How Many Types of Dystonia? Pathophysiological Considerations

Abstract

Dystonia can be seen in a number of different phenotypes that may arise from different etiologies. The pathophysiological substrate of dystonia is related to three lines of research. The first postulate a loss of inhibition which may account for the excess of movement and for the overflow phenomena. A second abnormality is sensory dysfunction which is related to the mild sensory complaints in patients with focal dystonias and may be responsible for some of the motor dysfunction. Finally, there are strong pieces of evidence from animal and human studies suggesting that alterations of synaptic plasticity characterized by a disruption of homeostatic plasticity, with a prevailing facilitation of synaptic potentiation may play a pivotal role in primary dystonia. These working hypotheses have been generalized in all form of dystonia. On the other hand, several pieces of evidence now suggest that the pathophysiology may be slightly different in the different types of dystonia. Therefore, in the present review, we would like to discuss the neural mechanisms underlying the different forms of dystonia to disentangle the different weight and role of environmental and predisposing factors.

Keywords: TMS; basal ganglia; dystonia; plasticity; tDCS.

Figure 1

Two factor hypothesis. FHD may develop in predisposed individuals where subtle abnormalities of plasticity may, in conjunction with repetitive hand training or other environmental factors, trigger the development of dystonic postures.

Figure 2

Impact of two factor hypothesis in dystonia spectrum. The role of environmental factors is less important in triggering generalized dystonias where important structural plasticity changes may take place.