1When the Cochlea Is Built Differently

FThe fraction you can see

Roughly one in five children with congenital sensorineural hearing loss has an inner ear that is structurally malformed in a way a radiologist can detect on CT or MRI; the remaining four in five have an inner ear that looks normal but is deaf at the membranous, cellular or molecular level. A radiologically normal-looking cochlea can still be profoundly deaf because the fault lies in the hair cells, the stria vascularis or a single gene — imaging maps the bony shell, not the organ of Corti inside it. The malformations matter out of all proportion to their frequency, because an abnormally shaped cochlea changes the surgical anatomy, the electrode that fits, the risk of complications and, sometimes, whether a cochlear implant can work at all. This chapter is about that visible minority: ears whose bony architecture announces, before the operation begins, that the standard playbook no longer applies.[2017][2009]

TThree governing questions

Every malformed ear can be interrogated with three questions, and the answers decide the entire management plan. First — is there a cochlea to stimulate? A common cavity or a tiny hypoplastic cochlea still has neural tissue an electrode can excite, but a complete labyrinthine aplasia (the Michel deformity) has none. Second — is there a cochlear nerve to carry the signal? A perfectly formed cochlea is useless if the nerve that should leave it is absent or too thin, and a small or atretic bony cochlear nerve canal warns of exactly that. Third — what will go wrong during surgery? Two recurring hazards dominate: a perilymph gusher when cerebrospinal-fluid pressure is transmitted into an abnormally open inner ear, and an aberrant facial nerve running where the surgeon does not expect it. Answer those three and you have, in order, the question of feasibility, the question of pathway, and the question of safety.[2017][1987]

CThe nerve, not the bone, decides CI versus ABI

The most important lesson of this chapter is that the cochlear nerve — not the shape of the bony cochlea — is the true determinant of whether a child receives a cochlear implant or an auditory brainstem implant. A grossly malformed cochlea with an intact nerve can often be implanted with a good outcome, because the electrode still has a population of nerve fibres to drive. A near-normal cochlea with an absent or aplastic cochlear nerve cannot transmit electrical hearing at all, and that child is pushed toward an auditory brainstem implant that bypasses the nerve entirely. This is precisely the blind spot of the classic morphology-based classifications, which name the bone in exquisite detail but say nothing about the nerve — a gap the later modules of this chapter exist to close.[2017][2019]