Objective: To compare the timecourse of and relationship between benefit and weakness after botulinum toxin (BoNT) injection.

Background: The timecourse of benefit onset and duration has been well reported in case series and clinical trials for many BoNT neurological indications [1, 2].  The clinical time course of BoNT-induced weakness is less well-defined. While muscle weakening is presumed to, at least partly, underlie BoNT clinical efficacy, there is little data on the degree to which weakness does or does not correlate with benefit.

Method: Patients in the NIH/NINDS Human Motor Control Section BoNT clinic complete a structured questionnaire about their previous BoNT injections at each visit. They self-rate the benefit and weakness on a visual analog scale and assign a numerical score of 0-100% to their response (For benefit:  0 = no benefit; 100% = complete relief. For weakness: 0 = no weakness; 100% = no voluntary movement).  They also record the time to onset of any benefit/weakness, time to maximal benefit/weakness, duration of maximal benefit/weakness and duration of any benefit/weakness.  We analyzed the most recent questionnaires for all patients injected in the past 2 year.

Results: Charts of 79 patients (63 dystonia, 9 spasticity and 7 hemifacial spasm) were reviewed. The mean benefit was 58% (SD=31%) and the mean weakness 19% (SD=23%). 43% of patients had no weakness despite benefit of 58% (SD=35%). We found no correlation between self-reported intensity of benefit and weakness (p=0.68). There was no difference between the time onset of benefit compared to weakness (p=0.26). There was a trend towards benefit peaking slightly later than weakness (22 days (SD=16) vs 15 days (SD=9), p=0.073). The duration of maximum benefit (mean 7.8  weeks, SD=3.4) and of any benefit (mean 11.3 weeks, SD= 5.3) was significantly longer than the duration of maximum weakness (mean 5.7 weeks, SD=3.9) (p<0.01) or any weakness (mean 9.3 weeks, SD=5.5) (p=0.03).

Conclusion: These data confirm previous observations that the extent of subjective benefit does not correlate with subjective weakness and that benefit can be achieved in the absence of weakness. Our data support that the latency of weakness/benefit onset and time to peak are the same but that benefit duration outlasts weakness by 2 weeks. These findings are useful for patient education and in setting expectations for the course of response to BoNT treatment.

References: 1. Lungu, C., et al., Long-term follow-up of botulinum toxin therapy for focal hand dystonia: outcome at 10 years or more. Mov Disord, 2011. 26(4): p. 750-3. 2. Karp, B.I., Botulinum toxin treatment of occupational and focal hand dystonia. Mov Disord, 2004. 19 Suppl 8: p. S116-9.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/benefit-and-weakness-with-botulinum-toxin-time-course-and-relationship/