Pathology as it relates to ear surgery. VI. Cochlear implantation

Formation of the Cochlear Nerve in the Modiolus of the Guinea Pig and Human Cochleae

BACKGROUND

Central processes of the bipolar neurons in the spiral ganglion converge in the modiolus to form the initial portion of the auditory branch (cochlear nerve) of the eighth cranial nerve. This occurs before the cochlear nerve passes through the internal auditory meatus. The neurons of the spiral ganglion send their central processes towards the internal acoustic meatus, through a single canal to form the cochlear nerve. These processes are described in many textbooks as running through numerous longitudinal small canals called canales longitudinales modioli before entering the internal acoustic meatus. Results of this study indicated that the term; "canalis longitudinalis modioli" was considered to be more appropriate than the former.

METHODS

Central processes of the bipolar neurons in the spiral ganglion of the guinea pig and human cochleae were investigated using stereo, light and electron microscopy.

RESULTS

Detailed examination of the guinea pig and human cochleae by light, electron and stereomicroscopy did not reveal multiple longitudinal canals but a single canal for the cochlear nerve.

CONCLUSIONS

The singular term canalis longitudinalis modioli is more appropriate than canales longitudinales modioli.

Cochlear hook anatomy: evaluation of the spatial relationship of the basal cochlear duct to middle ear landmarks

The cochlear hook is an important anatomical area for the otologist performing cochlear implants and other otological procedures, who requires knowledge of the basal cochlea. A total of 15 human temporal bones were dissected and the spatial relationship of the hook segment of the cochlear duct to the stapes, round window, cochleariform process and ductus reuniens were evaluated. Inter-individual variability was noted for widths of scala tympani (average width 1.36 +/- 0.25 mm) and scala vestibuli (average width 1.18 +/- 0.18 mm) in the region of typical cochlear implant placement, with the scala vestibuli occasionally being wider than the scala tympani. The cochlear duct was in closest proximity to the stapes at the midportion of the footplate, with an average distance of 1.23 mm at this narrowest width. A fibrous anchor, not previously described in otology literature, was identified securing the most basal end of the cochlear duct. Knowing the spatial relationship of the cochlear duct to the middle and inner ear structures could prevent damage to the basilar membrane in procedures around or involving the basal cochlear, such as cochlear implantation, stapedotomy, or implantable hearing devices.

Outcome analysis of cochlear implant reimplantation in children

OBJECTIVES

We review our experience gained in performing cochlear reimplantation in 25 children who have had multichannel cochlear implant device failure at the Cochlear Implant Center of the Manhattan Eye, Ear and Throat Hospital and Lenox Hill Hospital (New York, NY), to assess the feasibility of cochlear reimplantation in children and the effect of reinsertion on audiological performance.

STUDY DESIGN

We retrospectively analyzed the outcome of 27 consecutive multichannel cochlear implant reinsertions performed in 25 children at the Manhattan Eye, Ear and Throat Hospital and Lenox Hill Hospital. These reimplantations were performed between 1990 and 1999, with a minimum follow-up of 6 months for both surgical and audiological performance.

METHODS

Charts were reviewed for patient factors, findings at the time of initial operation and repeat operation, and results of audiological testing both before and after reimplantation.

RESULTS

Surgical complications of reimplantation included two intraoperative cerebral spinal fluid leaks and two late postoperative flap breakdowns with implant extrusions. For the most part, depth of electrode insertion was unchanged. Open-set speech recognition scores and speech perception abilities remained stable or improved compared with results before reimplantation.

CONCLUSION

Our results confirm that cochlear implant reimplantation is technically feasible and allows for continued auditory development for the child who has a cochlear implant device failure.

Cochlear implant revision: effects of reimplantation on the cochlea

The removal of an indwelling cochlear implant electrode followed by reinsertion of a new device has been a maneuver of uncertain consequences to the cochlea and its surviving neural population. The present study was conducted in an attempt to elucidate the factors that determine whether a reimplantation procedure will be successful. Cochlear implantation followed by explantation and subsequent reimplantation was performed in eight adult cats. Evaluation of cochlear histopathology suggested a significant increase in electrode insertion trauma when there was proliferation of granulation tissue in the round window area and scala tympani. In other cases, atraumatic reinsertion was achieved without apparent injury to the cochlea. The results of a survey of cochlear implant manufacturers and surgeons indicate that electrode replacement can usually be accomplished without adverse effects. Difficulties have been encountered, however, in removing implants with protuberant electrodes and when reimplantation was attempted on a delayed basis following explantation.