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HYPERKINETIC MOVEMENT DISORDERS
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Hyperkinetic movement disorders are characterized by involuntary movements unaccompanied by weakness (Table 428-1). 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 movements 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 consists of alternating contractions of agonist and antagonist muscles in an oscillating, rhythmic manner. 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 distribution, frequency, amplitude, and related neurologic dysfunction.
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PD (Chap. 427) is characterized by a resting tremor, essential tremor (ET) by a tremor that typically occurs while trying to sustain a posture coupled with an action tremor, and cerebellar dysfunction by a kinetic tremor 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 to occur in association with anxiety, fatigue, a metabolic disturbance (e.g., hyperthyroidism, electrolyte abnormalities), drugs (e.g., valproate, lithium), or toxins (e.g., caffeine, smoking, alcohol). Treatment is initially directed at control of any underlying disorder and, if necessary, can often be improved with a beta blocker.
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ET is the most common movement disorder, affecting ~5% of the population (an estimated 5–10 million persons in ...