Auditory Neuropathy Spectrum Disorder

Defining pattern

ANSD is defined electrophysiologically: absent or grossly disrupted ABR with preserved otoacoustic emissions and/or cochlear microphonic¹. Outer hair-cell function is intact (OAEs and CM present); synchronous neural transmission has failed somewhere between the inner hair cell and the brainstem.Foundation

The polarity-reversal test

The cochlear microphonic is a pre-neural alternating-current response that mirrors the stimulus waveform — it inverts cleanly when the click polarity is reversed². Running rarefaction and condensation as separate runs preserves the CM and confirms its identity by inversion. Alternating polarity cancels the CM and is contraindicated when ANSD is suspected.

How the OAE screen misses ANSD

OAEs reflect outer hair-cell function. In ANSD, outer hair cells are preserved; OAE screens pass³. The child is assigned a normal-screen result even though the auditory pathway cannot decode speech. AABR-based screens detect ANSD because they sample the synchronous neural response.

Risk factors and presentations

NICU graduates with hyperbilirubinaemia, hypoxia, or ototoxic exposure are over-represented. OTOF (otoferlin) mutations cause a temperature-sensitive synaptic ANSD with characteristically excellent cochlear-implant outcomes; a child with the OAE/ABR dissociation should have OTOF tested⁴. Older patients may present with disproportionate speech-in-noise difficulty relative to their audiogram.

Cochlear nerve deficiency Clinician

Congenital absence (aplasia) or under-development (hypoplasia) of the cochlear nerve can produce the same electrophysiologic pattern as ANSD — absent ABR with preserved OAEs — but the lesion is structural rather than synaptic⁷. High-resolution MRI of the internal auditory canal in the oblique-sagittal plane is the radiologic reference: the cochlear nerve should be the largest of the four nerves in the IAC; aplasia or marked hypoplasia is the radiographic diagnosis. Distinguishing cochlear nerve deficiency from synaptopathic ANSD is critical because the management diverges sharply — implantation outcomes are far poorer when there is too little nerve to drive.

Electrically-evoked ABR (EABR)

EABR uses an electrical stimulus delivered to the cochlear promontory, round-window niche, or via an implant electrode to bypass the cochlea entirely and test whether stimulable auditory neural tissue exists⁸. A reproducible EABR confirms that the cochlear nerve and brainstem can be driven; an absent EABR predicts poor cochlear-implant outcome and may redirect management toward an auditory brainstem implant. EABR is most useful in three scenarios: (1) suspected cochlear nerve deficiency after equivocal MRI, (2) post-meningitis cochlear ossification where acoustic ABR is unrecordable, and (3) implant programming for children too young or too disabled for behavioural threshold estimation.

Management implications

The diagnosis changes the entire intervention pathway. Conventional hearing aids deliver acoustic energy a damaged synapse cannot decode synchronously and so often produce limited functional benefit. Cochlear implants bypass the synaptopathic region in many ANSD subtypes — particularly OTOF-related — with excellent outcomes⁵. The decision to implant depends on the site of the lesion, the integrity of the cochlear nerve confirmed on MRI and EABR, and the developmental trajectory⁶.