Implant stimulation of vocal fold adduction (Heaton et al., 2023)
Purpose: Medialization procedures for unilateral vocal fold (VF) paralysis generally improve voice but do not fully replace dynamic VF adduction. Paralyzed VFs typically experience synkinetic reinnervation, which makes it feasible to elicit movement through electrical stimulation. We tested a novel laryngeal pacing implant capable of providing closed-loop (automatic) stimulation of a VF triggered by electromyography (EMG) potentials from the contralateral VF.
Method: A custom, battery-powered, microprocessor-based stimulator was tested in eight dogs with bipolar electrodes implanted for recording EMG from the left VF and stimulating adduction of the right VF. A cuff electrode on the left recurrent laryngeal nerve (RLN) stimulated unilateral VF adduction, modeling voluntary control in anesthetized animals. Closed-loop stimulation was tested in both acute and chronic experiments. Synkinetic reinnervation was created in two animals by right RLN transection and suture repair to model unilateral VF paralysis.
Results: In all animals, left VF activation through RLN stimulation generated a robust EMG response that rapidly triggered stimulation of contralateral thyroarytenoid and lateral cricoarytenoid muscles, causing nearly simultaneous bilateral adduction. Optimal triggering of VF stimulation from elicited EMG was achieved using independent onset and offset thresholds. Real-time artifact blanking allowed closed-loop stimulation without self-perpetuating feedback, despite the proximity of recording and stimulation electrodes.
Conclusions: Using a custom implant system, we demonstrated real-time closed-loop stimulation of one VF triggered by the activation of the contralateral VF. This approach could potentially restore dynamic glottic closure for reflexive behaviors or phonation in cases of unilateral VF paralysis with synkinetic reinnervation.
Supplemental Material S1. Surgical implantation of double-sided paddle electrodes into the paraglottis to stimulate the thyroarytenoid muscle (via upward-facing paddles) and lateral cricoarytenoid muscles (via downward-facing paddles).
Narration: “Dual-function recording and stimulating electrodes are being placed in a dog’s vocal folds through a horizontal incision at the level of the cricothyroid ligament. These multi-contact, double-sided paddle electrodes are being inserted between the TA muscles (superiorly) and LCA muscles (inferiorly) after blunt dissection just lateral to the cricothyroid membrane. Proper electrode position is confirmed by observing independent TA and LCA stimulation with the dog under suspension microlaryngoscopy.”
Supplemental Material S2. Closed-loop stimulation of bilateral VF adduction, with the BLS implant monitoring activity elicited in the left VF (via left RLN stimulation), which is used as a trigger for right VF stimulation. Closed-loop stimulation of the right VF by the implant was turned on and off, as indicated by the labels.
Narration: “In this example the left recurrent laryngeal nerve is periodically stimulated. EMG recorded from the left vocal fold is used to trigger stimulation of right vocal fold adduction. The stimulation implant is being turned on and off to demonstrate unilateral closure versus stimulated bilateral closure, thereby representing treatment for a patient with right vocal fold paralysis.”
Supplemental Material S3. Closed-loop stimulation of bilateral VF adduction, with the BLS implant monitoring reflexive motor activity in the left VF when the left laryngeal mucosa is touched, which is then used as a trigger for right VF thyroarytenoid muscle stimulation.
Narration: “In this example, the implant is monitoring activity in the left VF as a trigger for stimulating the right VF. Touching the right VF causes subtle twitching, but not adduction. Touching the left vocal fold elicits EMG on that side, which is detected by the implant, and triggers stimulation of the right VF until the twitches subside. This shows that the implant can be sensitive to relatively weak EMG activity.”
Supplemental Material S4. Closed-loop stimulation of bilateral VF adduction producing phonation. The BLS implant monitors reflexive motor activity from the left thyroarytenoid muscle when the left laryngeal mucosa is touched, which is used as a trigger for bilateral lateral cricoarytenoid muscle stimulation, causing glottic closure and phonation.
Narration: “EMG elicited in the left VF is being used to trigger stimulation of bilateral closure. A double-sided paddle electrode is recording EMG from the left TA from the paddle’s superior surface. The implant is then triggering bilateral LCA stimulation from the inferior paddle surfaces. Stimulation artifact blanking allows EMG recording and stimulation from within the same VF.”
Supplemental Material S5. Demonstration of unilateral and bilateral stimulation of the VF muscles using different electrode contacts on double-sided paddle electrodes implanted between the TA and LCA muscles. The drawing depicts the paddle electrodes positioned between the TA above and LCA below on the right and left sides. The animation of radiating electricity represents the hypothetical currents delivered by the superior and inferior paddle surfaces, corresponding to the VF movements occurring in the microlaryngoscopy video. Tracheal airflow during bilateral stimulation of TA and LCA muscles produced lower-frequency phonation (62 Hz) while the VFs were shortened, which increased in fundamental frequency (to 83 Hz) once TA stimulation was discontinued and the VFs lengthened.
Narration: “Here the [left] LCA is stimulated twice through the inferior paddle contacts, followed by left TA stimulation using the superior paddle contacts. Next the LCA muscles are stimulated bilaterally, followed by bilateral TA stimulation. Bilateral closure for phonation is shown with initially too much TA activation, which improves when turning off the TA stimulation.”
Heaton, J. T., Kobler, J. B., Otten, D. M., Tynan, M. A., Petrillo, R. H., Ottensmeyer, M. P., Slate, A. R., Hillman, R. E., & Zeitels, S. M. (2023). Electrical stimulation of vocal fold adduction triggered by laryngeal electromyography using a custom implant. Journal of Speech, Language, and Hearing Research. Advance online publication. https://doi.org/10.1044/2023_JSLHR-23-00377