New horizons in dynamic rehabilitation of paralyzed laryngeal functions

Bilateral vocal cord paralysis can result in inspiratory difficulty, while stroke can impair swallowing and be followed by severe aspiration. Ideally, these mutually exclusive functions must be rehabilitated dynamically. The principle of the artificial reflex arc (ARA) entails appropriate pick-up information, which is then sent to a modulator that in turn synchronously stimulates the impaired effector. In vocal cord paralysis (six dogs), respiratory information has been picked up via tracheal strain gauges and transthoracic impedance electrodes, producing an analog voltage proportional to respective changes during inspiration. A trigger and a stimulator circuit were used to drive an electrode placed around a nerve-muscle pedicle previously implanted into the posterior cricoarytenoid muscle. Two msec square waves with 1.8 V amplitude and 50 Hz frequency resulted in frank vocal cord abduction synchronous with inspiratory flow. Conversely, vocal fold closure can be paced from information originating from strain gauges placed on the hypopharynx (four dogs). Stretch stimulating deglutition resulted in controlled output of a constant current stimulator driving an electrode passed around both recurrent laryngeal nerves. An adjustable stimulus from 0 to 20 mA at 25 Hz produced tetanic closure of the vocal folds. This might offer promise for electronic control of the transplanted larynx.