Exploring asymmetry of electrical compound action potential latencies in sequential cochlear implants as a possible effect of the lateral olivocochlear bundle

Electrophysiological evaluations of the second cochlear implant (CI) in sequentially implanted patients usually consider the brainstem with electrically evoked auditory brainstem responses and the auditory cortex with slow responses. To further understand auditory electrophysiological behaviour after the second implantation, the present study investigated auditory nerve activity using electrical compound action potential (ECAP).

Of 167 bilaterally implanted patients, 73 were evaluated and divided into 2 groups: patients with simultaneous (33 children and 8 adults) and sequential (21 children and 11 adults) surgeries. The sequential surgeries were further grouped into "short delay" (< 4 years) and "long delay" (> 4 years), considering the time spent between procedures. Every patient was assessed by evaluating the N1 wave of ECAP, focusing on the latencies in both ears.

The ECAP's interaural latencies were not statistically different in simultaneous patients and sequential progressive/acquired deaf adults. In contrast, in sequential congenital deaf children, the latencies of the second implant were temporally earlier than those of the first implant, but symmetry recovered within 2 years after surgery.

The results revealed that late-sequential CI stimulation generates asymmetric interaural activity at the first neuron in congenitally deaf children. Such an asymmetry is due to the anticipation of the sequential N1 wave ECAP, rather than the expected delay. The hypothesis attributes it to a temporary lack of efferent system tuning, particularly the lateral olivocochlear bundle, which should still be active in implantees. These findings could enhance the benefits of sequential CI mapping by implementing a stimulation delay with a temporal duration similar to N1 anticipation until binaural processing normalises.