The effects of tetanus toxin on the orbicularis oculi muscle

Purpose: Tetanus toxin can cause localized neuromuscular weakness, but it a lso can produce systemic tetany. The action of tetanus toxin on the orbicul aris muscle has not been studied in animals immunized to prevent systemic t etany. Our objective was to determine whether tetanus toxin could be used t o treat orbicularis oculi muscle spasms. Methods: We analyzed the clinical, electrophysiologic, and histopathologic effects of tetanus toxin injected into the orbicularis oculi muscle of rabb its with passive immunity to tetanus toxin. In six rabbits, the orbicularis oculi function in both eyes was assessed clinically, and the baseline orbi cularis oculi muscle action potential was measured physiologically with ele ctromyography (EMG). The rabbits then were immunized against tetanus toxin with tetanus immunoglobulin for immediate and definitive immunity. Tetanus toxin was injected into the left orbicularis oculi muscles, leaving the rig ht eyes as controls. Ten days later, the rabbits were again assessed by cli nical examination and with EMGs on both the injected side and the noninject ed side. The animals were killed at 14 days, and the orbicularis muscle was removed from both sides. The injected and control tissues were examined mi croscopically for signs of neuromuscular denervation. Results: All six rabbits showed weakness in eye closure on the side injecte d with tetanus toxin. In addition, four rabbits developed complete ear ptos is on the tetanus toxin injected side because of spread of the toxin to adj acent ear muscles. EMGs showed both a denervation of the orbicularis oculi muscle and a poor blink potential on the side injected with tetanus toxin. Histopathologic studies of the orbicularis oculi muscle injected with tetan us toxin showed angulation of both slow and fast types of muscle fibers com patible with neuromuscular denervation. Conclusions: Tetanus toxin can cause localized orbicularis oculi weakness, as documented clinically, physiologically, and microscopically, without pro ducing systemic tetany in immunized rabbits. Tetanus toxin may have a poten tial application in the treatment of blepharospasm and hemifacial spasm.