- Approximately 5 minutes to read
- Learning objectives:
- Understand why and how impedances provide valuable clinical information
- Understand what factors impact impedances
- Understand how the clinician can use impedance measurements to determine the need for programming changes
Why are we talking about impedances?
As cochlear implant audiologists, the concept of impedances is integral to our clinical lexicon. Due to the growing interest in a more in-depth understanding of impedances, our clinical team has compiled a summary that delves into the science of impedances in cochlear implant care and outlines the essential information for programming cochlear implants.
What are impedances?
When electrical current flows in a conductor (in this case, the cochlea) there is a force that opposes the flow of the electrical current through the cochlea to the spiral ganglion cells. This is known as resistance. All materials naturally contain some resistance to the flow of an electrical current. Electrode impedances measures this resistance to the flow of electrical current from the cochlear implant electrode to the spiral ganglion cells.
Electrical impedance is calculated as the ratio of the applied voltage (driving force) to the resulting current that flows through the material or solution.
How does Cochlear calculate impedances?
Cochlear calculates the total impedance which includes the Polarization Impedance and the Access Impedance. Polarization Impedance looks at the electrode-electrolyte interface. Polarization impedances are caused by electrochemical effects at the electrode’s electrode-fluid interface. Access Impedance looks at the bulk material around the electrode. High access impedances are caused by the presence of fibrous tissue or new bone growth around the electrode array. To have an accurate understanding of the surface quality change of the electrodes, it’s necessary to assess both Access and Polarization Impedances. Total impedance offers the most complete picture of impedance and its effect on compliance, power requirements, and MAP parameters.
Impedance in a tissue-stimulating system is more complex than a simple material or solution due to the dynamic behavior of the electrode-tissue interface. Biological processes at a cellular level can change the electrode-tissue interface and therefore also affect the impedance of the system over time frames ranging from seconds to minutes to much longer periods.
Impedance measurements are included in Cochlear™ Nucleus® implants to perform three functions:
- To confirm the integrity of the electrode contacts through detection of open and short circuits.
- To ensure the map is set to maintain electrodes in compliance.
- To provide some insight into the characteristics of the local environment around the electrode to assist with clinical management.
What factors can impact impedances?
Conditions unrelated to a patient’s hearing have been associated with changes in hearing outcomes or impedance. In these cases, it may take an extended time for clinicians to find out that a recipient is on contributing drug therapy since the recipient may not associate other health conditions with challenges in their hearing. Papers have associated impedance rises to Meniere’s disease and tinnitus1, labyrinthitis2, otitis media3, 4, delayed hearing loss5, and episodes of vertigo2, 6. In these instances, the impedance change is an indicator of some process that is happening in the inner ear.
There are reports involving bilateral recipients where impedances are seen to rise in a single ear1, and others where the process involves both ears2 suggesting a more systemic involvement. Interestingly one bilateral case with different model implants experienced increased impedance in both implants at the same time2.
A number of papers report on the potential for pharmaceuticals to reduce electrode impedance. In one case treatment with cortisone, antibiotics and widening pulse widths resulted in impedances falling over 25 kohm to under 14 kohm within 24 hours2.
Why is it important to monitor impedances?
Alterations in impedance can lead to reduced performance in patients. By managing these changes through medical intervention or by adjusting the pulse width, these issues can be addressed, resulting in optimization of the recipient’s programming.
How do we use this information clinically?
Regular measurement of electrode impedances is considered good clinical practice, typically performed at each programming session. For adults and children capable of providing feedback on their cochlear implant’s sound quality, routine monitoring of the impedance profile, alongside speech perception measures and other case information, constitutes a best management approach. This practice establishes a historical record of impedance measurements, facilitating correlation with additional recipient data collected over time. Should any electrodes fall out of compliance, it is common practice to increase the pulse width across the array to return these channels to compliance, rather than deactivating.
Additionally, electrode impedance measurements detect circumstances under which certain electrodes should not be used, such as open or short circuits. Since each electrode interacts with its local environment, impedances can vary from electrode to electrode across the array. An unexpected impedance pattern calls for a detailed review of the electrode impedance data in context with the recipient’s imaging, programming, medical history, and auditory performance.
Please reach out to your local Cochlear representative to learn more about what impedance measurements can tell you about your patient’s cochlear implant.
1 McNeill, A., & Eykamp, K. (2016). Cochlear Implant Impedance Fluctuation in Meniere’s Disease: A Case Study. Otology & Neurotology, 37(7), 873-877.
2 Neuburger, J., Lenarz, T., Lesinski-Schiedat, A., & Büchner, A. (2009). Spontaneous increases in impedance following cochlear implantation: Suspected causes and management. International Journal of Audiology, 48(5), 233-239.
3 Vargas, J.L., Sainz, M., Roldan, C., Alvarez, I., & Torre, A. (2012) Long-tern evolution of the electrical stimulation levels for cochlear implant patients. Clinical and Experimental Otorhinolaryngology, 5(4), 194-200.
4 Dixon, J.F., Shinn, J.B., Adkins, M., Hardin, B.D., & Bush, M.L. (2015). Middle Ear Disease and Cochlear Implant Function: A Case Study. Hearing Balance and Communication, 12, 155-158.
5 Scheperle, R.A., Tejani, V.D., Omtvedt, J.K., Brown, C.J., Abbas, P.J., Hansen, M.R., Gantz, B.J, Oleson, J.J, & Ozanne, M.V. (2017). Delayed changes in auditory status in cochlear implant users with preserved acoustic hearing. Hearing Research, 350, 45-57.
6 Plontke, S.K., Götze, G., Rahne, T., & Liebau, A. (2017). Intracochlear drug delivery in combination with cochlear implants. HNO, 65, 19-28.
