By George Burwood, Ph.D.
A team of scientists and clinicians at Oregon Health & Science University recently published a study about how the inner ear responds to a cochlear implant—the first to use optical coherence tomography to analyze cochlear changes following implantation.
Our study, devised by Jeffrey Hyzer, M.D., in collaboration with Lina Reiss, Ph.D., was intended to explore the effects of age and steroid treatment on hearing threshold shifts following cochlear implantation in young and aged guinea pigs. Our results were published in the journal Otolaryngology–Head and Neck Surgery in March 2024.
Older guinea pigs were implanted and some were provided with a typical steroid taper treatment. It was hypothesized that older animals would benefit from steroid treatment. Additionally, a cohort of younger animals were implanted—here, the hypothesis was that younger animals would experience less threshold shift following implantation than the non-steroid treated older group.
Our study was also the first to feature optical coherence tomography (OCT) imaging to appraise the amount of fibrosis present in the basal turn of the guinea pig cochlea following cochlear implantation. The technique was part of my 2023 Emerging Research Grant project on changes in low frequency, inner ear mechanical function following cochlear implantation.
This optical coherence tomography image shows an older guinea pig that was given steroids with a yellow outline indicating the fibrosis/ossification seen in the scala tympani. Electrode is seen entering through an extended round window cochleostomy. Credit: Hyzer et al./Otolaryngology–Head and Neck Surgery
Our project concluded that, indeed, younger guinea pigs (8 weeks old at time of implantation) experienced less hearing loss due to cochlear implantation than older guinea pigs (19 months at the time of implantation). Further, animals treated with a steroid taper, beginning prior to surgery and ending days following surgery, experienced improved hearing preservation compared to animals without this drug intervention.
Interestingly, hearing thresholds, measured by auditory brainstem response, were not correlated with the amount of fibrosis estimated from the OCT scans of the guinea pig inner ear. There was a non-statistically significant association with age, fibrosis, and use of steroids, where younger animals appeared to develop more fibrosis than older animals, and those with steroids had the least fibrosis.
The implication from this data is that the amount of fibrosis does not predict how that fibrosis will affect residual hearing preservation following cochlear implantation. Other factors, such as the mechanical properties of the fibrosis and its distribution, may be more important. These were not examined in this study.
The questions left open by our study are being addressed by further experiments using the rodent model of cochlear implantation, where I am measuring the behavior of both the middle and inner ears mechanically, using OCT vibrometry, as well as measuring the amount of fibrosis present using OCT imaging.
The causes behind residual hearing loss following cochlear implantation are complex and varied. Studies such as this one provide clues as to what researchers and clinicians should focus on in order to improve the acoustic experience of cochlear implant recipients.
A 2023 Emerging Research Grants scientist, George Burwood, Ph.D., is a research instructor at Oregon Hearing Research Center, Oregon Health & Science University. His colleague Lina Reiss, Ph.D., is a 2012–2013 ERG scientist.
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