Laryngeal Reinnervation

Nerve Characteristics

The RLN contains 1000 to 4000 motor axons, efferent axons, and sympathetic and parasympathetic secretomotor fibers, depending on the level at which the count is made. The RLN gives off branches to the cricopharyngeus and upper esophageal muscles and also gives off a sensory branch that communicates with the superior laryngeal nerve (SLN) prior to entering the larynx. Five hundred to a thousand fibers are in the motor branches of the RLN. Prior to branching within the laryngeal framework, the motor fibers to the various muscles are intermixed throughout the RLN nerve trunk, making selective reinnervation at this level impractical. The anterior motor branch of the RLN enters the larynx posterior to the cricothyroid joint. The first branches that emerge from the RLN innervate the horizontal and oblique compartments of the posterior cricoarytenoid (PCA) muscle before entering the interarytenoid (IA) muscle. Separate branches innervate the adductor laryngeal muscles. In a majority of larynges, the nerve branch to the horizontal compartment of the PCA has a communicating branch to the otherwise separate nerve to the IA muscle, further complicating the separation of adductor and abductor innervation. The branch to the PCA has characteristics of a slow-twitch motor nerve with axons containing 200 to 250 muscle fibers in each motor unit. The IA muscle receives a separate branch from each RLN, resulting in bilateral innervation. The terminal branches of the RLN innervate the lateral cricoarytenoid (LCA) and thyroarytenoid (TA) muscles. The axons in these branches are more characteristic of fast-twitch fibers, with motor-unit sizes of 2 to 20 muscle fibers. In some larynges, a connection between the SLN and the terminal fibers of the RLN can be seen within the TA muscle.

Except for proprioceptive fibers carried within the nerve, the ansa cervicalis is a purely motor derivative of the ventral rami of the cervical plexus. Fibers from the first cervical nerve (C1) join the hypoglossal nerve until it curves anteroinferiorly. At this point the C1 fibers leave the hypoglossal nerve to form the superior root of the ansa cervicalis. The geniohyoid and thyrohyoid muscles are supplied entirely by C1 fibers. The branch to the superior belly of the omohyoid muscle typically originates from the superior loop of the ansa cervicalis. Because of its proximity to the thyroid ala and infrahyoid position, this branch has been frequently used as a donor for reinnervation using the nerve-muscle pedicle (NMP) technique. The other strap muscles receive motor fibers from second and third cervical nerves (C2 and C3), primarily via the inferior root of the ansa cervicalis. The lower portions of the sternothyroid and the sternohyoid muscles receive terminal nerves that branch off the loop of the ansa cervicalis as it passes deep to the omohyoid muscle. Although the above is a more classical description of ansa cervicalis anatomy, variations are common. Occasionally C1/C2 fibers may branch from the vagus nerve. Less commonly, C2/C3 fibers may branch directly off their respective ventral rami.

Muscle Characteristics

The performance of a particular muscle depends on the character of the individual muscle fibers it contains. Based on their histochemical and contraction properties, the fibers of skeletal muscle can be classified as slow fatigue-resistant (type 1), fast fatigue-resistant (type 2A), or fast fatigable (Type 2B). Several other classifications and subtypes exist (see Staron for review). Type 1 fibers are characterized by a low peak tension, slow contraction time, a low threshold of recruitment, fatigue resistance, and a dependence on aerobic metabolism. Muscles that are required to produce low tension over a prolonged period of time generally have a high proportion of type 1 fibers. Type 2 fibers are found to play a prominent role in muscles that can generate high tension for short periods: type 2B fibers have a fast contraction time and can generate a higher tension, but they are more easily fatigued, have a higher threshold of activation, and depend on anaerobic metabolism; type 2A fibers are intermediate in tension, fatigability, threshold, and make use of both aerobic and anaerobic metabolism. The laryngeal muscles also have high-velocity, low-tension fiber types (2L or mixed fiber type) that may be similar to those found in extraocular and jaw-closing muscles.

Most mammalian skeletal muscles are made up of multiple fiber types. Because of these combinations, the various whole muscles have a variety of contraction and relaxation characteristics. The peak contraction time of the extraocular muscles is the fastest (7 msec) while the soleus is the slowest (90 msec).

