11When the lesion changes the calculus

TIt depends where the lesion sits

Two ears with the same audiogram can call for completely different solutions, because the audiogram does not say where the lesion is. The useful question is whether amplifying sound through the ear, or stimulating the nerve directly, makes sense given the site of damage — and for several conditions the answer departs sharply from the textbook crossover.

CAuditory neuropathy

In auditory neuropathy spectrum disorder (ANSD) the outer hair cells often work — otoacoustic emissions are present — but the neural signal is desynchronised, so the timing the brain needs is scrambled. Hearing aids, which only make the (already audible) sound louder, frequently fail. A cochlear implant, by imposing its own synchronous pattern of nerve firing, can restore good speech — strikingly so for otoferlin (OTOF) ANSD, a pre-neural lesion (Chapter 6). Here the implant succeeds precisely where the aid cannot.

CSingle-sided deafness

In single-sided deafness, one ear is dead and the other normal. There is nothing in the dead ear for an acoustic hearing aid to amplify; a CROS aid merely routes sound to the good ear, restoring no true input to the deaf side. A cochlear implant in the deaf ear restores real input there, can quieten tinnitus, and gives back some binaural hearing — an indication that has grown as outcomes have improved.

CWhen neither has a target

Finally, the implant has its own limit: it needs a nerve to stimulate. Where the cochlear nerve is absent (cochlear nerve deficiency) or destroyed — as in neurofibromatosis type 2 — even the implant may give little, and an auditory brainstem implant, which bypasses the nerve to stimulate the brainstem directly, becomes the only option. The lesson of the module is symmetrical: just as the cochlea's failure sends a patient from aid to implant, the nerve's failure can send them beyond the implant altogether.