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Bioimpedance controlled neuro-prosthesis to support swallowing

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Subproject in the research area Controlled Functional Electric Stimulation for Rehabilitation Purposes

Contents

People involved

Cooperation

Funding

  • Federal Ministry of Education and Research (BMBF), Innovation Award 2009 for Medical Technology

Project homepage

Official website of the project BigDysPro

Description

Swallowing is a complex and vital process. Depending on the current phase of swallowing, it is either conscious or sub-conscious. Controlled by cortical processes which are coordinated in the brain stem (pattern generators), multiple muscles have to be activated in a timely manner for this purpose. Swallowing disorders (dysphagia) can lead to serious complications, including malnutrition and pneumonia, which may be fatal. The complete closure of the larynx and its timing take a central role in safe swallowing, especially since the larynx is a branching point between the trachea and the oesophagus. In case of closure failure, saliva, liquid or food may enter the airway (aspiration), with possible consequences as described above. The causes of swallowing disorders are mostly severe head injuries and strokes. Every second stroke patient suffers from dysphagia, and it is chronic in one quarter of the patients.

The primary objective of rehabilitation is the restoration of disturbed functions by, for example, sensory stimulation or teaching of special swallowing techniques. Necessary conditions for success are sufficient cortical potential after the injury and an existing connection from the cortex to the muscles. If this connection is lost or if the muscles cannot be sufficiently controlled, a rehabilitation of the swallowing process is not possible. Then, the patient is dependent on a diet via a feeding tube and a tracheal cannula. In these cases, electrical stimulation of the external laryngeal muscles as a therapeutic approach seems to enhance the swallowing process [1]. Another possibility is to stimulate the internal laryngeal muscles in such a way that the vocal cords close and aspiration can be prevented. In both cases, intramuscular stimulation seems to be superior to transcutaneous stimulation. Stimulation has to be applied in a timely manner. In previous studies, stimulation was triggered either by the patient himself via a hand-switch or by the electromyography (EMG) of the submental muscles. However, neither method is able to adapt to the swallowing success or skills of the patient. One approach to evaluate the swallowing success online is to measure bioimpedances (BI) at the neck. Impedance is defined as the relation of voltage to current over an electrical conductor. There are two possible methods to measure BI. In the two-point method, the voltage is measured directly over the current electrodes. The current, which is induced into the patient through the current electrodes, causes a voltage drop across the electrode-skin contact. As this resistance is time-variant, it will lead to a measurement error. This undesirable effect can be avoided by using the four-point measurement method, where voltage is recorded separately over additional electrodes by a high impedance instrumentation amplifier. Since no current can flow through the voltage electrodes, there is no disturbing voltage drop across the electrode-skin contact. We have developed a bioimpedance measurement instrument which supports both methods; however, the four-electrode method is preferred for measurements at the neck. In cooperation with the Unfallkrankenhaus Berlin (Trauma Hospital Berlin), we could demonstrate that a certain BI measurement (change in absolute value) correlates with the distance between the hyoid bone and the thyroid cartilage. This provides important information to assess the airway closure during swallowing. Another specific BI measurement at the neck can be used for the detection of aspiration events [2]. We believe that a system based on a combination of these BI measurements can serve as an adequate sensor for a neuro-prosthesis that supports swallowing by stimulating the appropriate muscles using intramuscular electrodes. The development of such a neuro-prosthesis is the goal of the research project BigDysPro, which received the Innovation Award 2009 for Medical Technology (Innovationspreis Medizintechnik 2009) from the Federal Ministry of Education and Research (BMBF). BigDysPro is a joint project with R. O. Seidl from the Trauma Hospital Berlin. The new system will continuously assess the success of stimulation by accessing BI measurements and adjust the stimulation intensity to the needs of the patient. To ensure synchronization between the start of the swallowing and stimulation processes, the BI-electrodes will also be used to record the remaining residual muscle activity by EMG and to trigger stimulation. Should the patient choke despite these measures, the device will be able to detect this from the BI measurements and will subsequently induce a protective or throat-clearing cough by another stimulation burst. In Fig. 1, the stimulation and measurement areas are illustrated. For the measurement of airway closure, changes in the BI at the level of the epiglottic vallecula are acquired. Aspiration detection takes place via electrodes on the thyroid cartilage at the level of the vocal cords.

Figure 1: Model of the measurement and stimulation system as basis for a swallowing neuro-prosthesis.

Publications

  1. Seidl, R. O., Nahrstaedt, H., Schauer, T.. Electric stimulation in dysphagia therapy – a review (Article in German). Laryngo-Rhino-Otologie, 88 (12):768–774, 2009.
  2. Nahrstaedt, H., Schauer, T., Seidl, R. O.. Bioimpedance based measurement system for a controlled swallowing neuro-prosthesis. In Proc. of 15th Annual International FES Society Conference and 10th Vienna Int. Workshop on FES, pages 49–51, Vienna, Austria, 2010.

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