Intraoperative objective measurements in synaptopathy/neuropathy: Extra-/intracochlear ECochG monitoring and E-CAP/-ABR diagnostics in cochlear implant recipients

The provision of a cochlear implant (CI) effectively helps patients with sensorineural hearing loss (SNHL). Auditory neuropathy spectrum disorder (ANSD) is a rare cause of SNHL and may affect CI outcome. A total of three patients, comprising one adult and two pediatric cases, exhibited indications suggestive of bilateral functional deafness caused by auditory neuropathy spectrum disorder (ANSD). Consequently, each of these patients was provided with a cochlear implant (CI) in both ears. Prior to CI surgery, auditory brainstem responses (ABR) and transtympanic electrocochleography (ECochG) were performed to confirm the diagnosis of ANSD. Intracochlear ECochG with either pure tone burst or broadband chirp was performed on each patient during electrode insertion via the implant electrode array. E-ABR recordings and measurement of electrical compound action potentials (E-CAP) were performed sequentially. Preoperative ECochG and ABR showed the presence of cochlear microphonics (CM) but the absence of CAP. The hypothesis underlying the study is that in ANSD the CM is detectable and the CAP and the ABR waves J1 and J5 are absent, whereas in hearing supported or restored by a CI all of these responses are present. Intraoperative measurements facilitate a differential diagnosis of perisynaptic disorders (postsensory and pre-neural) and allow for an immediate determination at the conclusion of the surgical procedure that CI fitting for these patients is highly probable to be successful.