What Happened to Masticatory Muscles after Botulinum-Toxin-Type A Injection into Masseter: A Systematic Review

Botulinum toxin type A (BoNT-A) is commonly used in neuromuscular disorders and cosmetic fields, but the detailed histological and subcellular changes in masticatory muscles, mainly in masseter, remain underexplored.

The literature review was conducted using the PubMed and Cochrane databases between January 2000 and September 2024. Studies of BoNT-A injection in masseters with available full- text and extractable data were included and were grouped depending on injected species. The sample size, dose of injection, follow-up duration, and detailed muscle changes were concluded.

Fourteen articles were eligible for reviewing, among which 2 were conducted on humans, and 12 were conducted on animals including rats (n=5), mice (n=2), rabbits (n=3) and pigs (n=2). Morphological analysis showed muscle atrophy with variable recovery. Histological findings included decreased fiber diameter, disrupted arrangements, and shifts in myosin-heavy-chain fiber types. Immunohistochemistry indicated increased cell proliferation and nuclear centralization. Electrophysiological assessments demonstrated reduced EMG amplitudes and shorter signal durations, correlating with muscle size and bite force. Ion analysis revealed increased sodium, chloride and sulfur, and decreased potassium and phosphorus.

BoNT-A injection causes dose-dependent denervation and dysfunction in masticatory muscles, with incomplete recovery over time. However, due to very limited human data, these findings should be interpreted cautiously, and strong clinical extrapolations are not justified. These findings provide valuable insights into the long-term effects of BoNT-A on muscle structure and function, highlighting the need for further research into the mechanisms underlying these changes and their clinical implications for both therapeutic and aesthetic applications.

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