1Overview — why the cause matters
A cochlear-implant candidate arrives with a single audiometric label — severe-to-profound sensorineural hearing loss — but that label is the end of many different stories. Genes, infections, drugs, noise, ageing and autoimmune disease all converge on the same silent audiogram, yet each gets there by injuring the ear in its own way, and leaves behind a different cochlea and a different nerve. This chapter is about those roads to deafness and the marks they leave, because the mark is what the implant has to work with. Knowing the cause turns a fixed diagnosis into a prediction — about the substrate, the surgery, and the result.
FWhat this chapter is
The earlier foundations chapters built the normal ear and brain, the population, the genes, and the way deafness remodels the pathway. This chapter asks the prior clinical question: how did the deafness happen? It surveys the major causes of sensorineural hearing loss — congenital and acquired, childhood and adult — and for each one keeps a single thread in view: what it does to the structures a cochlear implant depends on.
It is the pathology counterpart to the genetics chapter (Chapter 6) and the cellular chapter (Chapter 4). Where those explained the molecular and cellular machinery, this one names the diseases that break it, and connects each to candidacy and outcome.[2010]
FDeafness is a final common pathway
Sensorineural hearing loss is best thought of as a final common pathway: many causes, one end-point. The audiogram records how much hearing is lost and at which frequencies, but it is largely silent about why. Two people with the same flat profound loss may have an intact spiral ganglion behind a dead organ of Corti, or a ganglion that has itself degenerated — and the difference is invisible on the audiogram, yet decisive for the implant.
FTWhy the cause matters
The cause matters for four practical reasons. It predicts the substrate — how much of the nerve the electrode will find alive. It can dictate timing — meningitis ossifies the cochlea and turns implantation into an emergency. It changes the surgery — otosclerosis warns of facial-nerve stimulation, a deficient nerve calls for different imaging. And it grounds counselling — a progressive cause like CMV behaves differently from a fixed genetic loss. A cause is not a historical footnote; it is prognostic information.[1997]
TThe organising idea
One sentence runs through the chapter: every cause leaves a signature in the cochlea and the nerve, and that signature — not the audiogram — is what the implant must work with. Some causes spare the implant's target and implant beautifully; some damage it; some threaten the very road the electrode travels. Read this way, taking a history of the deafness becomes the first step of the implant work-up.
FChapter roadmap
| Movement | Modules | What they cover |
|---|---|---|
| The framework | 2 | Where the lesion sits — conductive, sensory, neural, central — and where the implant enters. |
| Childhood causes | 3–5 | Congenital and perinatal causes; meningitis and cochlear ossification; congenital CMV and the in-utero infections. |
| Acquired causes | 6–10 | Ototoxicity; noise; presbycusis; sudden and autoimmune loss; Ménière's and otosclerosis. |
| Reading & applying | 11–12 | What the temporal bone reveals; and how each cause translates into candidacy, surgery and prognosis. |
We begin with the framework that makes sense of all the rest — where the lesion sits (Module 2).
What is the best reason their prospects may differ?
Why is the cause of sensorineural hearing loss prognostically useful even when the audiogram is identical between patients?
Which phrase best captures sensorineural hearing loss as framed in this chapter?