Comparison Between Transimpedance Matrix (TIM) Measurement and X-ray Fluoroscopy for Intraoperative Electrode Array Tip Fold-Over Detection

Time course of transimpedances is affected by cochlea implant surgical technique

BACKGROUND

For cochlear implant (CI) users after subtotal cochlectomy for removal of intracochlear schwannomas, significantly different intracochlear potentials can be observed compared with cochleae of CI patients after round window electrode insertion. The time course of this difference after surgery is so far unknown.

PURPOSE

The change of intracochlear potentials over time and differences between surgical approaches are investigated.

METHODS

In a retrospective study of cochlear implant recipients, the electrode transimpedances were measured and compared between 19 patients after subtotal cochlectomy and 20 patients after round window insertion. Transimpedances were measured with pulse widths of 37 µs and current units of 100 CL to 110 CL using a monopolar stimulation and recording mode (MP2). For each patient, at least three time points over a period of up to four years were used for time course analysis.

RESULTS

The transimpedances of patients after subtotal cochlectomy were significantly smaller than those of the round window group. The largest transimpedance changes over time were observed for basal electrode contacts in patients after round window insertion.

CONCLUSIONS

For the interpretation of transimpedances, it is relevant to consider the time since surgery and the CI insertion technique. The width of the electric field may be related to loss of intracochlear conductive fluid due to surgical trauma and fibrotic processes.

Evaluation of Transimpedance Matrix Measurement (TIM) heatmap for the assessment of cochlear implant electrode placement in patients with Inner Ear Malformations

<b>Introduction:</b> It is estimated that 20% of pediatric patients eligible for implantation have various forms of Inner Ear Malformations (IEM). Transimpedance Matrix Measurement (TIM) is a relatively novel electrophysiological measurement protocol of the impedance patterns of electrode contacts within the cochlea. <br><br><b>Aim:</b> Presentation of TIM measurements, in the form of a heatmap, in patients with IEM. <br><br><b>Materials and methods:</b> Among 112 implantations performed between 2018 and 2024 in the pediatric group, 23 patients were diagnosed with IEM (20.5%). TIM and Stenver's plain X-ray were performed for all patients. <br><br><b>Results:</b> In 19 patients, standard approach with mastoidectomy and posterior tympanotomy was performed, in 2 cases retrofacial approach, in 2 petrosectomy with blind sac procedure, and in 2 "banana technique". In most patients, electrode Contour Advance (CA) was used. One case of tip fold-over showed a characteristic cross pattern. For all other IEM there were normal diagonals but voltage distribution showed abnormal distribution within the cochlea. <br><br><b>Discussion:</b> Most patients received a CA electrode because of its greater rigidity and the possibility of inhibiting CSF leakage by applying a piece of connective tissue for sealing to the electrode. Heatmaps for different IEM types differ from each other, but it is still possible to recognize TFO. The voltage spread in the cochlea in IEM shows different patterns for particular types of anomalies. <br><br><b>Conclusions:</b> TIM measurement as a heatmap seems to be a quick and reliable method for detecting electrode array tip fold-over intraoperatively for patients with inner ear malformations. Our study complements the knowledge about the use of research based on TIM heatmap.

Characterization of Tip Fold-Over Using Fluoroscopy and Intracochlear Pressure in Cadaver Specimens

OBJECTIVES

Cochlear implant array malpositioning is associated with impaired speech perception, vertigo, and facial nerve stimulation. Tip fold-over is a subset of malpositioning that occurs more often with perimodiolar electrodes, but historically it has not been characterized due to lack of knowledge regarding electrode movements of the electrode within the cochlea. The aim of this study was to characterize the mechanics of tip fold-over events and their associated insertion pressure profiles.

METHODS

Cadaveric human heads were surgically prepared with a mastoidectomy and facial recess. Fiberoptic pressure sensors were inserted into the scala vestibuli and tympani to measure intracochlear pressures. Perimodiolar CI electrodes (Cochlear Slim-Modiolar, CI532) were inserted via round window under fluoroscopy.

