Restoring abduction of paralyzed vocal cords in the cat using selective laryngeal reinnervation by phrenic motoneurons

Laryngeal reinnervation surgery - results of a selective approach in an animal study

Laryngeal reinnervation surgery is a difficult subject due to the fact that the recurrent laryngeal nerve (RLN) is responsible for both adductor and abductor laryngeal activity. Non-selective reinnervation procedures will result in laryngeal synkinesis with restoration of tonicity. Restoration of mobility requires selective reinnervation of the adductor and abductor branches with nerves with similar activity patterns as the initial abductor and adductor branches of the RLN.

Intrinsic muscles and distribution of the recurrent laryngeal nerve in the pig larynx

To use the pig larynx in studies of laryngeal reinnervation, it is essential to have a clear understanding of its anatomy. We aimed to define the macroscopic anatomy of the intrinsic muscles and the course of the recurrent laryngeal nerve (RLN) in the pig larynx. Twelve large white pig larynges were used. Five larynges were preserved in formalin, then dissected to study the anatomy of the intrinsic muscles. Seven larynges were stained using the modified Sihler's staining technique, which results in nerves being stained dark purple while the remainder of the larynx is rendered translucent. The intrinsic muscles of the pig larynx were similar to those in the human. The RLN gives off a branch that enters the posterior cricoarytenoid muscle (PCA) on its deep surface and supplies the entire muscle, although the branching pattern of the nerve within the muscle varies considerably. These results facilitate detailed reinnervation studies in the pig laryngeal transplant model.

Experimental reinnervation of a strap muscle with a few roots of the phrenic nerve in rabbits

In order to compare application of the roots of the phrenic nerve to the ansa hypoglossi for laryngeal muscle neurotization, 1 or more roots from the phrenic nerve were implanted into the right sternothyroid (RST) muscle of rabbits (n = 36). Controls were intact animals (in which RST innervation is provided by the ansa; n = 6) and denervated ones (n = 6). At 66 +/- 2 days (mean +/- SE) after neurotization, during quiet breathing, inspiratory electromyographic activity and isometric contraction force were observed in all reinnervated RST muscles (n = 24). During maximal inspiratory effort, electromyographic activity and force increased. In animals reinnervated by the C4 root alone, forces (46.22 +/- 7.8 g) were significantly higher than in intact animals (10.83 +/- 5.0 g). Retrograde labeling proved the phrenic origin of the neurotization. Electromyography of the diaphragm was recorded. We conclude that in rabbits, neurotization of a strap muscle by 1 or 2 roots of the phrenic nerve allows inspiratory contraction, even during quiet breathing. Such inspiratory activity is not observed in sternothyroid muscles of intact animals innervated by the ansa hypoglossi.

Cervical anatomy of phrenic nerve roots in the rabbit. European Group for Research on the Larynx

The cervical anatomy of the different nerve contributions that constitute the phrenic nerve (phrenic nerve roots and accessory phrenic nerve) were studied in rabbits. In 55 dissections, 6 main root arrangement types were observed. The roots that issued from the fourth and fifth cervical nerves (C4 and C5 roots) were constant. The C4 root was either short or long. The C6 root was at times absent, or sometimes double. An accessory phrenic nerve was present in 43% of the right and 28% of the left dissections. The distribution of the phrenic nerve roots often displayed left-right asymmetry. We conclude that a better knowledge of the cervical anatomy of the phrenic nerve is useful both in physiological studies involving diaphragm denervation and in experimental laryngeal reinnervation.

Phrenic nerve reinnervation of the cat's larynx: a new technique with proven success

Reinnervation of the posterior cricoarytenoid muscle (PCA) should provide vocal cord abduction on inspiration, and passive adduction to enable phonation. Previous investigators have shown that reinnervation is possible, but results have not been clinically encouraging. When reinnervation was successful, the question remained whether it was provided by the transplanted nerve or by the ingrowth of adjacent nerves. In this study the phrenic nerve was transplanted directly into the PCA in a series of 12 cats. Fibrin glue was used to overcome nerve trauma and to prevent retraction of the nerve from the PCA. Laryngoscopy, electromyography, and retrograde labeling of the phrenic motoneurons provided evidence of functional reinnervation in 9 cats. Partial or complete failure in the remaining 3 was due to retraction of the nerve from the muscle. These results appear to justify trials of the procedure in humans.

Selective reinnervation of the abductor and adductor muscles of the canine larynx after recurrent nerve paralysis

Functional rehabilitation of the larynx after unilateral vocal cord paralysis was attempted in the dog by selective reinnervation of the laryngeal muscles. The intralaryngeal branches of the right recurrent nerve were dissected. The adductor branch was anastomosed with the ansa cervicalis; the abductor branch was anastomosed with the trunk of the phrenic nerve either within the larynx or through the recurrent nerve, the adductor branch of which was sectioned. Results could be analyzed in seven dogs: mobility of the vocal cord was checked, and electromyography, stimulation of the nerves, and histologic studies were performed. Functional reinnervation of both the adductor and abductor muscles was obtained in only one case, with good abduction. Adduction was recorded in five cases. False-positive results emphasize the necessity of collecting several types of data before concluding that functional reinnervation has been accomplished. The reliability of the procedure can and must be improved.

Use of botulinum toxin to lateralize true vocal cords: a biochemical method to relieve bilateral abductor vocal cord paralysis

Using the mongrel dog as an animal model, we studied the effectiveness of botulinum toxin (Oculinum) to lateralize the true vocal cord. This experiment was undertaken in order to determine whether the toxin can improve the airway in subjects with bilateral abductor vocal cord paralysis. The toxin was injected into the cricothyroid muscle to block neuromuscular transmission at the motor end-plate. Paralysis of the cricothyroid muscle was achieved and documented by electromyography and videotaped endoscopy. Paralysis of the cricothyroid muscle decreases the tension of the true vocal cord and allows the cord to take a more lateral position. In this preliminary report, the literature is reviewed, the effect of and action of the toxin are discussed, and the results of the experimental protocol, establishment of dose-response curves, and techniques of injection are presented. Preliminary data suggest that there is an increase in the airway by lateralizing the true vocal cord with this biologic substance, and that this method may have many applications in clinical medicine for the otolaryngologist-head and neck surgeon.