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Normal motor function involves integrated muscle activity that is modulated by the activity of the cerebral cortex, basal ganglia, cerebellum, and spinal cord. Motor system dysfunction leads to weakness or paralysis, which is discussed in this chapter, or to ataxia (Chap. 373) or abnormal movements (Chap. 372). The mode of onset, distribution, and accompaniments of weakness help suggest its cause.

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Weakness is a reduction in the power that can be exerted by one or more muscles. Increased fatigability or limitation in function due to pain or articular stiffness often is confused with weakness by patients. Increased fatigability is the inability to sustain the performance of an activity that should be normal for a person of the same age, sex, and size. Increased time is required sometimes for full power to be exerted, and this bradykinesia may be misinterpreted as weakness. Severe proprioceptive sensory loss also may lead to complaints of weakness because adequate feedback information about the direction and power of movements is lacking. Finally, apraxia, a disorder of planning and initiating a skilled or learned movement unrelated to a significant motor or sensory deficit (Chap. 26), sometimes is mistaken for weakness.

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Paralysis indicates weakness that is so severe that a muscle cannot be contracted at all, whereas paresis refers to weakness that is mild or moderate. The prefix "hemi-" refers to one-half of the body, "para-" to both legs, and "quadri-" to all four limbs. The suffix "-plegia" signifies severe weakness or paralysis.

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The distribution of weakness helps to indicate the site of the underlying lesion. Weakness from involvement of upper motor neurons occurs particularly in the extensors and abductors of the upper limb and the flexors of the lower limb. Lower motor neuron weakness does not have this selectivity but depends on whether involvement is at the level of the anterior horn cells, nerve root, limb plexus, or peripheral nerve—only muscles supplied by the affected structure are weak. Myopathic weakness is generally most marked in proximal muscles, whereas weakness from impaired neuromuscular transmission has no specific pattern of involvement. Weakness often is accompanied by other neurologic abnormalities that help indicate the site of the responsible lesion. These abnormalities include changes in tone, muscle bulk, muscle stretch reflexes, and cutaneous reflexes (Table 22-1).

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Table 22-1 Signs that Distinguish the Origin of Weakness
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Tone is the resistance of a muscle to passive stretch. Central nervous system (CNS) abnormalities that cause weakness generally produce spasticity, an increase in tone associated with disease of upper motor neurons. ...

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