A Researcher and Surgeon’s Insights Into Cochlear Variability

Dr. Sumit Agrawal

Research in Cochlear Variability

Although there have been extensive studies into the variability of cochlear measures, such as cochlear duct length, these studies have typically had limitations in the modality or sample size of the data.

Histology was historically the only modality which allowed for accurate assessment and measurement of intracochlear anatomy, and multiple foundational studies demonstrated variability in cochlear anatomy using histology data.[1][2][3] However, due to the sample preparation and dissection required, the 3D anatomical structure is often not preserved in histology. Despite serving as an invaluable dataset for some cochlear anatomic analysis, it is not feasible to make certain measurements using these samples.

More recent imaging-based studies have provided 3D context for cochlear measurements. Computed tomography (CT) imaging has gained attention for its value in collecting cochlear measurements since it preserves 3D structures. Clinical and research CT imaging, such as micro-CT, have been used in numerous studies to measure cochlear length and dimensions.[4][5][6][7] The resolution and characteristics of these imaging modalities generally does not, however, contain adequate soft tissue contrast for ground truth intracochlear measurement.

In recent years, our Auditory Biophysics laboratory at Western University has used synchrotron imaging to successfully visualize intracochlear structures in intact cadaveric cochleae. This synchrotron imaging data has allowed for accurate soft tissue intracochlear measurements while preserving 3D morphology. Our previous publications have illustrated the use of synchrotron imaging for measuring cochlear length and scalar parameters accurately, but these studies have generally contained limited datasets, meaning they cannot capture the variability in cochlear size as extensively.[8][9][10][11]

The Latest Research: The Cochlear Morphometry Compendium

Our group has recently published a landmark reference measurement study titled “The cochlear morphometry compendium: High-resolution synchrotron measurements and normative refrence values.”[12] This study presents a large collection of reference cochlear measurements, derived from synchrotron imaging scans of 100 cadaveric cochleae.

This represents the first atlas of high-resolution measurements in a sample size this large and provides valuable data for the further development of individualized solutions. Reference measurements in the study include various cochlear duct lengths, tonotopic frequency distributions, cross-sectional scalar dimensions, scalar volume, and round window dimensions.

Cochlear Duct Length and Tonotopic Frequency

Our synchrotron reference study analyzed complete cochlear length as well as proportional cochlear lengths at each angular depth in the synchrotron data collected from the 100 cochleae. Additionally, measurements such as the number of cochlear turns and the length of the basal hook region were reported. Complete cochlear duct length measurements (at the level of the organ of Corti) span from 30 mm to 40 mm in the reported values. This demonstrates significant variability and aligns with normative ranges reported in previous work. This highlights the value of various arrays to achieve desired cochlear coverage no matter the size of the cochlea.

Using Greenwood’s function, cochlear depths were related to tonotopic frequency. The significant variability in cochlear length results in variability in tonotopic frequency along the cochlea and highlights the need for individualization to achieve appropriate frequency coverage. This data also reinforces the value of apical insertions to achieve low frequency stimulation in the second turn of the cochlea.

Scalar Dimensions and Volume

Both scalar cross-sectional measurements and overall scalar volumes are reported in the study. Scalar volumes demonstrated significant variability, with the total scala tympani volume ranging from approximately 25 to 45 mm. Despite this variability, cross-sectional measurements revealed that even the smallest cochleae maintain sufficient area to accommodate modern flexible electrode arrays in the second turn.

Round Window Dimensions

In addition to expanding the dataset size for common clinical measures such as cochlear length and scalar dimensions, the synchrotron reference study established reference data for round window size. Considerable variability was observed in round window dimensions, with average long-axis and short-axis measurements of 2.20 ± 0.21 mm and 1.64 ± 0.18 mm, respectively. Round window shape and symmetry change drastically, but the average size confirms accommodation of arrays without requiring potentially traumatic round window extensions or cochleostomies. The variability, however, highlights the value of preoperative planning to understand what will be encountered during the procedure.

The expanded dataset collected and measurements made from synchrotron imaging make the extent of cochlear variability clearer than ever before. With image-based electrode selection along with anatomy-based fitting, surgeons can do justice to this cochlear variability by providing cochlear implants individualized to patients’ cochleae.