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HYPERKINETIC MOVEMENT DISORDERS
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Hyperkinetic movement disorders are characterized by involuntary movements unaccompanied by weakness (Table 436-1). This term is somewhat arbitrary and potentially misleading as hypokinetic disorders such as Parkinson’s disease (PD) are often accompanied by tremor, which is a hyperkinetic feature, and hyperkinetic disorders such as dystonia may manifest slow or restricted movement because of the severe muscle contractions. Nonetheless, the terms continue to be used because of convention. The major hyperkinetic movement disorders and the diseases with which they are associated are considered in this section.
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Tremor is defined as an involuntary, rhythmic, oscillatory movement of a body part with alternating contractions of agonist and antagonist muscles. It can be most prominent at rest (rest tremor), on assuming a posture (postural tremor), on actively reaching for a target (kinetic tremor), or on carrying out a movement (action tremor). Tremor may also be characterized based on its distribution, frequency, amplitude, and related neurologic dysfunction. Tremor is classified along two axes: Axis 1 covers the clinical characteristics, including historical features (age at onset, family history, temporal evolution), tremor characteristics (body distribution, activation condition), associated signs (systemic, neurologic), and laboratory tests (electrophysiology, imaging). Axis 2 relates to the etiology of the tremor and distinguishes acquired, genetic, or idiopathic origins.
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PD (Chap. 427) is characterized by a predominant resting tremor, essential tremor (ET) characterized by a tremor that typically occurs while trying to sustain a posture coupled with an action tremor, and cerebellar dysfunction characterized by a kinetic tremor (brought out by trying to touch an object) and is usually associated with hypotonia and past pointing. Normal individuals can have a physiologic tremor that typically manifests as a mild, high-frequency (10–12 Hz), postural, or action tremor typically affecting the upper extremities. This tremor is usually of no clinical consequence and often is only appreciated with an accelerometer or under stress. An enhanced physiologic tremor (EPT) can be seen in up to 10% of the population, and tends ...