Effects of Botulinum Toxin Injection on Masticatory Muscle Activity in Sleep Bruxism: A Systematic Review and...

Background/Objectives: Bruxism, encompassing both awake (AB) and sleep bruxism (SB), is a prevalent condition associated with involuntary jaw muscle activity, leading to tooth wear and muscle hypertrophy. Botulinum toxin type A (BoNT-A) has emerged as a potential treatment by reducing masticatory muscle hyperactivity. This systematic review aimed to evaluate the efficacy of administering BoNT-A injections into the masticatory muscles for reducing bruxism-related muscle activity, as measured by any validated instrumental or questionnaire-based method. Methods: A systematic search of PubMed, Scopus, and Web of Science was performed according to PRISMA guidelines, with the search updated through July 2026. Eight studies evaluating bruxism/masticatory muscle activity after BoNT-A injection, using electromyography (EMG), polysomnography, or validated questionnaires/clinical global impression scales, met the inclusion criteria. Five placebo-controlled randomized controlled trials were synthesized quantitatively; a meta-analysis using a random-effects (DerSimonian-Laird) model, chosen a priori given the anticipated methodological heterogeneity across studies and confirmed by a fixed-effect model computed as a sensitivity analysis, was used to calculate the pooled Hedges' g standardized mean differences with 95% confidence intervals (CIs) for bruxism/muscle-activity outcomes. Three additional studies with non-placebo designs-two active-comparator RCTs and one large observational study-were synthesized narratively because their designs (no placebo arm) were not compatible with pooling. Risk of bias was assessed with the Cochrane Risk of Bias 2.0 tool and the Newcastle-Ottawa Scale. Results: Because the five placebo-controlled trials captured two related but distinct constructs of muscle activity, construct-specific estimates are presented as the primary quantitative findings: a fixed-effect pooled Hedges' g of -0.70 (95% CI: -1.04 to -0.37) for studies measuring bruxism event frequency, and -1.31 (95% CI: -1.95 to -0.68) for studies measuring EMG signal amplitude, in patients with sleep bruxism (SB) compared with placebo. As a secondary, exploratory summary across both constructs, the overall random-effects pooled estimate was Hedges' g = -1.09 (95% CI: -1.65 to -0.54), with moderate-to-substantial heterogeneity (I² = 58.6%). The formal test for the numerical difference between these two construct-specific estimates was not statistically significant (p = 0.096), reflecting the small number of studies per subgroup; the overall pooled estimate is therefore best interpreted as a secondary, composite signal across related constructs. The three non-pooled studies-two active-comparator RCTs and a large real-world observational series (number of partecipants = 304)-independently reported reductions in masticatory muscle EMG amplitude after BoNT-A injection, consistent in direction with the pooled estimate. In contrast, of the six studies that also assessed bruxism frequency by questionnaire, clinical global impression, or PSG scoring, only one found a clear BoNT-A-specific benefit; the remaining five found no significant difference from the comparator, even when an EMG-based measure from the same participants did show a BoNT-A-specific reduction. Mild and transient adverse effects were reported, with no serious events. The certainty of evidence was rated as low-to-moderate regarding the precise magnitude of EMG-based muscle activity reduction (downgraded for construct heterogeneity and imprecision) and very low regarding any corresponding reduction in questionnaire- or clinically-assessed bruxism frequency, and low concerning the short-term safety of BoNT-A administration (downgraded from the RCT-default starting point for imprecision, given the small number of controlled participants and short follow-up, and for indirectness, given heterogeneous dosing protocols across studies). Conclusions: BoNT-A injections into the masticatory muscles consistently reduce EMG-detectable masticatory muscle activity in patients with SB, although the magnitude of this effect varies depending on whether event frequency or signal amplitude is used to quantify it. This EMG-measured reduction did not consistently correspond to a reduction in bruxism frequency as assessed by questionnaires, clinical global impression, or PSG scoring, and the two types of measures should not be assumed interchangeable when interpreting treatment response. BoNT-A consistently reduces EMG-detectable masticatory muscle activity and, on this mechanistic basis, may be considered a candidate adjunctive therapy, particularly for individuals unresponsive to conservative treatments; claims of clinical efficacy should specify which type of bruxism measure they refer to, since this review does not provide direct evidence of benefit on pain, function, or other bruxism-related symptoms. Future research should focus on standardizing injection protocols and outcome measures-ideally reporting instrumental and questionnaire-based outcomes concurrently-and on evaluating long-term safety and efficacy.