The paths of the recurrent nerves from the central nervous system largely determine the causes of vocal fold paralysis. While the superior laryngeal nerve alone may be dysfunctional, recurrent nerve injury, either alone or in combination with the superior nerve, is necessary for gross vocal fold immobility.
The recurrent nerve may be subjected to a variety of mechanical trauma. It may be sectioned during surgery. It may be stretched or compressed acutely during surgical manipulation, or chronically by a mass growing adjacent to the nerve. It may be invaded by malignancy, and it may be devascularized as a result of nearby surgical dissection. Radiation produces fibrosis in and around nerves that may result in neuropathy as well.
Mild pressure to a peripheral nerve produces segmental demyelinization and impairs axonal transport, the degree and severity of the conduction block being proportional to the severity of the injury . Remyelinization over the preserved axon usually restores conduction and function. Injuries which interrupt the axon, such as nerve section or severe crush, produce complete wallerian degeneration along the entire length of the nerve distal to the injury beginning within 24 b of injury. Functional recovery depends on preservation of the neural conduits for axonal regeneration. The basal lamina of the original nerve fibers is usually preserved in severe crush (axonotme-sis), and proximal axonal sprouts may reach the appropriate muscle. If the nerve is transected (neurotmesis), the proportion of fibers that find their way to the original target is low and generally inversely proportional to the size of the gap between proximal and distal segments.
Regeneration of the recurrent nerve is more problematic than most peripheral nerves, because it carries mixed adductor and abductor fibers to a population of highly specialized muscles whose function must be precisely graded and tightly integrated for breathing, swallowing and phonation. The problems of synkinesis and misdirected reinnervation are detailed elsewhere in this volume.
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