table

The diagnostic approach follows a tiered logic, starting with screening and escalating to confirmatory testing.

Step 1: Newborn hearing screening. The best initial test for all newborns is either OAE or AABR, performed before hospital discharge and no later than age 1 month. OAE is fast and cheap, detecting outer hair cell dysfunction at a threshold of roughly 30-35 dB HL. AABR evaluates the entire peripheral auditory pathway up to the brainstem but has a slightly higher detection threshold (40-45 dB HL), meaning it may miss mild cochlear loss. The critical point: OAE alone will miss ANSD because outer hair cells are intact in that condition. For this reason, the guideline mandates AABR as the sole or combined screening modality for all NICU infants, given the higher prevalence of ANSD in that population.

Many programs use a two-stage protocol for well-baby nursery infants: OAE first, and if the baby refers, AABR is performed before discharge. A baby that fails OAE but passes AABR is considered to have passed the screen.

Step 2: Diagnostic confirmation. All infants who refer on screening must complete a comprehensive audiologic evaluation by age 2-3 months. The gold standard is diagnostic BERA with tone burst stimuli, which provides frequency-specific air- and bone-conduction threshold estimates per ear. Click-evoked BERA alone is insufficient because click is a broadband stimulus that cannot resolve hearing loss at individual frequencies, particularly sloping configurations.

When BERA shows no response at maximum output, high-intensity click testing with alternating polarity must be performed to evaluate for the presence of cochlear microphonic, the hallmark finding of ANSD.

ASSR is added when hearing loss is severe to profound, or when BERA provides no waveforms, because ASSR can estimate thresholds at higher stimulus intensities.

Step 3: Middle ear evaluation. Tympanometry is performed in every diagnostic workup to distinguish sensorineural from conductive or mixed hearing loss. For infants under 6 months of age, high-frequency tympanometry (1000 Hz probe tone) is required because the neonatal ear canal tends to collapse and the tympanic membrane resonates at higher frequencies due to its greater mass density. The standard 226 Hz probe tone may produce falsely normal results in this age group. Acoustic reflexes supplement the evaluation: an absent reflex predicts thresholds above 70 dB and is also a characteristic finding in ANSD.

Step 4: Behavioral cross-check. Electrophysiologic thresholds must be confirmed by behavioral testing as soon as the child's developmental age allows: BOA from 0-6 months (observational, not diagnostic), VRA from 6 months (conditioned response, yields reliable thresholds), and conditioned play audiometry from 24 months.

Step 5: Etiologic workup. All confirmed cases should be offered Connexin-26 genetic testing, urine/saliva CMV PCR (ideally within the first 3 weeks of life), and ECG to rule out long QT syndrome. MRI of the temporal bone is the first-line imaging study to visualize the cochlear nerve, membranous labyrinth, and brain parenchyma. HRCT is complementary when bony anatomy must be evaluated for cochlear implant surgery planning.

04Management & Treatment

Summary Table

Acute phase (0-6 months): The first priority after a confirmed diagnosis is ABD fitting, ideally by 4 months of age. The recommended type for children is the behind-the-ear (BTE) model because the ear continues to grow until puberty, and only the earmold (inexpensive) needs periodic replacement. Bilateral amplification is recommended for bilateral hearing loss regardless of symmetry. Fitting is verified using RECD (real ear to coupler difference) measurements or REAR (real-ear aided response). For ANSD, fitting is based on behavioral thresholds rather than BERA thresholds, and amplification is started at low gain and gradually increased.

Evaluation of ABD benefit (3-6 months post-fitting): The child should demonstrate aided thresholds of at least 40 dB across all frequencies and show measurable progress in auditory and language development assessed by the St. Gabriel Questionnaire at 3-month intervals. If two consecutive 3-month evaluations show no meaningful progress, the child should be referred for cochlear implant candidacy evaluation.

