Anatomical Variations of the Human Cochlea Using an Image Analysis Tool

Scala vestibuli cochlear implantation: exploring feasibility and outcomes- a systematic review

PURPOSE

While the scala tympani (ST) is usually the preferred site for electrode insertion in cochlear implantation, anatomical variations and cochlear ossification may require scala vestibuli (SV) insertion. This systematic review evaluates the feasibility, techniques, and clinical outcomes of SV insertions compared to ST insertions, focusing on their impact on auditory performance.

METHODS

A systematic review was conducted using PRISMA guidelines. Studies on SV cochlear implantation were identified from databases like PubMed and EMBASE. The review included research articles reporting on anatomical feasibility, surgical methods, postoperative outcomes, and complications. Data extraction focused on demographic details, electrode types, insertion depths, and clinical outcomes of SV insertions. A two-stage selection process was applied, and 17 studies with 72 cases were included.

RESULTS

The review covered 72 cases of SV insertions, with patients aged 18 months to 65 years. Bacterial meningitis was the leading cause of hearing loss (28%). Various electrode types were used, with insertion depths ranging from 12 to 31 mm. Approximately 75% of patients showed improved auditory performance. Complications, including vertigo and tinnitus, were reported in 10% of cases, with no significant difference between SV and ST insertions.

CONCLUSION

SV cochlear implantation is a feasible and effective alternative in cases where ST is inaccessible. Auditory outcomes are comparable to ST insertions, and the technique shows promise in challenging anatomical situations such as cochlear ossification. Further studies are required to optimize surgical approaches and confirm long-term outcomes.

Feasibility of Using Inertial Measurement Units (IMUs) to Augment Cadaveric Temporal Training

OBJECTIVE

Insertional speed of cochlear implant electrode arrays (EA) during surgery is correlated with force. Low insertional speed, and therefore force, may allow for preservation of intracochlear structures leading to improved outcomes. Given the importance of low insertional speeds, we investigate the feasibility of using inertial sensors for kinematic analysis during EA insertion to augment otolaryngology-head and neck surgery training.

METHODS

Practicing otolaryngology surgeons were recruited and inertial measurement units (IMU; Metamotions+, MBIENTLAB Inc, San Jose, CA) consisting of accelerometers were used to measure hand speed during EA (Cochlear™Nucleus®CI522 cochlear implant with Slim Straight electrode, Cochlear Limited, Sydney, Australia) insertion into a cadaveric cochlea. A mixed regression model was utilized to determine differences in speed across trials within a surgeon.

RESULTS

A total of nine trials were performed by three surgeons. The highest mean ± SD speed obtained was 8.4 ± 1.7 mm/s, and the highest speed was 22.5 mm/s. Mean speed was not significantly different across trials within surgeons (p > 0.05).

DISCUSSION

IMUs are relatively inexpensive and relatively easy to use sensors that provide information on variables that may be of interest for otolaryngology resident training. The use of IMUs as part of advanced temporal training for cochlear electrode insertion can provide insight into hand speed, thereby allowing residents to train with specific regard to this variable. Future randomized-controlled trials can be carried out to determine whether IMUs are conducive to lower insertional speeds.

LEVEL OF EVIDENCE

NA Laryngoscope, 2024.

Influence of Cochlear Anatomy on Intraoperative Electrically Evoked Compound Action Potentials

Objective: The electrically evoked compound action potential (ECAP) is an objective measure to indirectly assess spiral ganglion neurons. The ECAP provides inputs about the prognoses of cochlear implant (CI) recipients. Several factors such as cochlear morphology can affect ECAP measurements. This study aims to investigate the variation effect of cochlear parameters on intraoperative ECAP thresholds. Methods: This is a retrospective study on patients who underwent CI surgery with normal inner ear morphology at our center between 2017 and 2023. Cochlear anatomical parameters, including diameter (A value), width (B value), and height (H value), as well as cochlear duct length (CDL), were measured pre-operatively using OTOPLAN software (Version 3.0). Cochlear implant intraoperative objective measures were also collected. The correlation between the cochlear parameters and intraoperative objective measures was studied. Results: A total of 45 patients underwent cochlear implantation. The mean age was 2.4 ± 0.9 years. The mean CDL and cochlear coverage values were 33.2 ± 2.0 mm and 76.0 ± 5.7%, respectively. The ECAP threshold increased toward basal electrodes, with ECAP values as follows: apical 13.1 ± 3.8; middle 14.3 ± 3.7; and basal 15.6 ± 4.8. Additionally, the A, B, and H values showed a positive correlation with ECAP thresholds in different cochlear regions. The B value showed a significant moderate correlation with ECAP thresholds in the middle and basal electrodes but not in the apical electrodes. Conclusions: Cochlear anatomical parameters correlate with intraoperative ECAP thresholds. The B value showed a significant association with ECAP thresholds in the middle and basal electrodes. These findings could delineate the impact of the B value in CI and optimize electrode selection. Further research is required to study this correlation and its impact on postoperative outcomes.

