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MYOPATHY

ESSENTIALS OF DIAGNOSIS

  • Weakness, generally greater proximally than distally

  • Normal sensation (absent another cause for sensory loss)

  • Normal sphincter function

  • Relative preservation of deep tendon reflexes

  • Laboratory testing supportive of diagnosis and etiology

  • Electromyography testing is a key component of diagnosis

  • Muscle biopsy and genetic testing often definitive

General Considerations

Myopathies are disorders that predominantly affect the striated muscles of the body, sometimes cardiac muscle and rarely, smooth muscle. These conditions result from primary structural or functional impairment of muscle and can result from a variety of inherited and acquired disorders (Table 23–1). Patterns of weakness are often similar despite the broad spectrum of etiologies; however, it is possible to narrow the differential diagnosis based on symptoms, distribution of weakness, time course, family history, precipitating factors, and presence of systemic signs. In general, proximal greater than distal muscle weakness leads to difficulty arising from a chair or commode, climbing stairs, reaching up for objects, or combing one’s hair. Specific patterns of proximal weakness may be seen in certain disorders discussed later in this chapter, and facial, extraocular, bulbar, cardiac, and respiratory muscles may sometimes be involved. Detailed family history, broad systemic review, and careful drug history are mandatory. Understanding of this complex group of diseases has been greatly enhanced by significant advances in molecular genetics and immunology.

Table 23–1.Classification of myopathies.

Clinical Findings

A. Symptoms and Signs

Muscle weakness and fatigability are the most frequent symptoms of myopathy. Fatigue in and of itself is a nonspecific symptom which can result from many different causes including many nonmyopathic etiologies. Fatigability is a common complaint in those with muscle diseases; however, excessive fatigability out of proportion to the degree of weakness should raise suspicion of a neuromuscular junction disorder. Muscle pain (myalgias), stiffness, spasms, or cramps may occur with varying severity, depending on the nature of injury. Atrophy or hypertrophy of muscles can be present in some instances. Patients should be asked about the color of their urine, which, when dark red, suggests myoglobinuria. Typical symptoms of myopathies are listed in Table 23–2. Double vision, difficulty swallowing, and shortness of breath may be present in certain diseases. Distribution of weakness may vary among diseases, with some muscles affected more than others. Typical patterns for various myopathies are outlined in Table 23–3.

Table 23–2.Symptoms of myopathies.

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