RESULTS

Three types of tip fold-over events were observed: anterior-posterior C-shaped, medial-lateral C-shaped, and S-shaped roll-overs. The largest transient pressures occurred with anterior-posterior and S-type roll-over, and were associated with rotation or twisting inside the cochlea.

CONCLUSIONS

Results demonstrate at least three subtypes of tip fold-overs. Elevated pressure transients were noted before and during the tip fold-over event related to electrode twisting. The characterization of tip fold-over into subtypes is novel and may aid identification of tip fold-over events intraoperatively in the future. It remains important to identify tip fold-over events, and they should be recognized early using a multimodal verification system. Further investigation is still required to determine the significance of these changes and other possible patterns of intracochlear electrode movement.

LEVEL OF EVIDENCE

NA: Cadaver study Laryngoscope, 135:1795-1802, 2025.

Sensitivity and Specificity of Intraoperative TransImpedance Matrix Recordings Compared With X-ray Imaging in Detecting Perimodiolar Cochlear Implant Tip Foldovers: A Multicenter Study

OBJECTIVE

Characterize the sensitivity and specificity of TransImpedance Matrix (TIM) recordings compared with x-rays in detecting cochlear implant tip foldovers.

STUDY DESIGN

Retrospective.

SETTING

Multi-institutional, academic and private surgery centers.

PATIENTS

Patients 6 months and older undergoing cochlear implantation.

INTERVENTIONS

After cochlear implant array insertion, intraoperative TIM recordings and x-rays were conducted to confirm appropriate placement. If a foldover was identified, repeat insertion, followed by TIM and x-ray, was performed.

MAIN OUTCOME MEASURES

Presence of tip foldovers as determined by TIM recordings and x-rays.

RESULTS

There were 13 tip foldovers out of 484 insertions, yielding a 2.7% tip foldover rate. Using x-rays as the "gold standard," TIM recordings showed 100% sensitivity, 99.6% specificity, 84.6% positive predictive value (PPV), and 100% negative predicative value in detecting tip foldovers. The x-ray images for two TIM-identified tip foldovers were interpreted as normal intraoperatively (apparent false positives), but then were interpreted as foldovers when presented to the operating surgeon postoperatively in a blinded fashion. If these false positives had been reclassified as true positives, TIM specificity and PPV would both improve to 100%.

CONCLUSIONS

These results provide further support regarding the utility of TIM recordings to evaluate appropriate cochlear implant electrode array position. Despite the low tip foldover rate reported across the literature and within this study, this multicenter patient group provided a large sample size to calculate the sensitivity and specificity of TIM recordings in identifying tip foldovers. Compared with the reference-standard, x-ray imaging, TIM recordings show equivalent, and in some cases superior, performance in identification of tip foldovers. Thus, use of TIM can potentially preclude the need for routine intraoperative x-rays without compromising patient care.

A Novel Trans-Impedance Matrix (TIM) Abnormality Pattern in Cochlear Implants

In our clinical setting, we have identified a novel pattern of Trans-Impedance Matrix (TIM) measurement that we call 'scatter', a measure characterised by a loss of definition in the heat and line maps. Objective: the aim of this study was to describe the basic characteristics of the anomaly pattern. The secondary purpose is to evaluate correlations between the "scatter" pattern and normal TIM by considering different parameters. Methods: the experimental sample, therefore, consisted of 565 patients (81.1% of people with a checked TIM at follow-up; M: 279, F: 286 and mean age: 27 years (sd 26). The scatter pattern was found in 55 devices (9.7%). We classified this pattern as severe (20 devices) or mild (35 devices) according to the visual extent of the abnormality. We considered the visual extension of the pattern, device lifetime, type of internal part of the CI, and auditory performance (speech audiometry in quiet at 65 dB and in noise-Ita Matrix Sentence Test). We also analysed two quantitive parameters: Shannon entropy and exponential decay. Results: a difference was found in these two quantitative parameters between the severe scatter, mild scatter, and normal TIM groups (p-value < 0.0001). The severe scatter group seems to be related to the type of device (CI24RE and CI512) and long device life (average 133 months); it did not show differences in audiology performances compared to the other groups. Conclusions: this result gives a numerical validation to the more subjective visual inspection approach. The scatter pattern is a novel, previously undescribed abnormality of TIM. It can vary from moderate to severe. A numerical basis to validate the inspection approach is described here.

