Botulinum Toxin Resistance: A Comprehensive Systematic Review of Mechanisms, Risk Factors, Diagnosis, and Management Strategies

Background: Botulinum neurotoxin (BoNT) has transformed the treatment of neurological, pain, and aesthetic conditions by inducing temporary muscle paralysis through inhibition of acetylcholine release at the neuromuscular junction. However, treatment failure or resistance poses a significant clinical challenge. Resistance manifests as primary non-response (innate insensitivity) or secondary non-response (loss of efficacy after initial success), driven by immunological factors like neutralizing antibodies (NAbs) or non-immunological factors such as suboptimal dosing or inaccurate muscle targeting. Factors like cumulative dose, injection frequency, and BoNT formulation influence NAb development, with newer formulations showing lower immunogenicity. Diagnosis integrates clinical and laboratory assays, though current NAb testing has limitations. Management strategies include optimizing treatment parameters, switching serotypes, or exploring novel toxins.Methods: A systematic search of PubMed, Embase, Web of Science, and Cochrane Library was conducted, focusing on literature from 2015–2025, with seminal older works included for foundational context. Keywords included "botulinum toxin resistance," "immunogenicity," and "neutralizing antibodies." Inclusion criteria encompassed peer-reviewed studies on human subjects addressing mechanisms, risk factors, diagnosis, or management of BoNT resistance. Data were extracted using a standardized form and synthesized qualitatively into themes: molecular mechanisms, immunological and non-immunological factors, diagnosis, and management.Results: Resistance is multifaceted, with NAbs causing secondary failure in 10.1–10.3% of cases, varying by indication (e.g., 2.1% in cervical dystonia, 26.7% in blepharospasm). Risk factors for NAb formation include high cumulative doses, frequent injections, and complexing proteins in formulations, though recent studies suggest HLA polymorphisms play a significant role. Non-immunological causes, such as suboptimal dosing or incorrect diagnosis, are prevalent and often correctable. Diagnostic approaches combine clinical assessment (e.g., dose creep) with assays like the Mouse Hemidiaphragm Assay (MHDA), though ethical and practical limitations highlight the need for in vitro alternatives. Management includes dose optimization, EMG-guided injections, switching to less immunogenic formulations (e.g., incobotulinumtoxinA), or alternative serotypes (BoNT/B, BoNT/F), alongside emerging strategies like novel serotypes (e.g., BoNT/X).Conclusions: BoNT resistance requires a systematic approach to distinguish immunological from non-immunological causes. Clinicians should prioritize correcting non-immunological factors before diagnosing NAb-mediated resistance. Advances in low-immunogenicity formulations and diagnostic tools are critical to sustain BoNT’s therapeutic efficacy. Future research should focus on in vitro NAb assays, genetic predictors of resistance, and novel formulations to ensure long-term benefits for patients.