Chapter 023. Weakness and Paralysis (Part 1)

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Harrison's Internal Medicine Chapter 23. Weakness and Paralysis Weakness and Paralysis: Introduction 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. 368) or abnormal movements (Chap. 367). The mode of onset, distribution, and accompaniments of weakness help to suggest its cause. 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...

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Chapter 023. Weakness and Paralysis (Part 1) Harrison's Internal Medicine > Chapter 23. Weakness and Paralysis Weakness and Paralysis: Introduction 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. 368) or abnormal movements (Chap. 367). The mode of onset, distribution, and accompaniments of weakness help to suggest its cause. 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 is often 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, gender, and size. Increased time is sometimes required for
full power to be exerted, and this bradykinesia may be misinterpreted as weakness. Severe proprioceptive sensory loss may also 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. 27), is sometimes mistaken for weakness by inexperienced medical staff. Paralysis indicates weakness that is so severe that the 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. Weakness or paralysis is typically accompanied by other neurologic abnormalities that help to indicate the site of the responsible lesion. These include changes in tone, muscle bulk, muscle stretch reflexes, and cutaneous reflexes (Table 23-1). Table 23-1 Signs that Distinguish Origin of Weakness Sign Upper Lower Myopathic Motor Neuron Motor Neuron
Atrophy None Severe Mild Fascicula None Common None tions Tone Spastic Decreased Normal/decr eased Distribut Pyramidal/re Distal/segm Proximal ion of weakness gional ental Tendon Hyperactive Hypoactive/ Normal/hypo reflexes absent active Babinski' Present Absent Absent s sign 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. Spasticity is velocity-dependent, has a sudden release after reaching a maximum (the "clasp-
knife" phenomenon), and predominantly affects the antigravity muscles (i.e., upper-limb flexors and lower-limb extensors). Spasticity is distinct from rigidity and paratonia, two other types of hypertonia. Rigidity is increased tone that is present throughout the range of motion (a "lead pipe" or "plastic" stiffness) and affects flexors and extensors equally; it sometimes has a cogwheel quality that is enhanced by voluntary movement of the contralateral limb (reinforcement). Rigidity occurs with certain extrapyramidal disorders such as Parkinson's disease. Paratonia (or gegenhalten) is increased tone that varies irregularly in a manner that may seem related to the degree of relaxation, is present throughout the range of motion, and affects flexors and extensors equally; it usually results from disease of the frontal lobes. Weakness with decreased tone (flaccidity) or normal tone occurs with disorders of motor units. A motor unit consists of a single lower motor neuron and all of the muscle fibers that it innervates. Muscle bulk is generally unaffected in patients with upper motor neuron lesions, although mild disuse atrophy may eventually occur. By contrast, atrophy is often conspicuous when a lower motor neuron lesion is responsible for weakness and may also occur with advanced muscle disease. Muscle stretch (tendon) reflexes are usually increased with upper motor neuron lesions, although they may be decreased or absent for a variable period immediately after onset of an acute lesion. This is usually—but not invariably— accompanied by abnormalities of cutaneous reflexes (such as superficial
abdominals; Chap. 361) and, in particular, by an extensor plantar (Babinski) response. The muscle stretch reflexes are depressed in patients with lower motor neuron lesions when there is direct involvement of specific reflex arcs. The stretch reflexes are generally preserved in patients with myopathic weakness except in advanced stages, when they are sometimes attenuated. In disorders of the neuromuscular junction, the intensity of the reflexes may be affected by preceding voluntary activity of affected muscles—such activity may lead to enhancement of initially depressed reflexes in Lambert-Eaton myasthenic syndrome and, conversely, to depression of initially normal reflexes in myasthenia gravis (Chap. 381). The distinction of neuropathic (lower motor neuron) from myopathic weakness is sometimes difficult clinically, although distal weakness is likely to be neuropathic and symmetric proximal weakness myopathic. Fasciculations (visible or palpable twitch within a muscle due to the spontaneous discharge of a motor unit) and early atrophy indicate that weakness is neuropathic.