Characterizing Cochlear Implant Trans-Impedance Matrix Heatmaps in Patients With Abnormal Anatomy

OBJECTIVE

To characterize transimpedance matrix (TIM) heatmap patterns in patients at risk of labyrinthine abnormality to better understand accuracy and possible TIM limitations.

STUDY DESIGN

Retrospective review of TIM patterns, preoperative, and postoperative imaging.

SETTING

Tertiary referral center.

PATIENTS

Patients undergoing cochlear implantation with risk of labyrinthine abnormality.

INTERVENTION

None.

RESULTS

Seventy-seven patients were evaluated. Twenty-five percent (n = 19) of patients had a TIM pattern variant identified. These variants were separated into 10 novel categories. Overall, 9% (n = 6) of electrodes were malpositioned on intraoperative x-ray, of which 50% (n = 3) were underinserted, 17% (n = 1) were overinserted, 17% (n = 1) had a tip foldover, and 17% (n = 1) had a coiled electrode. The number of patients with a variant TIM pattern and normal x-ray was 18% (n = 14), and the number of patients with normal TIM pattern and malposition noted on x-ray was 3% (n = 2; both were electrode underinsertions that were recognized due to open circuits and surgical visualization).A newly defined skip heat pattern was identified in patients with IP2/Mondini malformation and interscalar septum width <0.5 mm at the cochlear pars ascendens of the basal turn.

CONCLUSIONS

This study defines novel patterns for TIM heatmap characterization to facilitate collaborative and comparative research moving forward. In doing so, it highlights a new pattern termed skip heat, which corresponds with a deficient interscalar septum of the cochlea pars ascendens of the basal turn in patients with IP2 malformation. Overall, the data assist the surgeon in better understanding the implications and limitations of TIM patterns within groups of patients with risk of labyrinthine abnormalities.

Utility of Intraoperative Radiographs in Pediatric Cochlear Implant Surgery

OBJECTIVE

To evaluate the role of intraoperative radiographs to confirm electrode position following pediatric cochlear implantation (CI).

STUDY DESIGN

Retrospective chart review.

SETTING

Single tertiary care pediatric center.

METHODS

A retrospective chart review was conducted, including all pediatric patients undergoing CI at UPMC Children's Hospital of Pittsburgh over a 13-year period.

RESULTS

We identified 326 patients undergoing 492 procedures. Across the cohort, there were 7 cases that required intraoperative electrode reinsertion due to malposition or presumed malposition. For 6 of the 7 cases, intraoperative X-ray identified electrode malposition. Neural response telemetry (NRT) testing was also abnormal for 4 of these cases prior to reinsertion. Implantation of Cochlear's Slim Modiolar electrode was associated with an abnormal perioperative X-ray (odds ratio [OR]: 9.2, p = 0.03) and increased change in management (OR: 9.2, p = 0.03) compared to Cochlear's Contour Advance (CA). Incidence of abnormal X-rays was 1.24% overall, 4% in the Slim Modiolar group, and 0.3% in the CA group. The Slim Modiolar electrode accounted for 4 of 7 cases requiring reinsertion, and in all 4 of these cases, electrode fold-over was identified on the X-ray. NRT was normal in 1 of these 4 cases.

CONCLUSION

The use of Cochlear's Slim Modiolar electrode was associated with a significantly increased risk of abnormal intraoperative X-ray compared to the CA electrode. Given the risk of fold-over with routine insertion and normal electrical testing using the Slim Modiolar electrode, we recommend routine use of intraoperative skull X-ray to confirm electrode position.

