Objective: To compare and quantify the degree of muscular atrophy (MA) in botulinum toxin (BT) type A (Botox) and type B (Myobloc). A secondary endpoint is whether improvement in the Modified Ashworth Scale (MAS) has any correlation with MA.

Background: BT clearly has good therapeutic benefit, but the MA it causes can weaken an already compromised muscle. While both BT types A and B are used, they have never been compared in terms of which one causes a greater degree of MA. 

Methods: Ten patients with spasticity affecting the upper or lower extremity were enrolled.5 patients were randomized into groups A or B.The biceps or gastrocnemius muscle were injected with the same dose (type A: 100 U or type B: 5000 U) every 12 weeks over 36 weeks.MRI of the muscle was done prior to the first set of injections and then after the last set and the muscle was measured by a blinded radiologist.A MAS was performed at baseline and prior to each treatment.A Wilcoxian paired sum test was performed on the results.  

Results: Types A and B caused a statistically significant increase in MA. However, this was not statistically significant when compared against one another. Group A had larger muscle size on MRI prior to treatment suggesting that type A caused a greater rate of MA than type B. For group A, there was a decrease of ~ 9.1% of muscle size on MRI and an decrease of 80% on MAS compared to a group B where muscle size on MRI decreased 1.6% with a decrease in MAS of 40%. 

Conclusions: Despite the lack of significance between the two groups, type A causes a greater rate of MA compared to B. A larger trial is still needed to determine the clinical significance of muscle atrophy. 

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MDS Abstracts - https://www.mdsabstracts.org/abstract/a-comparison-of-muscular-atrophy-between-botulinum-toxin-types-a-and-b/