Movement disorders are characterized by either excessive (hyperkinetic) or reduced (bradykinetic) activity. Hyperkinetic disorders are reviewed here. Bradykinetic disorders are discussed separately. (See "Bradykinetic movement disorders in children".)
Hyperkinetic disorders are characterized by abnormal involuntary movement. These excess movements can be regular and rhythmic, as in tremor; more sustained and patterned, as in dystonia; brief and random, as in chorea; or jerk-like and temporarily suppressible, as in tics. Diagnosis of the specific condition depends primarily upon careful observation of the clinical features . Tics are the most common hyperkinetic disorder in children. Dystonia, stereotypies, choreoathetosis, tremors, and myoclonus also occur but are less common.
ANATOMY OF THE BASAL GANGLIA
A brief review of the anatomy of the basal ganglia is appropriate since this site is involved in many of the bradykinetic disorders. The basal ganglia regulate the initiation, scaling, and control of the amplitude and direction of movement. Movement disorders can result from biochemical or structural abnormalities in these structures.
The basal ganglia are a complex of deep nuclei that consist of the corpus striatum, globus pallidus, and substantia nigra. The corpus striatum, which includes the caudate nucleus and the putamen, receives input from the cerebral cortex and the thalamus and, in turn, projects to the globus pallidus.
The substantia nigra is divided into the dopamine-rich pars compacta and the less dense pars reticularis. The pars reticularis is similar histologically and chemically to the medial segment of the globus pallidus, and both project via the thalamus to the premotor and motor cortex. The substantia nigra pars compacta gives rise to the nigral-striatal pathway, which is the main dopaminergic tract.