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The myopathies are neuromuscular disorders in which the primary symptom is muscle weakness due to dysfunction of muscle fiber. In pediatric patients, myopathies can be inherited (such as the muscular dystrophies) or acquired. The distribution of muscle weakness is proximal but it could be generalized or distal. Patients may also complain of muscle pain and cramps. Presentation of weakness may be acute or insidious (Figure 15-1). Acute weakness can occur in neuromuscular junction disorders. Some diseases are present since childbirth, and in this group the differential diagnosis is important to distinguish the disorders from other causes of hypotonic infants. Figure 15-2 shows a diagnostic algorithm in infants with suspected neuromuscular hypotonia. Evidence of reduced fetal movement and polyhidroamnios could be found early. Also respiratory and feeding difficulties can occur. Other physical signs such as joint contractures (severe arthrogryposis), foot deformities, and facial, palatal, and pharyngeal abnormalities are also useful in detecting an inherited myopathy.

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Figure 15-1.
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Algorithm for diagnosis of muscle weakness in children.

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Figure 15-2
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Diagnostic algorithm in infants with suspected neuromuscular disorder.

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This chapter includes the principal inherited myopathies: congenital and acquired neuromuscular junction disorders, congenital myopathies, congenital muscular dystrophies, muscular dystrophies, inherited metabolic myopathies, mitochondrial myopathies, inflammatory myopathies, muscle channelopathies, and endocrine and toxic myopathies.

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Myasthenia Gravis

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Definitions and Epidemiology

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Myasthenia gravis (MG) is a disorder involving the neuromuscular junction that results in fatigable weakness as its primary symptom. The onset can be insidious, and it is often difficult to make the diagnosis initially (Table 15-1).1

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Table 15–1. Acquired Neuromuscular Junction Diseases in Infants and Children 
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Myasthenia gravis is a relatively common disorder, affecting approximately 2 to 10 per 100,000 people in the United States. Before age 40 years, MG is three times more common in women than men. It may begin at any age but is rare before age 10 years or after age 60 to 70 years. There are bimodal age-related peaks, with the earlier peak involving more women between ages 10 and 40 ...

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