Excitation thresholds for nerves reinnervating the paralyzed canine larynx

Electrical stimulation of paralyzed laryngeal muscles implanted with nerve-muscle pedicles (NMP) has resulted in documented return of motion. No study, however, has yet determined how NMP excitability correlates with that of normal muscle or nerve. In six anesthetized dogs, one hemilarynx was denervated and the paralyzed thyroarytenoid, cricothyroid, and posterior cricoarytenoid muscles were reinnervated via NMPs originating from the ansa hypoglossi nerve. After 4.6 to 5.7 months, an electric stimulator delivering biphasic pulses of variable amplitude and widths was used to test thresholds for contraction in nine stimulatable NMPs, six intact recurrent laryngeal nerves (RLN), and five normal cervical muscles. With one exception (2.1 mA), NMP rheobases varied between 0.0002 and 0.04 mA (mean = 0.020 SD +/- 0.012, n = 6). Two NMPs belonging to animals stimulated for several hours had higher values (0.1 mA). Rheobase varied from 0.01 to 0.09 mA for control RLNs (mean = 0.058 SD +/- 0.025), and from 0.05 to 0.35 mA for muscles (mean = 0.144 SD +/- 0.109). Histologic correspondence with reinnervation was established in implanted muscles by type grouping on ATPase stains. These data suggest that 1) nerve pedicles may offer promise for the eventual construction of implantable low energy consuming laryngeal devices, and 2) the appropriate charge to be injected over time remains to be determined.