Dystonia is a syndrome characterised primarily by unwanted muscle spasms giving rise to involuntary movements and abnormal postures (Hallet, 2011). There are many types of dystonia, and while some aspects of the physiology may well be shared among them, this is not at all certain. Dystonia, like almost any other disorder, is a product of genetic factors and modifiers, including environmental modifiers (Hallet, 2011). Focal hand dystonia commonly starts as an occupational cramp. This is a task specific disorder very much related to a repetitive action often completed in the context of an occupation or hobby. It is clear that poor motor control follows, particularly, control of individual fingers. Attempting to move one finger will result in multiple finger movement or proximal movement (overflow). Hallet (2011) suggests that the excess movement in dystonia is due to a loss of inhibition in motor control. The nervous system is composed of excitory and inhibitory circuits in balance with each other, and in dystonia it appears that inhibition is defective leading to a loss of selectivity and overflow.
Hallet, M. 2011. Neurophysiology of dystonia: The role of inhibition. Neurobiology of disease, 42, 177-184.