Muscle fiber-type changes induced by botulinum toxin injection in the rat larynx

This study examined muscle fiber-type alterations after single or multiple botulinum toxin (BT) injections to better understand possible morphologic c hanges induced by therapeutic BT injections in patients with spasmodic dysp honia. Muscle fiber staining was accomplished in rat intrinsic laryngeal mu scles with antibodies to specific myosin heavy chains. Results indicated th at the typical baseline distributions of type II muscle fibers (ie, types I Ia, IIb, IIx, and III) were altered by BT injection, while no change was ob served in type I fibers. Embryonic fibers were observed only along the need le insertion site at 7 days post BT injection. Although inferences from the se animal data to human neuromuscular function must be made with caution, o ur findings provide insight into the possible cellular and molecular change s characterizing BT-injected muscles.