An in vivo analysis of the effect and duration of treatment with botulinum toxin type A using digital image speckle correlation

Background: Use of Botulinum toxin type A (BTX-A) for facial wrinkles is well-documented, but current methods of subjective evaluation by clinicians and patients fail to objectively quantify the magnitude and duration of facial muscle paralysis. Objective: (a) Determine the locus of facial muscular tension; (b) Quantify and monitor muscular paralysis and subsequent return; (c) Continuously correlate the appearance of wrinkles and muscular tension using non-invasive digital image speckle correlation (DISC) to measure treatment efficacy; (d) Corroborate objective data with existing rating scales (subject global assessment and facial lines outcome-11). Methods: Two sequential images of slight facial motion (frowning, raising eyebrows) are taken with a camera for n = 6 patients pre- and post-treatment at different time points up to 24 weeks. DISC processes the images to produce a vector map of muscular displacement to obtain spatially resolved information regarding facial tension. Results: We observed maximum paralysis (≥70%) at 2 weeks, and the rate of recovery varied widely ranging from 2 to 5 months, with two patients continuing to exhibit reduced contraction at 24 weeks. Vector analysis of pre-treatment contraction correctly predicted injection site and illustrated lines of maximum tension. Conclusions: Digital image speckle correlation can precisely track the degree of contraction of different muscle groups following BTX-A injection. It can help predict injection site, quantify muscle paralysis, and monitor the recovery following BTX-A injection. Results were found to be reproducible across six patients.