Botox-Injected Muscle Damage is Not Confined to the Injection Site

Objectives: Paralysis of the masseter muscles using botulinum toxin (BTX) is a common treatment for cosmetic reduction of the masseters, muscle spasm, and pain. We previously reported muscle degeneration and collagen accumulation near the injection site in rabbit masseters (IADR 2013 #1016); however, it was not clear whether these changes were due to localized damage or whether they affected distant parts of the masseter. The purpose of the present study was to examine a site in the muscle distant from the injection site to determine if it also showed degenerative changes.
Methods: Rabbit masseters were injected unilaterally in the lower third of the muscle with BTX (n=8) or saline (n=4) and euthanized after 12 weeks of recovery. Muscles were cross-sectioned and stained with trichrome. Sections from the inferior third (injection) and superior third (distant) were compared for fiber size, fat and collagen content, and proportion of fibers with central nuclei.
Results: The distant location showed similar atrophy to the injection level (minimum fiber diameter 8µm±3 vs. 16µm±4 in controls, p<0.001), however, hypertrophied fibers were 2 times more frequent (p<0.03) and increased in diameter (maximum fiber diameter 89µm±18 vs. 66µm±10 in injection site, p<0.02 and 58 µm±6 in controls, p<0.001). Also similar to the level of injection, the number of fibers with central nuclei was 7 times greater than in the controls (p<0.01). Collagen and empty profiles resembling fat or degenerated muscle fibers appeared less prominent away from the injection site.
Conclusions: BTX had a diminished, yet still significant effect far from the injection site, with more unaffected fibers available to hypertrophy in order to compensate for the affected fibers. We speculate that the toxin diffuses within the muscle. Muscular damage from BTX injection is not a local phenomenon but is widespread in the masseter.