7Auditory neuropathy

ECochG at its most pathognomonic. Outer hair cells working — CM clearly present, inverts with click polarity. Auditory nerve transmission failed — CAP absent or severely reduced. The pattern, once seen, is unmistakable — and the polarity-reversal test is the diagnostic manoeuvre that separates real biology from stimulus artifact.

Starr's group first delineated auditory neuropathy as a clinical entity in 1996 — patients with abnormal speech understanding, absent or grossly abnormal ABRs, but preserved outer hair cell function as evidenced by otoacoustic emissions and cochlear microphonics.[1996] The condition has since been renamed several times: auditory neuropathy, auditory dyssynchrony, and most recently auditory neuropathy spectrum disorder (ANSD), formalised by the 2008 Colorado guidelines because the umbrella covers several distinct sites of lesion — IHC ribbon synapse failure, post-synaptic dendritic dysfunction, demyelination, and axonal loss — all producing the same audiological pattern.[2008]

FClinical presentation

The classical findings are deceptively simple but require a specific test battery to confirm:

The CM-present-CAP-absent dissociation is the ECochG signature, and it is essentially specific to ANSD among common audiological conditions. Almost every other cause of hearing loss takes down the cochlea and the nerve together; only ANSD spares the cochlear receptor and breaks transmission downstream.

FThe ECochG signature

Three features together make the diagnosis:

1. A clearly present cochlear microphonic. The CM rides on the trace during the stimulus, oscillating at the stimulus frequency (for clicks, a high-frequency damped sinusoid; for tone bursts, the carrier).
2. That CM inverts 180° when click polarity is reversed. This is the Berlin 1998 authenticity test: a true biological CM, generated by the OHC mechanoelectrical transduction current, follows the stimulus waveform — reverse the stimulus, reverse the CM. Stimulus artifact (radiated electrical signal from the transducer) does not reverse with polarity and is therefore unmasked by this manoeuvre.[1998]
3. An absent or grossly reduced CAP (N1). Even at high click intensities (90–95 dB nHL), the expected sharp negative deflection at 1.5–2.0 ms is missing or replaced by low-amplitude noise.

TPolarity-reversal explorer

The diagnostic manoeuvre in suspected ANSD is to record rarefaction and condensation click runs separately and look for the 180° flip of the early oscillation that confirms it is biological CM, not radiated artifact. The figure below overlays the two single-polarity runs from an ANSD ear: the CM clearly inverts between rarefaction and condensation, while the AP (N1) is essentially absent in both — the pathognomonic ANSD pattern.

Toggle between rarefaction and condensation to see the single-polarity runs in isolation, then overlay both to read off the Berlin 1998 authenticity criterion at a glance — the early CM oscillation inverts 180° when the click polarity flips for a true biological CM, and looks identical between the two runs for radiated stimulus artifact. The AP (N1) is essentially absent at the expected 1.5–2.0 ms latency in both polarities, completing the pathognomonic CM-present-AP-absent signature of ANSD.[1998, 2002]

TCM authenticity protocol

The full Berlin protocol has four steps. The first three confirm a CM is biological; the fourth confirms the AP is genuinely absent.[1998]

CSite of lesion — presynaptic vs postsynaptic

ANSD is a category, not a single disease. The audiological pattern (CM present, AP absent) is identical across at least four distinct biological lesions: (1) IHC dysfunction with intact ribbon synapses; (2) presynaptic failure at the ribbon synapse itself (the OTOF/otoferlin form is the prototype); (3) postsynaptic dysfunction of auditory nerve dendrites; (4) demyelination or axonal loss of the auditory nerve fibres (the OPA1 form, often part of dominant optic atrophy syndromes).[2008, 2019, 2006]

McMahon and colleagues showed in 2008 that frequency-specific round-window ECochG could discriminate these two sites of lesion in vivo by separating the hair-cell, dendritic, and axonal current contributions to the recorded potential. The presynaptic form preserved a large dendritic component (the auditory nerve dendrites are intact, they just receive no synaptic drive); the postsynaptic form lost it.[2008] This matters clinically because cochlear implants directly stimulate the auditory nerve, bypassing the IHC and the synapse. They work brilliantly in presynaptic ANSD — and may underperform in the postsynaptic form because the lesion is downstream of where the CI delivers current.[2019]

FManagement implications

The ECochG findings shape management more directly in ANSD than in any other condition covered in this atlas:

• Identifying ANSD changes the workup. A profoundly deaf infant whose ABR is absent could be assumed to need a cochlear implant straightforwardly. If the ECochG/CM is preserved, the diagnosis shifts to ANSD and the question becomes whether the lesion is pre- or post-synaptic — which changes the CI prognostic counselling, and adds genetic testing (OTOF, OPA1 panels) to the workup.[2006]
• Hearing aids are often disappointing. Amplification helps audibility but not temporal processing. Children with ANSD frequently outgrow hearing aids and move to CI before adolescence.
• Cochlear implantation is the default interventionfor ANSD with poor speech outcomes from hearing aids, with the caveat that postsynaptic ANSD may produce less benefit. Riggs et al. 2017 (Fitzpatrick group) used intraoperative round-window ECochG to measure the "total response" (ECochG-TR) in CI candidates with and without ANSD; the ECochG-TR is dominated by hair cell activity and accounted for ~40–50% of the variance in post-CI speech perception in adults — a substantial increase on the ~25% that traditional clinical predictors achieve.[2017]
• Some acquired adult ANSD recovers. Particularly when the trigger was hyperbilirubinaemia, hypoxia, or transient demyelination, partial recovery of synchrony is possible. Avoid early irreversible intervention until the clinical trajectory is clearer.

FClinical case

FTSelf-assessment