Use of Botulinum Toxin as a Muscle Relaxant

Injection of botulinum toxin is a rather innovative way to control localized muscle hyperexcitability. Botulinum toxin is a purified version of the toxin that causes botulism. Systemic doses of this toxin can be extremely dangerous or fatal because botulinum toxin inhibits the release of acetylcholine from presynaptic terminals at the skeletal neuromuscular junction. Loss of presynaptic acetylcholine release results in paralysis of the muscle fiber supplied by that terminal. Systemic dissemination of botulinum toxin can therefore cause widespread paralysis, including loss of respiratory muscle function. Injection into specific muscles, however, can sequester the toxin within these muscles, thus producing localized effects that are beneficial in certain forms of muscle hyperexcitability. 

FIGURE 13-4 Mechanism of action of botulinum toxin at the skeletal neuromuscular junction. At a normal synapse (shown on left], fusion proteins connect acetylcholine (ACh] vesicles with the presynaptic membrane, and ACh is released via exocytosis. Botulinum toxin (represented by BTX on the right] binds to the presynaptic terminal, and enters the terminal where it destroys the fusion proteins so that ACh cannot be released. See text for details. 

It has been suggested that botulinum toxin might have other effects on neuronal excitability. This toxin, for example, might also inhibit contraction of intrafusal muscle fibers that are located within skeletal muscle, and help control sensitivity of the stretch reflex.33 Inhibiting these intrafusal fibers would diminish activity in the afferent limb of the stretch reflex, thereby contributing to the antispasticity effects of this intervention.33 

Clinical use of botulinum toxin. Seven strains (serotypes) of botulinum toxin have been identified, 

Botulinum toxin has been used for some time to control localized muscle dystonias, including conditions such as spasmodic torticollis, blepharospasm, laryngeal dystonia, strabismus, and several other types of focal dystonias.6 25,26,87 93 When used therapeutically, small amounts of this toxin are injected directly into the dystonic muscles, which begin to relax within a few days to 1 week. This technique appears to be fairly safe and effective in many patients, but relief may only be temporary. Symptoms often return within 3 months after each injection, necessitating additional treatments.40 Still, this technique represents a method for treating patients with severe, incapacitating conditions marked by focal dystonias and spasms. 

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