An alarm tone warns the surgeon that the position of his scalpel is dangerously close to the nerve leading to the patient's vocal cords - the beep tone signals excessive pressure on the nerve.
Nerves are difficult to detect with the naked eye, because they have the same color and structure as connective tissue and minor blood vessels. And the risk is considerable: If, for example, the nerve serving the vocal cords is damaged during an operation on the thyroid gland, the patient might be plagued by chronic hoarseness, loss of voice or breathlessness.
The new alarm system combines the skills of numerous research partners. The Fraunhofer Institute for Biomedical Engineering IBMT was joined by the University Hospital in Mainz and the Robert Bosch Hospital in Stuttgart, along with the companies Dr. Osypka GmbH, Reinhardt Microtech GmbH and Inomed Medizintechnik GmbH. The project was one of the winners of the 2006 innovation competition for medical engineering. Taking the example of operations on the thyroid gland and on the minor pelvis, the researchers developed the first flexible electrodes capable of continuously monitoring the nerves and warning the surgeon on time. "At present, it is not possible to verify until after surgery whether the patient's nerves are still responding," says IBMT project manager Klaus Peter Koch. "The operating zone is too narrow to allow continuous use of conventional electrodes."
In a pilot project, the researchers have been able to demonstrate the basic feasibility of continuously monitoring the nerve to the vocal cords during thyroid surgery. The two pairs of electrodes are attached to the respiration tube inserted in the patient's trachea. Under software control, one pair of electrodes stimulates the nerve into contracting the laryngeal muscle. The second pair of electrodes records the muscle response, which is immediately evaluated by the software. The planned system even contains a safeguard against imperfect attachment or slippage of the electrodes: the software calculates which of the numerous electrode contacts is in the best position to stimulate the nerve.
Whereas there is only one nerve trunk leading to the vocal cords, there is a whole network of nerve fibers serving the bladder. A study is under way to identify the best nerve that the researchers could use to stimulate the plexus. If the monitoring system successfully passes its clinical trials, it could also be used to protect nerves during other types of operation. Accidental damage, Koch hopes, could be reduced to less than half the present number of cases.