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Botox is a well known neuromuscular modulating agent that produces a temporary muscle relaxation effect. It is mostly known for its positive aesthetic benefits in decreasing wrinkles in the forehead and around the eyes due to its effect on the muscles of facial expression. While the facial muscles may have a diminished action, the effect is temporary and is due to blocked axonal transmission across the neuromuscular synapses.

Botox is also effective on decreasing the size of masticatory muscles such as the masster and temporalis muscles. In this case the blocked synapses doe not necessarily decrease  muscle motion but cause a temporal atrophic effect where the muscle actually decreases in size. This effect has served as the basis for an injectable method to decrease the size of prominent masseter muscles and a wide lower face, most commonly used in Asian patients.

It has been postulated that this lower facial thinning effect is also due to a decreased thickness of the overlying fatty tissue as well as a decrease in the muscle’s thickness.

In the February 2018 issue of the Aesthetic Surgery Journal an article was published entitled ‘Does Botulinum Toxin Injection into Masseter Muscles Affect Subcutaneous Thickness?’ In this study the authors used ultrasonography to measure any changes in the muscle thickness as well as that of the overlying subcutaneous fat layers twenty patients. They divide into two groups of 10 who received either 25 units of Botox into each masseter muscle or a control group of saline injections. The tissue thicknesses were taken at one, two and three months after the injections. The subcutaneous thickness did not differ significantly over time either at rest or during muscle contraction or between the Botox-injected or saline-injected control groups at rest or during muscle contraction. This was in contrast to muscle thickness which did decrease significantly compared to the saline treated patients.

The effect of Botox injections into the masseter muscles is, perhaps unsurprisingly, a change in the size of the muscle. This decreased muscular size does not induce a concomitant overlying loss of fat. There does not appear to be any immediate trophic effect of muscle size and activity  on the fat that lies over it.

Dr. Barry Eppley

Indianapolis, Indiana

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