Vestibular Organ and Cochlear Implantation-A Synchrotron and Micro-CT Study

Abstract

Background: Reports vary on the incidence of vestibular dysfunction and dizziness in patients following cochlear implantation (CI). Disequilibrium may be caused by surgery at the cochlear base, leading to functional disturbances of the vestibular receptors and endolymphatic duct system (EDS) which are located nearby. Here, we analyzed the three-dimensional (3D) anatomy of this region, aiming to optimize surgical approaches to limit damage to the vestibular organ. Material and Methods: A total of 22 fresh-frozen human temporal bones underwent synchrotron radiation phase-contrast imaging (SR-PCI). One temporal bone underwent micro-computed tomography (micro-CT) after fixation and staining with Lugol's iodine solution (I₂KI) to increase tissue contrast. We used volume-rendering software to create 3D reconstructions and tissue segmentation that allowed precise assessment of anatomical relationships and topography. Macerated human ears belonging to the Uppsala collection were also used. Drilling and insertion of CI electrodes was performed with metric analyses of different trajectories. Results and Conclusions: SR-PCI and micro-CT imaging demonstrated the complex 3D anatomy of the basal region of the human cochlea, vestibular apparatus, and EDS. Drilling of a cochleostomy may disturb vestibular organ function by injuring the endolymphatic space and disrupting fluid barriers. The saccule is at particular risk due to its proximity to the surgical area and may explain immediate and long-term post-operative vertigo. Round window insertion may be less traumatic to the inner ear, however it may affect the vestibular receptors.

Keywords: cochlear implant; human; micro-CT; synchrotron; vestibular organ.

Figure 1

SR-PCI and 3D reconstruction of a left human inner ear (superior view) using 3D slicer (version 4.10; www.slicer.org). The cochlea, utricle, saccule, and saccular nerve are seen with cranial nerves in the fundus of the IAC.

Figure 2

(A) SR-PCI 3D modeling of a left human temporal bone with a surgical view through the facial recess. (B) The relationship between the RW and the saccule is seen. The cochlear aqueduct (CA) and a second accessory canal are seen. (C,D) show the facial recess anatomy with (C) and without (D) the facial nerve.

Figure 3

A posterior-inferior view of the specimen shown in Figure 2. The relationships between the RW and the posterior ampulla and saccular nerves are shown. The distance between the middle of the RW and the middle part of the posterior ampulla was 2.6 mm.

Figure 4

SR-PCI section at the level of the RW and vestibule (lateral view). The RW and the position of a virtual electrode (dashed red line) are shown. The saccule lies in the spherical recess in the medial bony wall of the vestibule. It consists of a thicker and thinner part limited by thicker bands (arrows). The macula is stained yellow. The position of the RD is shown. Inset shows the modeled 3D anatomy with the saccule, RW (red), and spiral ligament of the cochlear base (blue). The broken line represents Reissner's membrane.

Figure 5

Left human micro-dissected temporal bone (taken from the temporal bone collection of Uppsala Museum) shows the RW and acoustic crest from “inside” the labyrinth. The OSL was resected, with the secondary lamina partially preserved along the rim of the RW. The broken line shows the attachment of the BM around the RW. The electrode was inserted via the RW. It rides upon the acoustic crest before reaching the ST. An anterior CO was drilled. The CA and the inferior cochlear vein (ICV) channels were dissected as well as the RD.

Figure 6

Micro-CT and 3D reconstruction of a macerated human temporal bone (right ear). An anterior CO was made (red arrow), and its relation to the spherical recess (blue) and saccule was studied. (A) Surgical view shows the CO and the RW. (B) Lateral cropping demonstrates the entrance canal of the CO and its relation to the spherical recess. The bold arrow shows the tympanic sinus. (C) Medial cropping shows the spherical recess (blue) with bony foramina of the saccular nerve. The broken arrow shows the direction of the scala tympani. (D) Medial view shows CO and spherical recess (dashed line). OW, oval window; RW, round window; LSSC, lateral semicircular canal; PSSC, posterior semicircular canal.

Figure 7

Micro-CT 3D reconstruction of a left human temporal bone (lateral view). An ACO (3) was made, and the distances to the saccular (4) and utricular (5) nerve foramina can be assessed (inset). Virtual AICO (2) and ICO (1) are shown with fiducials.

Figure 8

3D model of a right human temporal bone from micro-CT. Increased penetration time of aqueous I₂KI improved visualization of soft tissue structures. The cochlea was virtually implanted with an electrode (El) through the RW. (A) With bone capsule. (B) Bone capsule was made transparent to visualize the inner ear soft tissues.

Figure 9

3D model in Figure 8 is shown at higher magnification. (A) The modeled RD is seen. (B) The posterior ampule and its relation to the RW can be seen. El, cochlear implant electrode.

Figure 10

Micro-CT cross-section of the cochlear base at the RW. A virtual CI electrode (el) is placed in the scala tympani. The RD can hardly be seen on the superior surface of the OSL. RM, Reissner's membrane; ST, scala tympani; RW, Round window.

Figure 11

(A) Distances from the (a) utricle macula, (b) posterior semicircular canal ampulla, (c) saccule macula, and (d) saccule membrane to the middle of the RW were assessed. (B) Box plot showing measurements in 22 temporal bones.