Postoperative Impedance-Based Estimation of Cochlear Implant Electrode Insertion Depth

OBJECTIVES

Reliable determination of cochlear implant electrode positions shows promise for clinical applications, including anatomy-based fitting of audio processors or monitoring of electrode migration during follow-up. Currently, electrode positioning is measured using radiography. The primary objective of this study is to extend and validate an impedance-based method for estimating electrode insertion depths, which could serve as a radiation-free and cost-effective alternative to radiography. The secondary objective is to evaluate the reliability of the estimation method in the postoperative follow-up over several months.

DESIGN

The ground truth insertion depths were measured from postoperative computed tomography scans obtained from the records of 56 cases with an identical lateral wall electrode array. For each of these cases, impedance telemetry records were retrieved starting from the day of implantation up to a maximum observation period of 60 mo. Based on these recordings, the linear and angular electrode insertion depths were estimated using a phenomenological model. The estimates obtained were compared with the ground truth values to calculate the accuracy of the model.

RESULTS

Analysis of the long-term recordings using a linear mixed-effects model showed that postoperative tissue resistances remained stable throughout the follow-up period, except for the two most basal electrodes, which increased significantly over time (electrode 11: ~10 Ω/year, electrode 12: ~30 Ω/year). Inferred phenomenological models from early and late impedance telemetry recordings were not different. The insertion depth of all electrodes was estimated with an absolute error of 0.9 mm ± 0.6 mm or 22° ± 18° angle (mean ± SD).

CONCLUSIONS

Insertion depth estimations of the model were reliable over time when comparing two postoperative computed tomography scans of the same ear. Our results confirm that the impedance-based position estimation method can be applied to postoperative impedance telemetry recordings. Future work needs to address extracochlear electrode detection to increase the performance of the method.

Robotic pullback technique of a precurved cochlear-implant electrode array using real-time impedance sensing feedback

PURPOSE

During traditional insertion of cochlear implant (CI) electrode arrays (EAs), surgeons rely on limited tactile feedback and visualization of the EA entering the cochlea to control the insertion. One insertion approach for precurved EAs involves slightly overinserting the EA and then retracting it slightly to achieve closer hugging of the modiolus. In this work, we investigate whether electrical impedance sensing could be a valuable real-time feedback tool to advise this pullback technique.

METHODS

Using a to-scale 3D-printed scala tympani model, a robotic insertion tool, and a custom impedance sensing system, we performed experiments to assess the bipolar insertion impedance profiles for a cochlear CI532/632 precurved EA. Four pairs of contacts from the 22 electrode contacts were chosen based on preliminary testing and monitored in real time to halt the robotic insertion once the closest modiolar position had been achieved but prior to when the angular insertion depth (AID) would be reduced.

RESULTS

In this setting, the open-loop robotic insertion impedance profiles were very consistent between trials. The exit of each contact from the external stylet of this EA was clearly discernible on the impedance profile. In closed-loop experiments using the pullback technique, the average distance from the electrode contacts to the modiolus was reduced without greatly affecting the AID by using impedance feedback in real time to determine when to stop EA retraction.

CONCLUSION

Impedance sensing, and specifically the access resistance component of impedance, could be a valuable real-time feedback tool in the operating room during CI EA insertion. Future work should more thoroughly analyze the effects of more realistic operating room conditions and inter-patient variability on this technique.

Detection of Tip Fold-Over of the Slim Modiolar Electrode Using Intraoperative Mobile Cone-Beam Computed Tomography

OBJECTIVE

This study aimed to evaluate the importance of mobile cone-beam computed tomography in detecting tip fold-over of a slim modiolar electrode within the cochlea during surgery.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary medical center.

