6The Ear and the Nerve: Peripheral Predictors

FResidual hearing and aetiology

Greater preoperative residual hearing, in the implanted ear, the contralateral ear, or both, predicts better postoperative speech recognition; residual hearing acts as a marker for a healthier auditory periphery and pathway. Aetiology matters chiefly through the neural signature it leaves: causes that spare ganglion cells and the nerve (for example many cases of genetic or noise-related loss) tend to do better than those that destroy neural elements. Meningitis is a special case, it can cause both profound deafness and cochlear ossification, and the inflammatory neural damage can cap outcome independently of how well the array is placed. Auditory neuropathy spectrum disorder with a structurally intact nerve can do well, whereas the same audiogram from cochlear nerve deficiency does not.[2020][2009]

TThe neural substrate: ganglion cells and the cochlear nerve

Spiral ganglion neurons and the cochlear nerve are the cells the implant actually drives; their survival is the biological substrate of performance and is reduced, often markedly, in profound deafness. Counterintuitively, temporal-bone studies of former implant users found either no relationship or even a negative correlation between counted ganglion-cell number and the word scores those patients achieved in life. This implies that above some low threshold of surviving neurons, the brain rather than the raw neuron count limits performance, so histology is a poor individual predictor. Electrically evoked compound action potentials index neural responsiveness and are valuable for fitting and integrity checks, but they correlate only loosely with speech outcome and cannot substitute for behavioural results.[2009][2014][2005]

CElectrode position: scala, translocation and insertion depth

Closer proximity of the array to the modiolus has been associated with better open-set word recognition, supporting perimodiolar placement when hearing preservation is not a goal. Scalar translocation, the array crossing from scala tympani into scala vestibuli or scala media, is consistently associated with poorer speech outcomes and is the position variable with the clearest negative effect. Insertion depth modifies outcome through frequency mismatch: shallow insertions that incompletely cover the array's assigned frequency range correlate with poorer word recognition, and very deep insertions can produce an apicalward pitch mismatch. Perimodiolar arrays achieve modiolar proximity but carry a higher translocation risk than lateral-wall arrays, so the position benefit is realised only with disciplined surgical technique.[2013][2020]

CHard ceilings: cochlear nerve deficiency

Cochlear nerve aplasia (absence) or hypoplasia (a small nerve) imposes a ceiling that no electrode optimisation can lift, since there are too few fibres to stimulate. A narrow internal auditory canal on CT or a small or absent nerve on high-resolution MRI flags this risk preoperatively and changes counselling toward guarded or, in aplasia, potentially toward auditory brainstem implantation. Nerve hypoplasia spans a spectrum: some hypoplastic nerves still support useful, if limited, open-set outcomes, so imaging findings guide counselling rather than dictating exclusion. Cochlear nerve status is one of the few peripheral predictors that can be assessed in life and that meaningfully caps the achievable range, making dedicated nerve imaging essential when deficiency is suspected.[2020][2014]