An electrically-evoked compound action potential marker for local spiral ganglion neuron degeneration: the Failure Index

Spiral ganglion neuron (SGN) degeneration is a candidate factor for reduced hearing outcomes in cochlear implant (CI) users. However, there is no procedure available to identify CI contacts close to focal SGN degeneration in human patients. In an animal model, we assessed the impact of focal SGN degeneration on electrical responsiveness and derived an electrophysiological marker for the presence, location, and size of such lesions. We introduced cochlear microlesions in 13 guinea pigs (six female) and recorded electrically evoked compound action potentials (eCAP) after 8-12 d. These were compared with recordings from controls ( N  = 8) and acutely lesioned cochleae ( N  = 12). We stimulated via 6-contact CIs in monopolar configuration with symmetric, biphasic pulses of alternating polarity. We histologically assessed the lesion and its relative position to the CI contacts. The lesions (230-850 µm) significantly elevated thresholds and reduced amplitudes. The effect was found at stimulation distances of 3.5 mm from the lesion. A novel eCAP marker, Failure Index (FI: maximal input/output ratio), was significantly elevated in the presence of degenerated SGN. It indicates the failure to efficiently transduce the stimulation current into neural activation (N1P1 amplitude). The FI showed classification accuracies of 80% and identified contacts closest to the lesion in ∼80% of cases within ±700 µm (∼electrode spacing) from the lesion site and was correlated with the lesion size. Thus, the FI is a clinically relevant, noninvasive marker that is suitable for clinical datasets without a priori knowledge on lesions, when combined with the statistical method of median splitting.