The mechanism of action of botulinum toxin type A in focal dystonia is most probably through its dual effect on efferent (motor) and afferent pathways at the injected site.

OBJECTIVE: To highlight some clinical and physiological features related to treatment with botulinum toxin type A (BTX-A) injections for focal dystonia that may suggest an effect through efferent (alpha motoneuron) and afferent pathways. DATA SOURCES: This review is based on published clinical and physiological studies as well as personal experience regarding the effect of BTX-A in focal dystonia. DATA SYNTHESIS: Long or short lag period between BTX-A injections and clinical improvement, remote effect, an effect on the basic physiological characteristics of dystonia, poor correlation between the local weakness and the clinical improvement and alleviation of pain are clinical observations which are difficult to explain on the basis of the known effect of BTX-A on the neuromuscular junction of the alpha motoneuron. These observations as well as recent scientific reports are used to discuss a hypothesis that in addition to its effect as local muscle relaxant, BTX-A acts at the level of the central nervous system (CNS) for 'reorganization'. Such an effect on CNS activity can be mediated through afferent pathways coming from the injected site--possibly originated in muscle spindles. Its effect through afferent pathways on the CNS may be considered as a long-term 'sensory trick'.