Structural alteration of hair cells in the contralateral ear resulting from extracochlear electrical stimulation

Abstract

Chronic electrical stimulation of the auditory nerve in patients with profound sensori-neural deafness is becoming increasingly routine. Therefore, it is important to understand more about the long-term consequences of this procedure. Hitherto, structural studies in animals after electrocochlear stimulation have concentrated on the stimulated cochlea. Here we have examined the effects of unilateral extracochlear electrical stimulation on the spiral organ of both the ipsilateral and contralateral ears of the mature guinea pig, and have found alterations in the structure of the outer hair cells and their efferent nerve terminals in the contralateral as well as the ipsilateral cochlea. This is the first evidence for a structural influence of efferent activity on the cochlea. Although the importance of the efferent system, consisting of the crossed and uncrossed olivo-cochlear bundles, is well established in providing central control of the sensory pathways, its exact role in hearing is incompletely understood. However, it is known that the outer hair cells and their efferent innervation are important in their contribution to inner hair cell responses and in modulating the micromechanics of the whole cochlea. These efferent functions now appear to be related to an important part of cochlear morphology, and are also relevant to our understanding of cochlear neurobiology, normal development and the management of hearing disability in both adult and child.

Interaction Between Electric and Acoustic Stimulation Influences Speech Perception in Ipsilateral EAS Users

“…This relation was stronger in subjects that received the individualized masking map due to significant electric masking. This effect might result in better overall performance after longer adjusting times and could help to preserve residual hearing by better training effects and less possible adverse electric stimulation ( Dodson et al 1986 ).…”

“…The results might indicate that a fitting strategy with less electric stimulation in the apical region is feasible for the clinical practice, as it reduces the amount of electric current the surviving hair cells are exposed to. As Dodson et al (1986) showed damage to the outer hair cells and the efferent functionality of the cochlear nerve due to loud electric stimulation, implementing a reduction of stimulation of apical electrodes in a standard fitting practice could help to protect outer hair cells in the long-term. Coco et al (2007) did not find an effect of chronic suprathreshold electric stimulation on the survival of inner and outer hair cells, however, they did not address the status of the hair cells.…”

“…At the same time, deeper insertion has been shown to adversely affect hearing preservation in the implanted ear ( O'Connell et al 2017 ). It increases the risk of insertion trauma ( Adunka & Kiefer 2006 ) resulting in hair cell apoptosis ( Eshraghi & van De Water 2006 ) as well as the possibility of adverse effects of electric stimulation on the survival of outer hair cells ( Dodson et al 1986 ).…”

Calcium signalling mediated by the α9 acetylcholine receptor in a cochlear cell line from the Immortomouse

“…At higher levels of ACh (100 μM), slow changes in the potassium currents and in the outer hair cell motility suggest that calcium release and activation of an enzyme cascade may occur (Dallos et al 1997). Evidence from whole‐animal experiments further suggests that an abnormal patterning of efferent stimulation may lead to degenerative changes in outer hair cells (Dodson et al 1986). In such experiments where the auditory system is stimulated by an indwelling unilateral electrode, alterations of the outer hair cells on the contralateral side are observed.…”

Effects of the intensified frequency and time ranges on consonant enhancement in bilateral cochlear implant and hearing aid users

“…The different effects of long-term electric and acoustic stimulation on peripheral and central auditory processing are also an important factor for influencing the ability to integrate auditory information (Gstoettner et al, 2006 ; Kronenberger et al, 2014 ; Skarzynski, 2014 ). Intense, long-term electric stimulation could cause damage in the outer hair cells and the efferent functionality of the cochlear nerve, particularly the apical regions, which results in negative neural processing in the higher auditory system (Dodson et al, 1986 ). One specific factor that affects integration for BCI users is binaural spectral mismatch, which can be evoked by various insertion depths of the electrode array into the cochlea of each ear (Yoon et al, 2011 ; Mukherjee et al, 2012 ; Mertens et al, 2020 ).…”