Modular prosthesis of the Larynx: Current Stage - Analysing the Information from the Recurrent Laryngeal Nerve

Abstract

In previous publications we outlined the feasibility of a modular prosthesis of the larynx for patients suffering from laryngeal carcinoma. The larynx is located between the trachea and the pharynx with its main functions being phonation, protection and regulation of the air ways. Patients suffer severely from the diagnosis of a laryngeal carcinoma of the stages pT3 and pT4. In most cases this diagnosis will lead to a total laryngectomy, which usually leads to a dissatisfying postoperative rehabilitation. The postoperative consequences include the loss of the voice, the regular air ways via mouth and nose, the sense of smell, and the inability to build up an abdominal pressure. This paper deals with the basic research on the nervus laryngeus recurrens for the development of a modular prosthesis of the larynx which eventually will enable the laryngectomee to talk with his native voice, to breathe via the regular air ways, and to build up abdominal pressure. The voice reproduction will be performed by a vocoder. Vocoders are electronic devices which emulate the human vocal tract by using adequate filter settings on discrete filter units thus consisting of both variable and permanent filter settings. The signal pattern in the nervus laryngeus recurrens will affect the variable filter settings of the vocoder according to the current voice intonation whereas the vocal range of the patient's voice gives the limit of the permanent filter settings. In this work we put emphasis on the recording and analysis of the signal pattern in the nervus laryngeus recurrens. Therefore we tried to record the nerve potentials of this nerve first in a non-invasive approach by using skin electrodes along the anatomic course of the nerve. A sophisticated amplifier (Intan ® Technologies RHA2000-Series Amplifier and RHA2116 Amplifier Board) was used to amplify the weak nerve potentials, but we had to cope with a poor SNR and strong signal distortions mostly due to muscle potentials from large muscles like the sternocleidomastoid muscle eventually delivering us only microphone potentials of the voiced parts of the speech generated by the muscle potentials of the laryngeal and pharyngeal muscles and not representing action potentials on the nerve thus rendering them useless for our needs. Further basic research in this field therefore is necessary, even taking an invasive approach into account.