According to an investigation published in the January issue of Anesthesia & Analgesia, official journal of the International Anesthesia Research Society (IARS), researchers have discovered that injecting Botox (botulinum neurotoxin type A) affects muscles other than those it’s injected into.

The study, led by Dr Christiane G. Frick of Massachusetts General Hospital, Boston, demonstrates that rats injected with Botox display lasting effects on muscles after injection, even in muscles far from where Botox was injected.

Furthermore, Botox appears to have “unique” effects on the way muscles respond to a commonly used muscle relaxant. This may affect monitoring patients during mechanical ventilation or during surgery.

The team conducted tests in order to evaluate the local and distant, immediate and delayed effects Botox has. In addition to being used for cosmetic purposes, Botox is used to treat neuromuscular disorders. Botox temporarily paralyzes the injected muscle – whether in muscles that cause forehead wrinkles or spastic muscles in individuals with cerebral palsy – by blocking the signals between nerves and muscle tissue.

In the study, the researchers injected Botox into the tibialis muscle of the rat’s hind leg. As anticipated the injected muscle was completely paralyzed four days later. However, when the team used electric stimulation on the opposite leg, they discovered twitch responses also decreased, even without any considerable effect on muscle function.

Decreased function, muscle tension, and twitch responses were still observed in the Botox-injected muscle 16 days later. Furthermore, in the opposite leg, although to a lesser degree, twitch responses and muscle tension remained considerably decreased. This finding indicated that, as well as temporarily paralyzing the injected muscle, Botox effects lasting changes in “un-injected” muscles as well.

The team also discovered alterations in responses to atracurium (the muscle relaxant medication). This finding may have vital clinical implications, as the drug is commonly used to relax patients’ muscles during surgery or mechanical ventilation. The muscles injected with Botox demonstrated a uniquely increased sensitivity to the effects of the drug, even after 16 days.

Physicians usually monitor muscle “twitch” responses in order to evaluate atracurium is at relaxing muscles. The novel discoveries raise the possibility that recent Botox injections – including Botox for cosmetic treatments – may affect patient monitoring during surgery. This appears to have occurred in one recently reported case, in which the anesthesiologist used twitch responses in the forehead in order to monitor the extent of atracurium block in other parts of the body.

Dr. Steven L. Shafer of Columbia University, Editor-in-Chief of Anesthesia & Analgesia, explains:

“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. 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.”

Written by Grace Rattue