Applications of Phenomenological Loudness Models to Cochlear Implants

Developing a Virtual Model of the Rhesus Macaque Inner Ear

A virtual model of the rhesus macaque inner ear was created in the present study. Rhesus macaques have been valuable in cochlear research; however, their high cost prompts a need for alternative methods. Finite Element (FE) analysis offers a promising solution by enabling detailed simulations of the inner ear. This study employs FE analysis to create a virtual model of the rhesus macaque's inner ear, reconstructed from MRI scans, to explore how cochlear implants (CIs) impact residual hearing loss. Harmonic-acoustic simulations of sound wave transmission indicate that CIs have minor effects on the displacement of the basilar membrane and thus minimally impact residual hearing loss post-implantation, but stiffening of the round window membrane worsens this effect. While the rhesus macaque FE model presented in this study shows some promise, its potential applications will require further validation through additional simulations and experimental studies.

A new objective method to estimate the charge integration efficiency in cochlear implant users

OBJECTIVE

The present study aimed to objectively assess the charge integration efficiency (CIE) of the auditory nerve using electrically-evoked stapedial reflex threshold (eSRT) measurements in paediatric cochlear implant users.

DESIGN

The eSRT was estimated in two ways: by keeping pulse width constant while increasing pulse amplitude and vice versa. The electrical dynamic range (EDR) obtained for eSRT was measured with increasing pulse amplitude (EDRPA) and pulse width (EDRPW) by calculating the difference in charge units between eSRT and behavioural thresholds; further, CIE was estimated.

STUDY SAMPLE

Fifteen paediatric cochlear implant users.

RESULTS

EDRPW was 1.4 to 1.7 times larger than EDRPA. The EDRPW was significantly greater than the EDRPA. A significant correlation was observed between both EDR measures. However, the CIE measure did not significantly correlate with the behavioural threshold. Further, there was no significant effect of electrode location on the CIE.

CONCLUSIONS

The CIE estimated with objective measures such as eSRT is comparable to those estimated using behavioural loudness judgments. eSRT can serve as a quick method to estimate the CIE, which can be easily implemented in the paediatric population. The CIE could be a valuable tool for differentiating electrodes with excellent and poor charge integration efficiency.

Management of Severe Facial Nerve Cross Stimulation by Cochlear Implant Replacement to Change Pulse Shape and Grounding Configuration: A Case-series

OBJECTIVES

To investigate the combined effect of changing pulse shape and grounding configuration to manage facial nerve stimulation (FNS) in cochlear implant (CI) recipients.

PATIENTS

Three adult CI recipients with severe FNS were offered a replacement implant when standard stimulation strategies and programming adjustments did not resolve symptoms. Our hypothesis was that the facial nerve was less likely to be activated when using anodic pulses with "mixed-mode" intra-cochlear and extra-cochlear current return.

INTERVENTION

All patients were reimplanted with an implant that uses a pseudo-monophasic anodic pulse shape, with mixed-mode grounding (stimulus mixed-mode anodic)-the Neuro Zti CI (Oticon Medical). This device also allows measurements of neural function and loudness with monopolar, symmetric biphasic pulses (stimulus MB), the clinical standard used by most CIs as a comparison.

MAIN OUTCOME MEASURES

The combined effect of pulse shape and grounding configuration on FNS was monitored during surgery. Following CI activation, FNS symptoms and performance with the Neuro Zti implant were compared with outcomes before reimplantation.

RESULTS

FNS could only be recorded using stimulus MB for all patients. In clinical use, all patients reported reduced FNS and showed an improvement in Bamford-Kowal-Bench sentences recognition compared with immediately before reimplantation. Bamford-Kowal-Bench scores with a male speaker were lower compared with measurements taken before the onset of severe FNS for patients 1 and 2.

CONCLUSIONS

In patients where CI auditory performance was severely limited by FNS, charge-balanced pseudo-monophasic stimulation mode with a mixed-mode grounding configuration limited FNS and improved loudness percept compared with standard biphasic stimulation with monopolar grounding.