Botox can have unsuspected 'systemic' effects on muscles

Botulinum neurotoxin type A—better known as Botox—has previously unsuspected 'systemic' effects on muscles other than the ones it's injected into, reports a study in the January issue of Anesthesia & Analgesia, official journal of the International Anesthesia Research Society (IARS).

Experiments in rats show that Botox has lasting effects on muscles after injection—even muscles distant from the injection site. In addition, Botox seems to have "unique" effects on muscle responses to a widely used muscle relaxant, which could affect patient monitoring during surgery or mechanical ventilation.

Botox Injection Can Have 'Distant' Effects
Led by Dr Christiane G. Frick of Massachusetts General Hospital, Boston, the researchers performed experiments to assess the immediate and delayed, local and distant effects of Botox injection. Although more familiar from its use in cosmetic procedures, Botox is also used for treatment of neuromuscular disorders. Botox works by interrupting the signals between nerves and muscle tissue, causing temporary paralysis of the injected muscles—whether in the facial muscles that produce forehead wrinkles or spastic muscles in patients with cerebral palsy.

In the experiment, rats underwent a single injection of Botox into the tibialis muscle of the hind leg. Four days later, as expected, the injected muscle was completely paralyzed. However, the tibialis muscle in the opposite leg also showed decreased twitch responses to electric stimulation—even in the absence of any significant effect on muscle function.

Sixteen days later, the Botox-injected muscle still had decreased function, twitch responses, and muscle tension. Although to a lesser extent, twitch responses and muscle tension in the opposite tibialis muscle also remained significantly reduced. This suggested that, in addition to causing temporary paralysis of the injected muscle, Botox causes lasting changes in "distant" muscles as well.

The researchers also found changes in responses to the muscle relaxant drug atracurium. That could have important clinical implications, because atracurium is widely used to relax the muscles of patients undergoing surgery or mechanical ventilation. Specifically, muscles injected with Botox showed a uniquely increased sensitivity to the effects of atracurium, which was still present after 16 days.

Muscle "twitch" responses are commonly monitored to assess the effectiveness of muscle relaxation with atracurium. The new findings raise the possibility that recent Botox injections—including Botox treatment for cosmetic purposes—could affect patient monitoring during surgery. This seems to have happened in one recently reported case, where the anesthesiologist was using twitch responses in the forehead to monitor the degree of atracurium block elsewhere in the body.

"Although we knew that Botox has lasting effects on muscle function, this study suggests that these muscle effects may be seen quite distant from the injected muscle," comments Dr. Steven L. Shafer of Columbia University, Editor-in-Chief of Anesthesia & Analgesia. "If you're a patient undergoing surgery who has had a recent Botox injection, it might be a good idea to mention it to your anesthesiologist."