Cochlear implant candidacy: Criteria include (1) bilateral SNHL with threshold >70 dB on electrophysiologic testing confirmed by behavioral assessment, (2) failure of ABD trial as described above, (3) no medical or radiologic contraindications, (4) demonstrated family commitment to long-term habilitation, and (5) psychological/behavioral evaluation. For thresholds above 110 dB, the child may proceed directly to CI without an ABD trial. Implantation should occur as early as possible, with literature supporting surgery before 12 months of age for optimal language outcomes comparable to normal-hearing peers. The maximum recommended age for CI in a pre-lingual child without prior optimal habilitation is 7 years.

Bilateral CI is superior to unilateral for speech perception and sound localization. Simultaneous bilateral implantation is preferred. If sequential, the second implant should be placed within 1 year and no later than 5 years after the first.

Speech-language habilitation must begin immediately after device fitting or CI switch-on. The primary method recommended for children pursuing auditory-oral communication is Auditory Verbal Therapy (AVT), delivered by a certified AVT practitioner. AVT focuses on developing listening skills through the hearing device and building spoken language through a parent-centered coaching model. Other methods include sign language, total communication, and preverbal communication skills training. The choice of communication modality is made collaboratively with the family.

Contraindications and special considerations: Conductive hearing loss should be medically or surgically treated before ABD fitting. Children with cochlear nerve aplasia on MRI are not CI candidates and should be evaluated for Auditory Brainstem Implant (ABI). For ANSD, some children show spontaneous improvement by 18 months, so CI timing may be deferred with close monitoring of language development. Vaccination (particularly pneumococcal vaccine) is recommended prior to CI surgery to reduce the risk of post-implant meningitis.

05Newborn Hearing Screening Program

EHDI 1-3-6 Framework

Screening Protocol by Population

Screening Flowchart

Post-Screening Surveillance (Even If Pass)

The guideline emphasizes that all children, regardless of screening outcome and regardless of risk factor status, must receive ongoing communication development surveillance starting at age 2 months. This is because:

(a) Screening can miss mild hearing loss (below OAE/AABR thresholds), (b) progressive or late-onset hearing loss (e.g., CMV, enlarged vestibular aqueduct) may not be present at birth, and (c) ANSD may show transient improvement and later regression. Any child showing delayed speech/language milestones or whose parents express concern about hearing must be referred for full audiologic evaluation regardless of a prior pass result.

06Differential Diagnosis & Distractors

07Traps & High-Yield Pearls

The most common way students get questions on congenital SNHL wrong is by confusing the capabilities and limitations of OAE versus AABR. A vignette describing a NICU infant with hyperbilirubinemia requiring exchange transfusion who "passes OAE" but has delayed speech is testing your knowledge of ANSD. The student who sees "passed OAE" and concludes that hearing is normal will miss the diagnosis entirely. The correct reasoning is that OAE only tests outer hair cell function and does not evaluate neural transmission; AABR is required for NICU babies precisely because ANSD is prevalent in this population.

Another frequent trap involves the timeline. Test-writers will present a child diagnosed at 9 months of age with bilateral profound SNHL and ask for the next best step. The distractor answer is "start ABD." The correct answer for thresholds above 110 dB is direct CI referral without an obligatory ABD trial, because the guideline allows bypassing the trial at that severity. Conversely, for thresholds between 90 and 110 dB, an ABD trial is mandatory, and the student who jumps to CI referral without a documented ABD trial is wrong.

A third trap is the ECG question. A vignette describing a child with bilateral profound SNHL who suddenly collapses during exercise is testing whether you recognize Jervell and Lange-Nielsen syndrome (autosomal recessive SNHL + prolonged QT interval). The student who attributes the syncope to an unrelated cardiac condition without connecting it to the hearing loss will miss the underlying syndrome.

Finally, students must recognize that passing a newborn hearing screen does not rule out hearing loss permanently. Progressive or late-onset causes (CMV, enlarged vestibular aqueduct, certain genetic mutations) can develop after a normal screen. The core competency tested here is understanding that ongoing developmental surveillance with communication milestone monitoring is mandatory for every child, regardless of screening result, and any parental concern about hearing or language at any age warrants a full audiologic re-evaluation.