The Impact of Strength Changes on Active Function Following Botulinum Neurotoxin-A (BoNT-A): A Systematic Review

Botulinum neurotoxin-A (BoNT-A) injections are effective in reducing focal limb spasticity; however, their impact on strength and active function needs to be established. This review was a secondary analysis aimed at evaluating changes to active function in the context of muscle strength changes following BoNT-A intramuscular injection for adult upper and lower limb spasticity. The original review searched eight databases (CINAHL, Cochrane Central Register of Controlled Trials (CENTRAL), Embase, Google Scholar, MEDLINE, PEDro, PubMed, Web of Science) and was conducted with methodology that followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines as described in section 6.2 of Gill et al. For this secondary analysis, no databases were searched; only further data were extracted. The current and preceding review were registered in the Prospective Register of Systematic Reviews (PROSPERO: CRD42022315241). Twenty studies were screened for inclusion, and three studies were excluded because active function was not assessed in all participants. Seventeen studies (677 participants) met the inclusion criteria for analysis. Quality was examined using the PEDro scale and modified Downs and Black checklist and rated as fair to good. Pre- and post-BoNT-A injection strength (agonist, antagonist, and global), active function (activity), participation, and quality-of-life outcomes at short-, mid-, and long-term time points were extracted and analysed. Significant heterogeneity and limited responsiveness in strength and active function outcome measures limited the ability to determine whether changes in strength mediate an effect on active function. Further, variability in BoNT-A type and dose, adjunctive therapies provided, and variability in reporting limited analyses. Overall, no clear relationship existed between the change in muscle strength and active function following BoNT-A injections to the upper and lower limbs for focal spasticity in adult-onset neurological conditions.