Journal Club: Comparison of intracochlear electric potential between different cochlear array

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Today's journal article

Söderqvist S, Sivonen V, Huber A, Sinkkonen ST, Sijgers L. Intracochlear electric potential measurements for estimating electrode array position in cochlear implantation: The in vivo utility of an ex vivo model. 

Why I picked this article

This research explores how we can use intraoperative electrical data, something already collected during cochlear implant surgery, to estimate electrode placement and cochlear anatomy in real time to improve the outcome of the cochlear implant surgery. 

Cochlear implant surgery is a highly successful surgery to restore the sense of hearing in those with severe or profound sensorineural hearing loss. The cochlear implant, an electrode array, is inserted along the cochlear spiral, and the electrical current will directly stimulate the auditory nerve, bypassing damaged sensory cells of the cochlea. The electrical current is applied between an intracochlear electrode and the reference electrode outside the cochlea, in a monopolar fashion; a fraction of the current will reach the auditory neurons, and the rest flows towards the base of the cochlea. To make the most of the limited current flowing towards the auditory neurons, establishing good contact between the implant is very important for effective use. 

Most cochlear implants are able to record "the stimulation-induced intracochlear electric potentials", which, across non-stimulating contacts, are referred to as transimpedances by the researchers. In this research, the researchers developed and verified a model that predicts the cross-sectional area of the cochlea (Ascala) and the distance between each electrode and the medial wall (dEM). This could enable the use of transimpedances more effectively to get feedback on the surgery and effective use of the cochlear implant. 

From pixabay.com

Some of the research findings

Input data:

  • clinical data from twenty-nine adult patients (age 55.4 ±16.5 years; mean ±SD)
  • recipients of either Cochlear Nucleus CI622 Slim Straight or CI612 Contour Advance electrode array (Cochlear Ltd)
  • These two are also compared against each other; Slim Straight (lateral wall), Contour Advance (perimodiolar)

Measurements: 

  • Transimpedance Matrix (TIM): measures how current spreads between electrodes
  • Four-point impedance (Zfp): measures local voltage changes around each contact 
  • Both metrics depend on the cochlear size and how close the electrode is to the medial wall of the scala tympani.

Finding: 

  • The researchers first used temporal bone samples to develop a predictive model using TIM and Zfp data to estimate Ascala and dEM, and compared with the patient data. 
  • There was a difference between specimens in how impedance changed with different array type. 
  • Predicted local potential predicted using Zfp was lower with a lateral wall electrode array in vivo (in patient), compared to the perimodiolar array ex vivo (temporal bone). 
  • Lateral wall arrays showed lower impedance and slower potential decay compared to perimodiolar arrays. 
  • The model predicted Ascala with good accuracy: 73% variance explained (with random effects). Overestimation was most noticeable in the apical region. 

Authors suggest that this model may allow Ascala (the cochlear cross-section) to be reliably estimated intraoperatively using TIM and Zfp, and that array-specific calibration (different models for different electrode types) could improve prediction accuracy in the future. 

Haruna's takeaway

The introduction of this article was very helpful to understand and view the cochlear implant and the cochlea as an electric current-producing device and an environment! This was another publication about the use of "live" measurement obtainable from a cochlear implant. I wonder how much impact this "feedback" from cochlear implants will have in the future, for the better outcome for patients and subsequent future cochlear implant design. 

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This is Haruna's 45/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.