Botulinum toxin: from poison to remedy

Botulinum toxins, exotoxins of Clostridium botulinum, are the most toxic naturally occurring substances known to man. For more than a century they are known to be the cause of botulism, a nowadays rare intoxication with spoiled food that leads to generalized flaccid weakness of striated muscle including pharyngeal and respiratory musculature. The toxins act primarily at peripheral cholinergic motor nerve endings by blocking the release of the neurotransmitter acetylcholine. As a consequence, action potentials in the motor nerve can no longer be transmitted to the muscle. This lack in transmission, clinically appearing as weakness, may disable or actually critically endanger affected patients. However, in certain neurological diseases characterized by an abnormal increase in muscle tone or activity, for example dystonia or spasticity, a reduction in signal transmission may actually be beneficial. Around 1980 local injections of minute amounts (in the order of 0.5 ng) of Botulinum toxin type A were first successfully used in a neurological disorder named blepharospasm which is characterized by an involuntary squinting of the eyes. Since then Botulinum toxin has developed rapidly from a frightful poison to a safe therapeutic agent with a remarkable beneficial impact on the quality of life of many thousands of patients worldwide. This review tries to outline in brief the characteristics of Botulinum toxins, their mechanism of action and the various indications for clinical use as a therapeutic agent.