Four-point impedance as a biomarker for bleeding during cochlear implantation

Abstract

Cochlear implantation has successfully restored the perception of hearing for nearly 200 thousand profoundly deaf adults and children. More recently, implant candidature has expanded to include those with considerable natural hearing which, when preserved, provides an improved hearing experience in noisy environments. But more than half of these patients lose this natural hearing soon after implantation. To reduce this burden, biosensing technologies are emerging that provide feedback on the quality of surgery. Here we report clinical findings on a new intra-operative measurement of electrical impedance (4-point impedance) which, when elevated, is associated with high rates of postoperative hearing loss and vestibular dysfunction. In vivo and in vitro data presented suggest that elevated 4-point impedance is likely due to the presence of blood within the cochlea rather than its geometry. Four-point impedance is a new marker for the detection of cochlear injury causing bleeding, that may be incorporated into intraoperative monitoring protocols during CI surgery. The preservation of cochlear structure and residual functional hearing has become the standard of care for cochlear implantation (CI). Hearing preservation is important to facilitate combined electrical and acoustic stimulation of the cochlea, as this improves speech recognition in noise and music appreciation 1-4. Cochlear structural preservation will ensure that the ear is ready for future, regenerative therapies 5,6. Structural and functional preservation of the cochlea depends not only upon the electrode design, but also the surgery. Electrodes must be introduced into the cochlea without causing injury. Until recently, technologies have not existed to guide the surgeon during the implant procedure; the operation has been conducted "blind" without the provision of feedback. Over recent years, we and others have begun to monitor cochlear function during cochlear implantation 7-10 , using the CI's own electrodes to monitor the electrophysiological response of the ear to acoustic stimulation. This technique, known as electrocochleography, has provided valuable information to guide surgeons during the operation; if the electrophysiological response is preserved during surgery, residual hearing is better after implantation 7-10. This paper is motivated by a desire to increase the scope of intraoperative monitoring during CI surgery. Current methods allow real-time detection of cochlear dysfunction, but these do not assess cochlear injury directly. Here we report on a method that has this potential. We have monitored "four-point" electrical impedance (4PI) from the implant's intracochlear electrodes during CI surgery. This impedance measurement is acquired by passing current between two outer electrodes whilst the voltage (from which the impedance may be inferred) is measured between two inner electrodes (Fig. 1A). The method is believed to assess the bulk impedance between the two inner electrodes, and has been used to differentiate between tissue and fluid ...

Cochlear Implant Electrode Impedance as Potential Biomarker for Residual Hearing

“…In these cases, the ear can be stimulated both acoustically (via the hearing aid) and electrically (via the implant), resulting in even better speech understanding (6,7). In case of unsuccessful hearing preservation, intraoperative loss or residual hearing is thought to be the result of traumatic events during the insertion and implantation of the electrodes (8)(9)(10)(11). Postoperative hearing loss, on the other hand, is suspected to be caused by an immune reaction to the electrode array (12), intracochlear inflammatory responses, e.g., to blood components entering the inner ear (13), intracochlear scar tissue formation (14), or a progression of hearing loss independent of the implanted device (1).…”

“…Sudden increases in postoperative impedance values (so-called “impedance spikes”) were linked with inner-ear events leading to loss of residual hearing and vertigo ( 18 , 19 ). In an animal model, Bester et al ( 8 ) reported higher impedances in the presence of blood clots around the electrode, which could initiate an inflammatory response that affects the residual function of the inner ear. However, despite the promising indication, impedance telemetry remains a non-established monitoring tool.…”

Predicting Cochlear Implant Electrode Placement Using Monopolar, Three-Point and Four-Point Impedance Measurements

“…When repeatedly inserting CI electrode arrays into the cochleas of cadaveric temporal bones, it is likely that pieces of tissue, blood, or foreign material enters the cochlea at some stage. This may confound impedance changes resulting from variations in electrode-modiolar proximity as four-point impedances have been shown to be sensitive to intracochlear bleeding [21]. Impedances are further confounded by translocations of the electrode array into the scala vestibuli, which we do not control for in our analysis.…”

“…1, bottom right). The method is believed to assess the bulk impedance between the two inner electrodes and was recently used to detect intracochlear bleeding [21]. We hypothesized that three-point and four-point impedance measurements are less confounded by patient-specific electrical tissue properties and hence could be better predictors of electrode-modiolar distances than monopolar impedances.…”

Prevalence of Endolymphatic Hydrops in Cochlear Implant Candidates with Idiopathic Profound Sensorineural Hearing Loss

“…The preservation of remaining cochlear and vestibular function has become a goal of all CI operations. Surgical factors such as entering of blood or bone dust into the cochlea (35) as well as trauma elicited by the insertion of the CI electrode array are important factors in regard of function preservation (36). Furthermore, the postsurgical inflammation plays a major role (19).…”