Recovery of rat muscle size but not function more than 1 year after a single botulinum toxin injection.

INTRODUCTION: Neurotoxin injection is used to treat a wide variety of neuromuscular disorders. The purpose of this study was to measure the functional and structural properties of botulinum toxin-injected adult rat skeletal muscle over nearly the entire lifespan. METHODS: Ten groups of animals were subjected to either neurotoxin injection [Botox, Type A (BT-A); Allergan, Irvine, California] or saline solution injection. Neurotoxin-injected animals (n = 90) were analyzed at different time-points: 1 week; 1 month; 3 months; 6 months; 12 months; or 18 months. RESULTS: In spite of the recovery of structural features, such as muscle mass and fiber area, dorsiflexion torque production remained significantly depressed by 25%, even at 12 months after neurotoxin injection. DISCUSSION: The data demonstrate that, after a single BT-A injection, although gross muscle morphology recovered over a 12-month time period, loss of contractile function did not recover. Muscle Nerve 57: 435-441, 2018.

Dramatic changes in muscle contractile and structural properties after 2 botulinum toxin injections.

INTRODUCTION: Botulinum toxin is frequently administered serially to maintain therapeutic muscle paralysis, but the effect of repeated doses on muscle function are largely unknown. This study characterized the muscle response to 2 onabotulinum toxin (BoNT) injections separated by 3 months. METHODS: Animal subjects received a single toxin injection (n = 8), 2 BoNT injections separated by 3 months (n = 14), or 1 BoNT and 1 saline injection separated by 3 months (n = 8). RESULTS: The functional effect of 2 serial injections was exponentially greater than the effect of a single injection. While both groups treated with a single BoNT injection had decreased torque in the injected leg by approximately 50% relative to contralateral legs, the double BoNT injected group had decreased torque by over 95% relative to the preinjection level. Both single and double BoNT injections produced clear signs of fiber-type grouping. CONCLUSIONS: These experiments demonstrate a disproportionately greater effect of repeated BoNT injections.

Systematic test of neurotoxin dose and volume on muscle function in a rat model.

INTRODUCTION: Onabotulinum toxin serotype A (BT-A) is used for a variety of motor and sensory disorders related to abnormal muscle activity. METHODS: We developed a high-resolution rodent model to allow precise determination of the effect of BT-A dose (measured in units) and injectate volume (measured in µl) on the efficacy of the injection and systemic side effects. Dorsiflexion is the best indicator of injected and contralateral muscle function. RESULTS: One month after injection, dorsiflexion torque of BT-A-injected limbs was decreased significantly in all experimental groups compared with saline controls (P < 0.05). Torque was also compared among the BT-A groups, which demonstrated a significant effect of dose (P < 0.001), but no effect of volume (P > 0.2) and no dose × volume interaction (P > 0.3). Similar results were observed for other parameters measured. CONCLUSIONS: These data demonstrate that injection dose and not volume or concentration is the primary determinant of neurotoxin efficacy in a rodent model.

Increased efficacy and decreased systemic-effects of botulinum toxin A injection after active or passive muscle manipulation.

The effect of physical manipulation on the outcome of neurotoxin (NT) injection was studied in a rat tibialis anterior (TA) model system where dorsiflexion torque could be measured precisely. After determination of initial torque, all rats received a one-time botulinum toxin A (BTX-A) injection (dose 6.0 units/kg in a volume of 100 microL) into the TA midbelly. Four experimental groups were studied: one group was subjected to BTX-A injection alone (BTX-A only, n=8), one was subjected to BTX-A injection followed immediately by 10 isometric contractions (ISO; n=9), and the third was subjected to BTX-A followed immediately by 10 muscle passive stretch/release cycles (PS; n=10). After 1 month, maximum dorsiflexion torque of the injected and contralateral legs was determined followed by quantification of TA fiber area. Post-injection torque was significantly reduced by around 80% in all NT-treated extremities 1 month after injection (p<0.05). While all NT-treated extremities demonstrated a significant torque decrease relative to their pre-injection levels, ISO and PS groups demonstrated significantly lower torques compared with the BTX-A only group which received no physical manipulation (p<0.05) indicating greater efficacy. Perhaps even more surprising was that the ISO and PS groups both demonstrated a significantly smaller contralateral effect compared with the BTX-A only group that received no manipulation (p<0.05) indicating a decreased systemic-effect. Muscle fiber size generally correlated with dorsiflexion torque. These data demonstrate that both neuromuscular activity (seen in the ISO group) and muscle movement (seen in the PS group) increased the efficacy of BTX-A and decreased the systemic side effects.