The auditory brainstem implant (ABI) is a neurostimulator that bypasses the cochlea and auditory nerve and directly stimulates the cochlear nucleus (CN) in the auditory brainstem. Under experimental protocols with Investigational Device Exemption (IDE), ABIs recently have been used in the U.S. as a treatment option for children who have contradictions for cochlear implantation (CI), including absent or abnormally small auditory nerves or complete cochlear ossification. Programming speech processor of the ABI in prelingually deaf children is challenging due to their limited listening experiences and language skills. Moreover, many pediatric ABI users, regardless of age, have multiple disabilities or medical conditions that limit their abilities to provide reliable behavioral responses. Having an objective tool for optimizing programming parameters for these patients is clinically important. To date, results of two objective measures in pediatric ABI users have been reported: the electrically evoked auditory brainstem response (eABR) and the electrically evoked auditory event-related response (eERP).
ELECTRICALLY EVOKED AUDITORY BRAINSTEM RESPONSE
The eABR can be recorded intra- and post-operatively in pediatric ABI users under general anesthesia. eABRs recorded in adult and pediatric ABI users show similar morphological characteristics (Ear Hear. 2011;32:286 http://bit.ly/2cWZqQK; Ear Hear. 2011;32:300 http://bit.ly/2cWZW1h). In both patient populations, the eABR can consist of up to four vertex positive peaks occurring within the first five ms after stimulus onset (Ear Hear. 2015;36:368 http://bit.ly/2cX08xp; Int J Pediatr Otorhinolaryngol. 2012;76:257 http://bit.ly/2cX0150). The eABR waveform shows substantial variations across patients and across stimulating electrodes within individual pediatric ABI patients. In addition, eABR waveforms recorded intra- and post-operatively can be different.
During ABI surgery, the eABR can be used to guide electrode pad placement to maximize the number of stimulating electrodes that elicit auditory perception. The absence of the eABR typically suggests poor contact between the stimulating electrode and neural structures. This information will be useful if adjusting electrode placement is desired and feasible. However, the eABR appears to be poorly associated with patient's perception. For example, a robust eABR response does not guarantee auditory perception and vice versa (Ear Hear. 2011;32:300 http://bit.ly/2cWZW1h).
ELECTRICALLY EVOKED AUDITORY EVENT-RELATED RESPONSES
eERPs, including the onset response and auditory change complex (eACC), are cortically generated neural responses that can be recorded from surface electrodes placed on the scalp. The onset eERP and the eACC indicate sound detection and auditory discrimination capability at the auditory cortex, respectively (Ear Hear. 2008;29:285 http://bit.ly/2dleCd5). In contrast to the eABR, eERPs can only be recorded postoperatively when patients are awake.
The literature related to eERPs in ABI patients is scarce. Onset eERPs recorded in pediatric ABI users have only been reported in two studies (Ear Hear. 2015;36:377 http://bit.ly/2dldKoX; Ear Hear. 2016 http://bit.ly/2dler1F). Results of these studies showed that (1) there were substantial variations in response morphology across patients and across stimulating electrodes within individual patients; (2) the onset eERP demonstrated good test-retest reliability; and (3) auditory detection thresholds assessed using the onset eERP were strongly associated with those measured using behavioral procedures. Unfortunately, the results of these studies did not provide conclusive evidence of any association between eERP morphology and auditory vs. non-auditory perception.
The feasibility of measuring the eACC evoked by temporal gaps in pediatric ABI users has been recently established (He et al. Poster presented at ASA/ASJ Meeting, 2016). Figure 1 shows the eACCs recorded for stimulating electrode 19 in one pediatric ABI user. Gap durations used to evoke the eACC are labeled for these traces. The onset eERP is indicated using black triangles; the eACC is indicated using red triangles. For this patient, the objective gap detection threshold (i.e., the shortest gap duration that can be used to evoke the eACC) is 4 ms. The association between the eACC and auditory perception in ABI patients is under investigation.
In summary, the eABR and the eERP can be recorded in pediatric ABI users. The intraoperative eABR can be used to guide electrode pad placement. Its clinical application in optimizing programming parameters is limited. The onset eERP can potentially be used to estimate detection levels (i.e., T levels) in pediatric ABI users. Other potential clinical applications of the eERP need further investigation.