Model-based Vestibular Afferent Stimulation: Modular Workflow for Analyzing Stimulation Scenarios in Patient Specific and Statistical Vestibular Anatomy

Abstract

Our sense of balance and spatial orientation strongly depends on the correct functionality of our vestibular system. Vestibular dysfunction can lead to blurred vision and impaired balance and spatial orientation, causing a significant decrease in quality of life. Recent studies have shown that vestibular implants offer a possible treatment for patients with vestibular dysfunction. The close proximity of the vestibular nerve bundles, the facial nerve and the cochlear nerve poses a major challenge to targeted stimulation of the vestibular system. Modeling the electrical stimulation of the vestibular system allows for an efficient analysis of stimulation scenarios previous to time and cost intensive in vivo experiments. Current models are based on animal data or CAD models of human anatomy. In this work, a (semi-)automatic modular workflow is presented for the stepwise transformation of segmented vestibular anatomy data of human vestibular specimens to an electrical model and subsequently analyzed. The steps of this workflow include (i) the transformation of labeled datasets to a tetrahedra mesh, (ii) nerve fiber anisotropy and fiber computation as a basis for neuron models, (iii) inclusion of arbitrary electrode designs, (iv) simulation of quasistationary potential distributions, and (v) analysis of stimulus waveforms on the stimulation outcome. Results obtained by the workflow based on human datasets and the average shape of a statistical model revealed a high qualitative agreement and a quantitatively comparable range compared to data from literature, respectively. Based on our workflow, a detailed analysis of intra- and extra-labyrinthine electrode configurations with various stimulation waveforms and electrode designs can be performed on patient specific anatomy, making this framework a valuable tool for current optimization questions concerning vestibular implants in humans.

Visualization of the Membranous Labyrinth and Nerve Fiber Pathways in Human and Animal Inner Ears Using MicroCT Imaging

“…We were able to segment the outline of the sensory epithelium in the vestibular end organs as distinct structures although we meet the limit of resolution of this cell layer. For the purpose of finite element simulation of current spread with electrical stimulation we introduced a gap region between the space of the sensory epithelium and peripheral nerve fiber bundles that give higher contrast ( Handler et al, 2017 ). This allows setting conductivity parameters for this region that contains myelinated and unmyelinated nerve fibers (close to the basal pole of the hair cells) as well as loosely arranged fibrous tissue.…”

“…This shall allow engineers and surgeons to find best ways to access and stimulate malfunctional sensory structures. The data from 52 human temporal bones shall provide a basis for a morphometric analysis of the variability of human inner ear anatomy ( Johnson Chacko et al, 2018 ) and simulation of current spread with finite element analysis ( Handler et al, 2017 ).…”

Model-Based Vestibular Afferent Stimulation: Evaluating Selective Electrode Locations and Stimulation Waveform Shapes

“…With this 3D imaging data as foundation, a highly precise tetrahedral mesh of a human inner ear was constructed (Handler et al, 2017). We realized this finite element model (see section 2.2) to compute the potential distribution of stimulation electrodes.…”

“…Additionally, a saline layer with a thickness of 10 mm surrounding the bone sphere was introduced to ensure proper model boundary conditions as proposed by Marianelli et al (2015) (see Figure 2B). This was done using a semi-automatic algorithm developed by Handler et al (2017) which included the mesh tools TetGen (Hang, 2015) as well as CGAL (The CGAL Project, 2016) and resulted in a volume mesh with approximately 18 million elements.…”

“…A principal component analysis of several shapes of vestibular systems showed that the specimen used in this study is close to the “mean shape” of all the vestibular systems that were examined and is therefore considered appropriate for this type of study (Fritscher et al, under review). With this 3D imaging data as foundation, a highly precise tetrahedral mesh of a human inner ear was constructed (Handler et al, 2017 ). We realized this finite element model (see section 2.2) to compute the potential distribution of stimulation electrodes.…”