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New study: Botox affects 'distant' muscles, too

Article-New study: Botox affects 'distant' muscles, too

Boston — Results of a new study suggest that Botox (onabotulinumtoxinA, Allergan) has previously unsuspected and long-lasting systemic effects on muscles other than the ones into which it's injected.

In experiments with rats, a research team led by Christiane G. Frick, M.D., of Massachusetts General Hospital, examined the immediate, delayed, local and distant effects of Botox injections. Investigators found that the lasting effect of Botox injections occurs in muscles distant from as well as near the injection site, and that injections appear to cause a unique effect on muscle responses to a commonly used relaxant. Medical News Today reports that this could affect patient monitoring during surgery or mechanical ventilation.

Researchers injected rats with a single dose of Botox into the tibialis muscle of the hind leg. As expected, the injected muscle was completely paralyzed after four days — but researchers also noted that the tibialis muscle in the other hind leg also displayed lower twitch responses to electric stimulation, although no substantial effect on muscle function was observed. At 16 days, the Botox-injected muscles still showed decreased function, twitch responses and muscle tension, and the non-injected tibialis muscles’ twitch responses and muscle tension also remained substantially lower, though to a lesser extent.

The finding indicates that Botox causes long-lasting changes in distant muscles in addition to the temporary paralysis it causes in the injected muscles.

Investigators also noted that there were changes in responses to the muscle relaxant atracurium, which could result in significant clinical implications, given that atracurium is commonly administered to patients undergoing surgery or mechanical ventilation to relax the muscles. Researchers observed that in particular, muscles injected with Botox showed a uniquely higher sensitivity to atracurium that lasted longer than 16 days.

"Botulinum toxin has local and distant effects on muscle," the authors wrote in the study abstract. "The decrease in specific twitch tension indicates that the muscle atrophy alone cannot explain the functional changes; neuromuscular transmission is also impaired. An increased sensitivity to atracurium on the toxin-injected side ... seems unique to botulinum toxin."

Kellie Lao, manager, corporate affairs and public relations, Allergan, notes the difficulty in translating results of an animal study to humans, but says the research "generally supports the known pharmacology of Botox, and possible risks are noted in the package insert." She says the insert states that "co-administration of Botox and aminoglycosides or other agents interfering with neuromuscular transmission (e.g., curare-like compounds) should only be performed with caution, as the effect of the toxin may be potentiated."

The study appears in the January issue of Anesthesia & Analgesia.

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