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Movement disorders are conditions that produce either reduced or excessive movement. Neurologic disorders that result in a paucity or slowness of movement are termed hypokinetic disorders. The category of hypokinetic disorders is represented by Parkinson disease and other causes of parkinsonism. Hyperkinetic disorders are characterized by excessive, involuntary movements. Hyperkinetic disorders can usually be placed into one of five main categories of abnormal movement: dystonia, chorea, tremor, myoclonus, or tic.

Abnormal movements may be difficult to recognize or categorize because of their unusual appearance, complexity, subtlety, or variability. Movement disorder specialists tend to isolate or reduce abnormal movements to their unitary components, but often it is the pattern of the movement and its body part distribution that provides the important diagnostic clue. In addition, many diseases cause abnormal movements that can be fit into two or more categories or abnormal movement phenomenology. Table 15–1 provides descriptions of the main categories of movement disorders.

Table 15–1.General classification of abnormal movements.

There are many other types of abnormal movements that do not fit cleanly into a simple classification of phenomenology. Athetosis, meaning “no fixed posture,” was first coined in reference to postanoxic birth injury to denote a quivering “fibrillary” movement of the limbs and digits. In modern usage, the term describes a slow, continuous, writhing movement that bears similarities to both chorea and dystonia. Ballism refers to large-amplitude random flinging movements of the limbs and represents a proximal form of chorea. Unilateral ballism is termed hemiballism and is most often caused by an infarct of the contralateral subthalamic nucleus. Akathisia, meaning “inability to sit,” describes inner restlessness and intolerance of remaining still, together with repetitive fidgeting, squirming, and pacing movements.

Many, but not all, movement disorders result from disordered function of the basal ganglia, a group of interconnected subcortical nuclei. The basal ganglia comprise the substantia nigra, putamen, caudate, globus ...

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