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.