METHODS

From January 2020 to June 2022, 33 ears of 30 patients with normal cochlear morphology underwent cochlear implantation with slim modiolar electrodes and intraoperative mobile cone-beam computed tomography imaging. Furthermore, we retrospectively reviewed the medical records and images.

RESULTS

The tip fold-over of the electrodes was detected using mobile cone-beam computed tomography in 3 out of 33 ears (9.1%). We could not identify the tip fold-over by scouting plain X-ray images in 2 out of 3 cases before taking the cone-beam computed tomography images. Electrode removal and reinsertion were performed before wound closure and the successful reinsertion was confirmed by mobile cone-beam computed tomography. The folded electrode tips were located at 238.8°, 152°, and 185.8°.

CONCLUSION

Intraoperative mobile cone-beam computed tomography is useful in detecting the tip fold-over of the slim modiolar electrodes during surgery. Therefore, it was possible to reinsert the electrodes in all cases before closing the wound, eliminating the need for revision surgeries. Moreover, the analysis of mobile cone-beam computed tomography images may help to elucidate the mechanisms of electrode tip fold-over.

An optically-guided cochlear implant sheath for real-time monitoring of electrode insertion into the human cochlea

In cochlear implant surgery, insertion of perimodiolar electrode arrays into the scala tympani can be complicated by trauma or even accidental translocation of the electrode array within the cochlea. In patients with partial hearing loss, cochlear trauma can not only negatively affect implant performance, but also reduce residual hearing function. These events have been related to suboptimal positioning of the cochlear implant electrode array with respect to critical cochlear walls of the scala tympani (modiolar wall, osseous spiral lamina and basilar membrane). Currently, the position of the electrode array in relation to these walls cannot be assessed during the insertion and the surgeon depends on tactile feedback, which is unreliable and often comes too late. This study presents an image-guided cochlear implant device with an integrated, fiber-optic imaging probe that provides real-time feedback using optical coherence tomography during insertion into the human cochlea. This novel device enables the surgeon to accurately detect and identify the cochlear walls ahead and to adjust the insertion trajectory, avoiding collision and trauma. The functionality of this prototype has been demonstrated in a series of insertion experiments, conducted by experienced cochlear implant surgeons on fresh-frozen human cadaveric cochleae.

Predictive Value of Transimpedance Matrix Measurements to Detect Electrode Tip Foldover

OBJECTIVE

To evaluate the ability of the transimpedance matrix (TIM) measurement to detect cochlear implant electrode tip foldover by comparing results to a "gold standard," the intraoperative plain film radiograph.

STUDY DESIGN

Retrospective case series.

SETTING

Tertiary referral hospital.

PATIENTS

One hundred three patients who underwent cochlear implantation between June 2020 and August 2021.

INTERVENTIONS

Intraoperative electrophysiologic monitoring (electrode impedances, neural response telemetry, and TIM measurement) and modified Stenver's view plain film radiographs.

MAIN OUTCOME MEASURES

Identification of tip foldover on both TIM and plain films.

RESULTS

In total, 103 patients (117 ears) had both a TIM measurement and intraoperative X-ray available for review, including 68 adults and 35 children. One hundred patients (85%) received the Cochlear Slim Modiolar electrode. Tip foldovers were noted in three of 117 implants (2.5%). In all cases, TIM was able to detect the foldover, and the electrode arrays were reinserted with the patients still under anesthesia, with repeat X-ray demonstrating a normal configuration. Two other abnormal TIM patterns were identified. One was in a patient with an obstructed cochlea in whom only 10 electrodes could be inserted, the other was in a patient with a common cavity abnormality. One additional patient underwent electrode repositioning intraoperatively because of overinsertion. In this patient, the TIM appeared to be within normal limits, but the over-insertion was apparent on X-ray. Overall, the sensitivity and specificity of TIM measurements in detecting electrode tip foldover were both 100%.

CONCLUSION

TIM measurements were able to accurately identify tip foldovers. More research is needed to define the adjunctive role of TIM as an intraoperative measure.