Effects of Denervation

With denervation, the area of sensitivity to acetylcholine, which is limited to the endplate region in the intact muscle, spreads out over most of the external membrane. Denervation also eliminates both the trophic and activity-related influences on the muscle, resulting in muscle atrophy. Without reinnervation, there is progressive atrophy and eventual destruction of the muscle, despite an adequate supply of all nutrients.

Loss of innervation will result in a change of muscle fiber type composition. In denervated human PCA muscles, the percentage of type 1 fibers decreased while type 2 fibers increased when compared to controls. The maximal transition of fiber types in human PCA muscle occurs between 1 and 2 years after denervation. Embryonic, perinatal, and IIx myofibers were significantly up-regulated in denervated muscle.

Human skeletal muscle, in general, apparently is able to survive at least 3 years after denervation. Successful laryngeal reinnervation has been claimed at much longer times after onset of laryngeal paralysis. Muscle survival in the larynx may be helped by a low-grade but functionless reinnervation via the RLN that was found to occur in the range of 2.9% to 39.4% of the average fiber count of a normal RLN. Innervation of laryngeal muscles can also come from misdirected reinnervation from autonomic nerves.

Restoration of Function

Successful regeneration of injured nerves requires (1) a neuron responding to the injury with the metabolic changes necessary to support axonal regrowth, (2) an environment around the injured axon permitting axon growth, and (3) guidance clues for restoration of function.

Regeneration is most effective when the regenerating axons grow back to reinnervate the original targets. Motor axons will preferentially reinnervate distal motor branches of a severed nerve. The specificity is gained primarily through pruning of misdirected fibers. Some evidence for a neurotrophic mechanism for directional nerve regrowth exists, but the usual outcome following nerve injury suggests that the specificity is low. A major problem is that after division of a nerve, a functional specificity in motor neuron regeneration for the original muscle does not exist. The result, in a regenerating nerve supplying several muscles such as the RLN, is synkinesis.

Substantial evidence suggests that reinnervated muscle takes on the characteristics of the donor nerve. Muscle fiber types can change under the influence of activation pattern and altered innervation. Thus the selection of a donor nerve should ideally take into account the fiber type and contraction characteristics of the muscle to be reinnervated. Reinnervation with a new donor nerve would be expected to change the fiber type composition of the reinnervated muscle and thus contraction characteristics of the donor muscle.

Selective Reinnervation

Because of the difficulty in obtaining motion of the vocal folds in coordination with respiration, reinnervation at the level of the nerve trunk has not gained acceptance as a clinical modality in bilateral vocal fold paralysis. Selective innervation of individual laryngeal muscles—in particular the PCA muscle—is required; this can be achieved by neuromuscular pedicle (NMP) technique, selective anastomosis, or direct nerve implantation.

The activity and anatomy of the phrenic nerve makes it an excellent candidate for reinnervation of the PCA. A similar muscle fiber type profile between the diaphragm and the PCA exists as well. Abductor mobility of the vocal fold has been obtained in multiple animal models. Experimental reinnervation after a 9 month delay is successful but the functional recovery is reduced. Attempts at creating vocal fold motion with phrenic nerve reinnervation in humans were initially unsuccessful.

More recent attempts at selective reinnervation have shown promise of restoring vocal fold motion. Innervation of both PCA muscles can be accomplished using a single upper phrenic nerve root by using an interposition nerve graft to the contralateral PCA. C1 fibers innervating the thyrohyoid muscle are used to innervate the adductor muscles. With use of this technique three of six evaluable patients achieved active arytenoid abduction. Section and reimplantation of the RLN between the PCA and IA branches with implantation of the proximal stump into the PCA may help reduce synkinesis. Li and colleagues were able to demonstrate some ipsilateral abductive vocal fold motion in 38 of 44 patients (87%) with return of normal pulmonary function at 1 year following surgery using a unilateral phrenic nerve transfer and sectioning of all adductor RLN branches. A free nerve graft was used to innervate the contralateral PCA. Contralateral vocal fold excursion was significantly less but was present in 32 patients (72%).