3Auditory Neuropathy Spectrum Disorder

FA lesion the audiogram hides

ANSD is diagnosed by a characteristic dissociation: present otoacoustic emissions and/or a cochlear microphonic (proving working outer hair cells) alongside an absent or grossly abnormal auditory brainstem response. The clinical hallmark is speech understanding that is far worse than the pure-tone audiogram predicts, so audibility is preserved but intelligibility collapses. Prevalence estimates among children with hearing loss have ranged from roughly 1% up to around 8% of profoundly deaf children, with the figure rising as awareness and screening improve. Reported series describe onset before age 2 in most cases, with frequent neonatal risk factors such as hyperbilirubinaemia and hypoxia, and a subset showing a reverse-sloping audiogram. Because emissions are present, ANSD can be missed by emission-only newborn screening and is detected only when the ABR is included.[2009][2020]

TSite of lesion: inner hair cell, synapse, or nerve

The combination of normal outer-hair-cell function with an abnormal ABR localises the fault to one of three places: the inner hair cell, the ribbon synapse between inner hair cell and spiral ganglion neuron, or the auditory nerve fibres themselves. Two mechanisms explain the percept: dyssynchrony, where surviving fibres fire out of step so temporal cues are scrambled, and reduced neural input, where too few fibres carry the signal. Dyssynchrony accounts for the disproportionate loss of speech understanding, because pitch, loudness, and temporal fine structure all depend on precisely synchronised neural firing. ANSD is heterogeneous: a presynaptic lesion at the inner hair cell or synapse leaves the downstream nerve intact, whereas a postsynaptic or true neural lesion damages the very fibres an implant must stimulate. This presynaptic-versus-postsynaptic distinction, rather than the audiogram, is what predicts implant benefit, which is why ANSD is the population where site-of-lesion thinking matters most.[2009][2020]

CWhy the implant can rescue a dyssynchronous synapse

A cochlear implant bypasses the inner hair cell and the ribbon synapse entirely and stimulates the spiral ganglion and nerve directly with electrical pulses, replacing a dyssynchronous biological synapse with synchronous artificial input. Consequently, when the lesion is presynaptic and the cochlear nerve is intact, implanted ANSD patients often achieve speech-perception outcomes comparable to children with other causes of deafness. Outcomes are poor and unpredictable when the lesion is the nerve itself, for example with cochlear nerve deficiency or a syndromic peripheral neuropathy, because there is little healthy substrate to drive. Pre-implant tools help predict this: electrical stimulation responses and the cortical auditory evoked response can indicate whether the pathway can produce a synchronous response, and an aided cortical response suggests amplification alone may suffice. The decisive investigations are therefore high-resolution MRI to confirm an adequate cochlear nerve and electrophysiological probing of the pathway, not the behavioural audiogram. A trial of well-fitted hearing aids with cortical-response monitoring is reasonable first in milder cases, with implantation pursued when amplification fails to restore a synchronous, useful signal.[2020][2009][2002]

COTOF and the favourable synaptic subtype

Biallelic mutations in OTOF, which encodes otoferlin, cause a recessive non-syndromic ANSD (DFNB9) in which otoferlin's role in synaptic-vesicle fusion at the inner-hair-cell ribbon synapse is lost. The OTOF lesion is purely presynaptic: the inner hair cell cannot release neurotransmitter, but the spiral ganglion and auditory nerve downstream are structurally intact. This makes OTOF-related ANSD the prototypical favourable subtype for implantation, because the implant supplies the synchronous neural drive the failed synapse cannot, and reported cochlear-implant speech outcomes are excellent. Genetic confirmation of an OTOF cause therefore both explains the audiogram-speech mismatch and supports a confident recommendation to implant. By contrast, OTOF biology is the reason emerging gene-therapy approaches target this presynaptic defect specifically, since the rest of the pathway is healthy. Clinically, identifying an OTOF genotype shifts counselling from uncertainty toward a strong expectation of benefit, illustrating how molecular site-of-lesion data now informs candidacy.[2009][2020][2020]