Quantitative mapping of the effect of botulinum toxin injections in the thyroarytenoid muscle.

Spasmodic dysphonia has been successfully treated by thyroarytenoid muscle injections of botulinum toxin (Botox) with dosages ranging from 0.625 to 25 U. In some patients, excessive paralysis with resulting breathiness and aspiration have been noted. In order to maximize the efficiency of Botox injections, the histologic effects of various Botox dosages were examined in the dog. Nine canine thyroarytenoid muscles were injected with 0.5 to 12.5 U of Botox. After 24 hours, the recurrent laryngeal nerve to the injected muscle was electrically stimulated in order to deplete the glycogen within the muscle fibers. Frozen sections of this muscle were then stained for glycogen. Those fibers that retained their glycogen were presumed paralyzed by the Botox injection. The extent of paralysis was found to be dose-related from 1.0 to 7.5 U. At 10 U and above the muscle was completely paralyzed. Spread of the toxin to the lateral cricoarytenoid muscle was seen at doses as low as 1.0 U. Clearly, doses less than 10 U appear sufficient for clinical paralysis.