Dystonia : emerging concepts in pathophysiology.

The essential pathophysiological feature of dystonia is co-contraction of antagonistic muscles. This may be due to derangement of the spinal cord or cortical mechanism. In the cord, there is disruption of the normal reciprocal inhibition of antagonists during agonist contraction. This decreased reciprocal inhibition is due to reduced presynaptic inhibition of muscle afferent input to the inhibitory interneuron. The reduced presynaptic inhibition may in turn be either due to defective suprasegmental control or to changes in the tonic afferent input to the interneuron from cutaneous and muscle afferents. Alternatively, genesis of dystonia may entirely be a cortical mechanism. Overactivity of the premotor cortices, which receive projections from basal ganglia via ventral thalamus, could result in dystonia by abnormal activation of cortical motor neurons. This may again be due to a dopaminergic dysfunction of basal ganglia.