Journal Club: Cochlear implant electrical impedance; from recording to analysis and modelling as a mean to understand what's happening in the cochlea

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Today's journal article

Veloso de Oliveira J, Weiss NM, Wimmer W. Comprehensive decomposition of cochlear implant electrode impedances. 

Why I picked this article

This research evaluates the effectiveness of algorism to analyse the electrical impedance of the cochlear implants. 

Cochlear implant surgery is a highly successful intervention where an electrical wire is surgically inserted into the cochlea and used to stimulate the auditory nerve directly to restore the sense of hearing. Cochlear implants can restore hearing in those with profound hearing loss. Since its effectiveness and safety have been established, the current research effort surrounding cochlear implantation is how to improve the clinical outcome further for the patients and to improve the cost and safety profiles. 

It has been known that the performance of a cochlear implant can be variable depending on some surgery-associated factors, such as how deeply the cochlear implant was inserted (i.e. how closely the cochlear implant sits next to the auditory neurons). The response from the host cochlea has also been demonstrated to contribute to a decrease in the performance of a cochlear implant; the mechanical stress from the surgery and/or foreign body response can trigger inflammation in the inner ear and form scarring (= fibrosis) around the cochlear implant, which decreases the effectiveness of the cochlear implant.  

Electrical impedance was measured for the cochlear implants and analysed into different components that correlate with some of the performance variability associated with cochlear implants. As the technology emerges to monitor the impedance of the cochlear implant live in the patient through clinical telemetry, there is a potential that the electrical impedance model can provide diagnostic insights into the cochlea following cochlear implantation surgery. 

Some of the research findings

Patient data:

  • n = 24 individuals 12 years old or older with a MED-EL brand cochlear implant (received between Dec 2023 - May 2025)
  • Cochlear implant electrodes are inserted through the round window
  • Average age was 62.6 years old, a mixture of males and females
  • 23 unilateral implants, one bilateral 

Impedance models used in this study are based on a number of literature. Basic concepts are: 

  • A single stimulating electrode has: 
    • capacitance and resistance at the electrode - fluid contact site
    • resistance from the vicinity of the electrode (further divided into near-field and far-field)
    • longitudinal resistance along the cochlea
  • There are longitudinal currents along the cochlea from the apex of the cochlea towards the ground electrode at the base, and the leak current that moves towards the outside of the cochlea. 
Part of Figure 1. Impedance network model (de Oliveira et al. 2025)

Data collection and analysis:

  • The impedance telemetry (Maestro 9.0, MED-EL)
  • The electrode impedance can be determined by using the same electrode for stimulation and recording.
  • Recording made "in a monopolar configuration using biphasic charge-balanced stimuli with cathodic-leading polarity and an inter-phase gap of 2.1 μs."
  • This normalized voltage is commonly referred to as the clinical impedance 𝑍c.
  • 12-electrode array cochlear implant system. 
  • Multi-step algorithm was used to estimate the electrical components. 
  • The fit quality of the models to recorded data was evaluated first, then the change over time following cochlear implants was estimated. 
  • Components from each of 12 electrodes were explained
Key findings:
  • Zc measured across pulse duration (24.2 - 212.5us) ranged between 1 - 8kΩ in day 0, peaking between 6 - 18 kΩ in 26- and ranged between 5 - 13kΩ in day 27-216- post surgery. 
  • Warburg capaci-tance averages at24.0 nF ± 12.1 nFover all electrodes
  • Far-field resistance component was more stable long term
  • There was a good agreement between different models tested.
  • Comparison between 12 electrodes along the cochlear turn shows that the far-field sub-component of the resistance increases from the basal to the apical region, likely corresponding to the narrowing of the cochlear geometry. 
  • Measurement takes only ~2 min in total, and can be easily integrated into the clinical routine. 

Haruna's takeaway

I appreciated this publication for its clarity and very good figures, especially given my lack of background in modelling. While acknowledging that I probably didn't understand everything, it was a very interesting read.  It is incredible that the cochlear implant can provide some live recording to provide a parameter that can correlate with the performance of the cochlear implants. Two key developments almost here in cochlear implants are: the telemetry, live monitoring of the cochlear impedance as used in this publication to provide input data, and the drug-eluting cochlear implant that has "add-on" drug administration  capabilities. Very exciting. 

I wonder if affiliated researchers are planning any use of clinical telemetry cochlear implants in the sheep or pig large animal model, as that could be very interesting as a live recording opportunity with some experimental manipulations. 

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This is Haruna's 27/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.