4Cochlear Nerve Deficiency

FAplastic versus hypoplastic: a spectrum, not a switch

Cochlear nerve deficiency (CND) describes a cochlear nerve that is either entirely absent (aplasia) or thinner than its neighbours in the internal auditory canal (hypoplasia); the distinction predicts outcome, so it must be made explicitly rather than recorded as a single label. The clinical importance of the aplasia/hypoplasia split is that they behave differently: even a hypoplastic nerve carries some fibres, whereas a truly aplastic nerve offers no afferent pathway from a cochlear electrode. CND is frequently unilateral; one report found a deficient nerve in only one ear in about half of affected children, which makes side selection, not just the decision to implant, the central problem. Up to half of children with congenital single-sided deafness have CND, and in one infant series every infant with profound unilateral sensorineural loss had a deficient nerve, so CND should be assumed until imaging proves otherwise in profound unilateral loss. Syndromic CND (notably CHARGE) carries both a higher chance of an absent nerve and frequent cognitive or neurological involvement, compounding a poorer prognosis.[2020][2009]

TThe imaging work-up: a small canal, a missing nerve

High-resolution MRI is mandatory before implantation whenever CND is possible; CT alone cannot confirm the presence of a cochlear nerve and will miss the diagnosis. The nerve is best judged on oblique-sagittal (Stenver's-plane) CISS or FIESTA images cut perpendicular to the canal, where the four nerves of a normal canal (facial, cochlear, superior and inferior vestibular) should each be identifiable. On CT, a narrow bony cochlear nerve canal (the cochlear aperture) below about 1.4 mm is suggestive of nerve hypoplasia or aplasia and should trigger dedicated MRI nerve imaging. A narrow internal auditory canal is a surrogate clue rather than the diagnosis itself; canal width raises suspicion but the nerve must still be visualised directly. Because CND is so often asymmetric, the imaging goal is not a yes/no answer for the patient but identification of the better ear, with the larger and more clearly present nerve favoured for implantation.[2020][2009]

COutcomes: a wide range and a real ceiling

Outcomes in CND are markedly more variable and on average poorer than in children with normal nerves, but they are not uniformly futile, and many children gain at least sound awareness. In one series of 50 children, roughly 47% of ears with an aplastic nerve showed some ability to understand spoken language compared with about 89% of ears with a hypoplastic nerve. A literature review of 97 subjects found that about 54% developed no speech discrimination at all, yet 65% of hypoplastic-nerve children reached closed- or open-set speech recognition versus only 30% of aplastic-nerve children. Hypoplastic nerves consistently outperform aplastic ones, which frames counselling: the thinner-but-present nerve justifies a trial of implantation; the absent nerve carries a much higher chance of minimal benefit. Useful benefit from an apparently absent nerve is biologically plausible because cochlear fibres can travel with other nerves in the canal, but this is the exception and should not be promised. An additional disability or a syndromic diagnosis lowers the expected ceiling further, with close to 43% of syndromic CND children showing no auditory response to electrical stimulation in one review.[2020][2009][2016]

CWhen to look past the cochlea

Present electrically evoked responses, transtympanic or implant-elicited auditory brainstem response and intraoperative neural-response telemetry, favour proceeding, whereas their absence supports a more cautious prognosis and may prompt reconsideration of the implant target. A staged plan is reasonable in equivocal cases: implant the better-nerve ear, document electrical responses and behavioural progress over months, and let the response guide expectations rather than committing to a single outcome at the outset. When the cochlear nerve is genuinely absent and a cochlear implant produces no auditory response, the auditory pathway can instead be stimulated centrally, the rationale developed in the auditory brainstem implant population module. Counselling families should set realistic expectations up front, emphasise spoken-language outcome is uncertain in aplasia, and keep a manual-communication pathway open so the child is not left without language while a device trial proceeds.[2020][2009]