Pediatric Movement Disorders - Dystonia

Mechanism

The mechanism of dystonia is one of the most poorly understood issues in movement disorders. Studies in humans and animals have not been able to find a good explanation that can relate particular injuries to the emergence of dystonic symptoms. Dystonia is frequently associated with injury to the basal ganglia, in particular the sensory-motor regions of the putamen. In children, dystonia may also occur with decreased dopamine as occurs in dopa-responsive dystonia (DRD) or in response to dopamine-blocking medications.

In adults with dystonia, measurement of the cellular activity in the basal ganglia shows that these cells often respond to movements of multiple limbs. This suggests that there is confusion or "cross-talk" between different body parts. It is possible that this confusion relates to the involuntary activation of normally suppressed muscles. Human and animal research has shown that for adult-type focal dystonia, there is also confusion of the cell responses in cerebral cortex. Therefore, it is possible that abnormalities in the cortex may be one cause of dystonia.

Understanding the role of dopamine remains elusive. An abnormally low level of dopamine causes many childhood dystonias; however, in parkinsonism, it is possible to cause dystonic symptoms by administering large amounts of dopamine. On the other hand, acute dystonic reactions in children and adults are caused by medications that selectively block the dopamine receptors in the indirect pathway. These reactions are treated with anticholinergic medications that may increase the effectiveness of dopamine in both the direct and indirect pathways. (See the section on Anatomy for an explanation of these pathways.)

When muscle activity is recorded using electromyography (EMG) electrodes, many children and adults with dystonia have a rapid, machine-gun-like, staccato firing of muscle fibers. These firings are involuntary and completely unlike the normal patterns of muscle electrical activity. In some cases, similar rapid repetitive firing has been found in the basal ganglia. On the other hand, this type of activity is not always found, and, in some cases of dystonia, the involuntary muscle activity has a pattern that is essentially identical to normal muscle activation.