Histology of nerves and muscles in adductor spasmodic dysphonia

To elucidate the etiology and pathophysiology of spasmodic dysphonia, we examined the adductor branch of the recurrent laryngeal nerve and the lateral cricoarytenoid muscle from 9 consecutive patients with this disorder who were previously treated with botulinum toxin. Histologic examination revealed average muscle fiber diameters ranging from 21 to 57 pm. Botulinum toxin treatment-related muscle atrophy was observed up to 5 months after injection. Endomysial fibrosis was present in all samples. Histochemical analysis in 8 patients revealed type 2 fiber predominance in 7 patients and fiber type grouping in 2. Type-specific muscle fiber size changes were not present. Nerve samples were examined in plastic sections. In 8 patients the nerves contained homogeneous, large-diameter myelinated nerve fibers and sparse small fibers. One patient had a relatively increased proportion of small myelinated nerve fibers. Overall, the nerve fiber diameter was slightly larger in patients than in controls. These findings may implicate the central nervous system in the pathophysiology of adductor spasmodic dysphonia.