Round window versus cochleostomy technique in cochlear implantation: histologic findings

Abstract

Hypothesis: Cochleostomy or round window enlargement techniques for cochlear implant electrode insertion result in more abnormal tissue formation in the basal cochlea and are more apt to produce endolymphatic hydrops than round window electrode insertion.

Methods: Twelve temporal bones from implanted patients were examined under light microscopy and reconstructed with 3-dimensional reconstruction software to determine cochlear damage and volume of neo-ossification and fibrosis after electrode insertion. Amount of new tissue was compared between 3 groups of bones: insertion through the round window (RW), after enlarging the RW (RWE) and cochleostomy (Cochl). The probable role of the electrode was evaluated in each case with hydrops.

Results: More initial damage occurred in the Cochl and RWE groups than in the RW group, and the difference was significant between RWE and RW in cochlear segment I (p < 0.026). The volume of new bone in Segment I differed significantly between groups (p < 0.012) and was greater in the RWE group than in either the Cochl or RW groups (post hoc p's < 0.035 and 0.019, respectively). Hydrops was seen in 5 cases, all in the Cochl and RWE groups. Blockage of the duct was because of new tissue formation in 4 of the 5 hydrops cases.

Conclusion: With the electrodes in this series, implantation through the RW minimized initial intracochlear trauma and subsequent new tissue formation, whereas the RW extension technique used at the time of these implantations produced the greatest damage. Future studies may clarify whether today's techniques and electrodes will produce these same patterns of damage.

Figure 1

Cochlear rendering showing the four segments (14).

Figure 2

a & b. Initial histological changes in Segment I (a) and Segment II (b). Larger circles represent multiple cases. Initial histological changes: 0 = no observable trauma; 1 = elevation of the basilar membrane; 2 = rupture of basilar membrane; 3 = electrode in scala vestibuli; and 4 = severe trauma such as fracture of the osseous spiral lamina or modiolus or tear of stria vascularis (13).

Figure 2

a & b. Initial histological changes in Segment I (a) and Segment II (b). Larger circles represent multiple cases. Initial histological changes: 0 = no observable trauma; 1 = elevation of the basilar membrane; 2 = rupture of basilar membrane; 3 = electrode in scala vestibuli; and 4 = severe trauma such as fracture of the osseous spiral lamina or modiolus or tear of stria vascularis (13).

Figure 3

Cochlear hydrops. Reissner’s membrane is distended by the hydrops (black arrows); no hydrops was seen into the saccule. Only a small portion of the stria vascularis was evident in the anterior middle turn (black star: cochlear implant). (Hematoxylin & Eosin × 20)

Figure 4

a & b. (a) Case 1R. The electrode was inserted through the round window (grayed-out path), into the basal turn. The only new tissue seen is an areolar fibrosis (black arrow) surrounding the electrode path. (Skeletonized preparation, [H&E] × 200). (b) Case 5. The cochleostomy path is filled with dense fibrosis (black star), which extends into the cochlea (black triangles). A great amount of new bone is present in the scala tympani of the basal turn (black arrow). (H&E × 10).

Figure 4

a & b. (a) Case 1R. The electrode was inserted through the round window (grayed-out path), into the basal turn. The only new tissue seen is an areolar fibrosis (black arrow) surrounding the electrode path. (Skeletonized preparation, [H&E] × 200). (b) Case 5. The cochleostomy path is filled with dense fibrosis (black star), which extends into the cochlea (black triangles). A great amount of new bone is present in the scala tympani of the basal turn (black arrow). (H&E × 10).

Figure 5

a & b. (a) Case 6. A three-dimensional reconstruction (Amira 4.1) of the first segment of a left cochlea was performed on a case with the electrode inserted through the round window. Normal tissues are represented in blue, with the cochlear implant path, fibrosis and new bone delineated in green, red, and white, respectively. (b) Case 7R. A three-dimensional reconstruction (Amira 4.1) of the first segment of a right cochlea was performed on a case with the electrode inserted through a round window enlargement. Normal tissues are represented in blue, with the cochlear implant path, fibrosis and new bone delineated in green, red, and white, respectively.

Figure 5

a & b. (a) Case 6. A three-dimensional reconstruction (Amira 4.1) of the first segment of a left cochlea was performed on a case with the electrode inserted through the round window. Normal tissues are represented in blue, with the cochlear implant path, fibrosis and new bone delineated in green, red, and white, respectively. (b) Case 7R. A three-dimensional reconstruction (Amira 4.1) of the first segment of a right cochlea was performed on a case with the electrode inserted through a round window enlargement. Normal tissues are represented in blue, with the cochlear implant path, fibrosis and new bone delineated in green, red, and white, respectively.

Figure 6

Graphic depiction of likely electrode path with the different insertion techniques.