A full-head model to investigate intra and extracochlear electric fields in cochlear implant stimulation

Objective.Despite the widespread use and technical improvement of cochlear implant (CI) devices over past decades, further research into the bioelectric bases of CI stimulation is still needed. Various stimulation modes implemented by different CI manufacturers coexist, but their true clinical benefit remains unclear, probably due to the high inter-subject variability reported, which makes the prediction of CI outcomes and the optimal fitting of stimulation parameters challenging. A highly detailed full-head model that includes a cochlea and an electrode array is developed in this study to emulate intracochlear voltages and extracochlear current pathways through the head in CI stimulation.Approach.Simulations based on the finite element method were conducted under monopolar, bipolar, tripolar (TP), and partial TP modes, as well as for apical, medial, and basal electrodes. Variables simulated included: intracochlear voltages, electric field (EF) decay, electric potentials at the scalp and extracochlear currents through the head. To better understand CI side effects such as facial nerve stimulation, caused by spurious current leakage out from the cochlea, special emphasis is given to the analysis of the EF over the facial nerve.Main results.The model reasonably predicts EF magnitudes and trends previously reported in CI users. New relevant extracochlear current pathways through the head and brain tissues have been identified. Simulated results also show differences in the magnitude and distribution of the EF through different segments of the facial nerve upon different stimulation modes and electrodes, dependent on nerve and bone tissue conductivities.Significance.Full-head models prove useful tools to model intra and extracochlear EFs in CI stimulation. Our findings could prove useful in the design of future experimental studies to contrast FNS mechanisms upon stimulation of different electrodes and CI modes. The full-head model developed is freely available for the CI community for further research and use.

Evaluation of automatic cochlear dimension measurement using ALPACA: a comparative study

BACKGROUND

Cochlear dimension measurements are critical in diagnosing and managing congenital sensorineural hearing loss.

OBJECTIVES

To evaluate the feasibility and reliability of an automated landmark approach for measuring cochlear dimensions (A-, B- and H-values).

MATERIAL AND METHODS

Cochlear parameters from 100 patients were measured by MPR, manual three-dimensional and ALPACA. We assessed intra- and inter-observer reliability as well as inter-method reliability. Statistical analyses were conducted to detect differences between the right and left ears, as well as to assess the relevance of the values obtained using ALPACA.

RESULTS

All A-, B-, and H-values measured by the various methods showed a high intra-observer reliability with intra-class correlation coefficients (ICC) ranging from 0.70 to 0.99, and values gained by ALPACA reaching the highest ICC. Inter-method reliability was at a good level with ICC ranging from 0.51 to 0.86. There were no significant differences between the right and left ears' measured values. Obvious positive correlations existed among cochlear dimensions measured by ALPACA.

CONCLUSIONS AND SIGNIFICANCE

The ALPACA method can be used to measure cochlear dimensions. Values obtained by the method demonstrate high reliability and consistency with a significant reduction in intra-observer variability compared to results from conventional MPR and manual 3D measurements.

Morphologic Analysis of the Scala Tympani Using Synchrotron: Implications for Cochlear Implantation

OBJECTIVES

To use synchrotron radiation phase-contrast imaging (SR-PCI) to visualize and measure the morphology of the entire cochlear scala tympani (ST) and assess cochlear implant (CI) electrode trajectories.

METHODS

SR-PCI images were used to obtain geometric measurements of the cochlear scalar diameter and area at 5-degree increments in 35 unimplanted and three implanted fixed human cadaveric cochleae.

RESULTS

The cross-sectional diameter and area of the cochlea were found to decrease from the base to the apex. This study represents a wide variability in cochlear morphology and suggests that even in the smallest cochlea, the ST can accommodate a 0.4 mm diameter electrode up to 720°. Additionally, all lateral wall array trajectories were within the anatomically accommodating insertion zone.

CONCLUSION

This is the first study to use SR-PCI to visualize and quantify the entire ST morphology, from the round window to the apical tip, and assess the post-operative trajectory of electrodes. These high-resolution anatomical measurements can be used to inform the angular insertion depth that can be accommodated in CI patients, accounting for anatomical variability.

LEVEL OF EVIDENCE

N/A. Laryngoscope, 134:2889-2897, 2024.

Optimization of pharmacological interventions in the guinea pig animal model-a new approach to calculate the perilymph volume of the scala tympani

Objective

The guinea pig serves as a well-established animal model for inner ear research, offering valuable insights into the anatomy, physiology, and therapeutic interventions of the auditory system. However, the heterogeneity of results observed in both in-vivo experiments and clinical studies poses challenges in understanding and optimizing pharmacotherapy outcomes. This heterogeneity may be due to individual differences in the size of the guinea pig cochlea and thus in the volume of the scala tympani (ST), which can lead to different drug concentrations in the ST, a fact that has been largely overlooked thus far. To address this issue, we aimed to develop an approach for calculating the individual volume of perilymph within the ST before and after cochlear implant insertion.

Method

In this study, high-resolution μCT images of a total of n = 42 guinea pig temporal bones were used to determine the volume of the ST. We compared fresh, frozen, and fixed tissues from both colored and albino strains to evaluate the potential influence of tissue condition and strain on the results.

Results

Our findings demonstrate a variability in mean ST volume with a relative standard deviation (RSD) of 14.7%, comparable to studies conducted with humans (range RSD: 5 to 20%). This indicates that the guinea pig cochlea exhibits similar variability to that of the human cochlea. Consequently, it is crucial to consider this variability when designing and conducting studies utilizing the guinea pig as an animal model. Furthermore, we successfully developed a tool capable of estimating ST volume without the need for manual segmentation, employing two geometric parameters, basal diameter (A) and width (B) of the cochlea, corresponding to the cochlear footprint. The tool is available for free download and use on our website.

Conclusion

This novel approach provides researchers with a valuable tool to calculate individual ST volume in guinea pigs, enabling more precise dosing strategies and optimization of drug concentrations for pharmacotherapy studies. Moreover, our study underscores the importance of acknowledging and accounting for inter-individual variability in animal models to enhance the translational relevance and applicability of research outcomes in the field of inner ear investigations.