gms | German Medical Science

Introduction: Cochlear implant (CI) provision effectively aids patients with sensorineural hearing loss (SNHL). Auditory neuropathy spectrum disorder (ANSD) is a rare cause of SNHL, characterized by intact outer hair cells in the cochlear but unresponsive inner hair cells, the synaptic transmission, or spiral ganglion neurons. ANSD diagnosis is based on abnormal patterns on the results of otoacoustic emissions (OAE), electrocochleography (ECochG), and auditory brainstem responses (ABR). In the case of cochlear microphonic (CM) responses without action potential and/or OAE and an unusual ABR, ANSD is suspected.

Inserting the CI electrode risks the cochlear function. ECochG recording by means of CI telemetric measurement may provide information of inner ear damage by assessing acoustically evoked potentials from cochlear hair cells and compound action potentials (CAP) from auditory nerve fibres. We aimed to assess hearing preservation in CI recipients with synaptopathy/neuropathy by reviewing case histories and using diverse ECochG stimuli during and after CI surgery.

Methods (case series): Three CI recipients (one adult, P1, and two children, P2/3) were diagnosed with bilateral ANSD. P1 suffered from progressive hearing loss with tinnitus, while P2 and P3 developed no hearing function despite the presence of OAE. Preoperative ECochG of all patients showed present CM, but ABR responses were either dysmorphic or completely absent.

All patients received FLEX electrodes (MED-EL, Innsbruck) inserted via the round window. Intracochlear ECochG recordings of CM responses were obtained during and after full insertion. Acoustic stimuli were 500 Hz tone bursts, narrowband chirp (2 octaves), and broadband chirp (4 octaves) acoustic stimuli. Furthermore, P1 and P2 were evaluated postoperatively at 4 and 6 months after surgery with intracochlear ECochG.

Result and discussion: CI insertion was performed without complication. Intracochlear ECochG showed detectable CM amplitudes (up to 50 µV) in all cases. Latency and phase changes were observed. In addition, broadband chirp stimuli provide multi-frequency responses, while sinusoidal tone bursts only preset single frequency responses. On postoperative follow-up recording, P1 & P2 still show CM responses along all electrode contacts.

Conclusion: CI implantation is a viable therapy for inner hair cell/synaptopathy disorders. Intracochlear ECochG measurements are feasible using either chirp or tone burst stimuli. However, the results are currently open to interpretation and not yet applicable for reliable hearing monitoring during surgery. Further studies are necessary to investigate the correlation between changes in ECochG response and insertion trauma, thereby inducing hearing loss.