Implantierbare Hörgeräte - der aktuelle Stand

Implantable Hearing Devices - State of the Art

Hearing aids may have fundamental disadvantages: (1) stigmatization of the patient; (2) the sound may be found unsatisfactory due to the limited frequency range and undesired distortion; (3) in some patients, the ear canal fitting device generally necessarry leads to an occlusion effect; (4) acoustic feedback may occur when amplification is high. Conventional hearing aids transmit sound into the ear canal via a small microphone. Sound has the disadvantage of requiring high output sound pressure levels for its transmission. This along with the necessary miniaturization of the loudspeaker as well as the resonances and reflections in the closed ear canal contribute to the possible disadvantages mentioned. In contrast, implantable hearing aids do not make sound signals but micromechanical vibrations. An implantable hearing aid has an electromechanical transducer instead of the loudspeaker of a conventional hearing aid. The hearing signal does not leave the transducer as sound but as a mechanical vibration which is directly coupled to the auditory system bypassing the air. This implantable hearing aid is either coupled to the tympanic membrane, the ossicular chain, the perilymph of the inner ear, or the skull. Requirements on electronic hearing implants designed for patients with conductive hearing loss differ from those on implants for sensorineural hearing loss. Conductive hearing loss requires the implant to replace the impedance transformation, thus being an impedance transformation implant (ITI). In various respects, the demands on an ITI are lower than the demands on an electronic hearing aid for patients with sensorineural hearing loss. The latter are mostly patients with a failure of the cochlea amplifier (CA). A damage to the CA is clinically discernible by a positive recruitment and loss of oto-acoustic emissions (OAE). Since these patients form the majority of cases with sensorineural hearing loss, an active hearing implant for such patients should partially replace the function of the CA. Therefore, the suggestion is to refer to an AI (amplifier implant). The implant expressions ITI (for patients with conductive hearing loss) and AI (for patients with sensorineural hearing loss) used in this context allow nomenclatural association with the CI (cochlear implant) for complete inner ear failure as well as with the BSI (brainstem implant) in the case of acoustic nerve failure.

Einleitung

Noch 1997 wurde die chirurgische Versorgung von Innenohrschwerhörigen mit implantierbaren Hörgeräten in einer umfassenden Übersicht über aktive elektronische Hörimplantate lediglich als eine Vision der Zukunft angesehen [85 - 87]. In kürzester Zeit ist diese Vision Realität geworden und hörverbessernde Operationen bei Innenohrschwerhörigkeiten sind heute klinische Wirklichkeit [34, 82, 83]. Die vorliegende Arbeit greift die o. g. Übersicht auf und geht auf die aktuelle rasante Entwicklung einer neuen Ära der Ohrchirurgie ein. Dabei wird zwischen Implantaten für Mittelohrschwerhörige und Implantaten für Innenohrschwerhörige unterschieden [87]. Cochlear Implants für Gehörlose sind nicht Thema dieser Übersicht.