Changes in contractile properties of muscles receiving repeat injections of botulinum toxin (Botox)
Introduction
Botulinum toxin type A (BTX-A) has become an accepted and frequently used therapeutic tool to chemically denervate muscles in the treatment of neuromuscular disorders (Antonucci et al., 2008, Brin, 1997). BTX-A is one of seven neuromuscular blocking agents produced by the bacterium Clostridium botulinum. It has an affinity for motor nerve endings where it inhibits acetylcholine release, exerting its paralytic effect on injected muscles (Antonucci et al., 2008, Brin, 1997, Borodic et al., 1990, Yaraskavitch et al., 2008).
BTX-A is commonly used to treat disorders characterized by hyperfunction of cholinergic terminals, such as dystonia and spasticity (Lange et al., 1987, Lange et al., 2007). Because of its time limited action, repeat injections are routinely performed at intervals of 3–4 months (Brin, 1997, Dutton, 1996; Inagi et al., 1999, Garner et al., 1993, Cichon et al., 1995, Girlanda et al., 1992). Although regional or systemic adverse effects have been reported, local application of botulinum toxin is considered to be safe by the US Food and Drug Administration (FDA; Cote et al., 2005). Regional adverse effects of toxin application include weakness in neighboring non-target muscles (Yaraskavitch et al., 2008), while possible systemic effects include general muscle weakness (Cote et al., 2005). Other side-effects include dysfunction of non-target neuromuscular junctions (Lange et al., 1991) and atrophy of non-target muscles (Ansved et al., 1997, Girlanda et al., 1992). With an increased use of BTX-A for an ever growing number of disabilities, including repeat injections into young children with cerebral palsy, the effect of repeat injections on muscle mass, strength and muscle structure need to be known.
Although decreases in muscle mass and strength have been reported for specific time points following BTX-A injections (Dodd et al., 2005, Herzog and Leonard, 2002; Longino et al., 2005a, Longino et al., 2005b, Longino et al., 2005c, Rehan Youssef et al., 2009), the associated structural changes for single and repeat injections remain unknown. Therefore, the primary purpose of this study was to investigate the changes in muscle mass and strength in BTX-A treated muscles while simultaneously quantifying cross-sectional histology in order to determine possible changes in the amount of contractile material. A secondary purpose was to identify whether changes in the outcome measures are muscle specific and affect non-target muscles; therefore, all mass and histological outcomes were determined for three target and three non-target muscles following 1, 3 and 6 monthly injections of BTX-A.
Section snippets
Experimental design
Twenty skeletally mature, 1 year old, female NZW rabbits weighing an average of 5.5 kg were used with the Animal Care Committee approval of the University of Calgary. Animals were allowed normal activity in a cage (65×45×30 cm3), and received a standard diet.
Animals were divided into four study groups as follows:
- (1)
Control group—saline injection unilaterally (n=5).
- (2)
1-month post-single BTX-A injection unilaterally (n=5).
- (3)
3-months post-repeated monthly BTX-A injections unilaterally (n=5).
- (4)
6-months
Results
The injection procedures were well tolerated by all rabbits. Data were analyzed from all animals, except for the percentage of contractile material from the rectus femoris of the control rabbits, as these samples were of bad quality.
Discussion
With the increased use of BTX-A in medical applications, its mechanism of action has become well understood. However, its long-term effect on structure and function of target and non-target muscles has not been studied systematically. Repeated use of BTX-A, especially in children and adolescents suffering from neuromuscular disorders, such as cerebral palsy, makes understanding of BTX-A’s effects on muscles essential.
Here, we demonstrate that BTX-A not only creates great loss of strength due to
Conclusions
The results of this study lead to the conclusion that BTX-A injections cause severe muscle atrophy and loss of contractile tissue, thereby affecting the strength and structure of the target muscles, while also causing atrophy and contractile tissue loss in non-target muscles far removed from the injection site. It is not known if chronic BTX-A induced atrophy and contractile tissue loss is reversible and over what time frame a full muscle recovery might be possible. However, when using BTX-A
Conflict of interest statement
All five authors declare that they do not have any financial and personal relationships with other people and organisations that could inappropriately influence our work on this manuscript.
Acknowledgements
The authors would like to thank CIHR of Canada, the Canada Research Chair Programme, CAPES-Brazil and CNPq-Brazil for financial support (MAV).
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