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.
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.
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.
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).
Table 22-1 Signs that Distinguish the Origin of Weakness
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Table 22-1 Signs that Distinguish the Origin of Weakness
|Sign||Upper Motor Neuron||Lower Motor Neuron||Myopathic|
|Distribution of weakness||Pyramidal/regional||Distal/segmental||Proximal|
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 the muscle fibers that it innervates.
Muscle bulk generally is not affected in patients with upper motor neuron lesions, although mild disuse atrophy eventually may occur. By contrast, atrophy is often conspicuous when a lower motor neuron lesion is responsible for weakness and also may 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. 367) 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 generally are preserved in patients with myopathic weakness except in advanced stages, when they sometimes are attenuated. In disorders of the neuromuscular junction, the intensity of the reflex responses may be affected by preceding voluntary activity of affected muscles; that 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. 386).
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.
Upper Motor Neuron Weakness
This pattern of weakness results from disorders that affect the upper motor neurons or their axons in the cerebral cortex, subcortical white matter, internal capsule, brainstem, or spinal cord (Fig. 22-1). These lesions produce weakness through decreased activation of the lower motor neurons. In general, distal muscle groups are affected more severely than are proximal ones, and axial movements are spared unless ...