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Disorders of the peripheral nervous system can generally be divided into 3 major categories, based on anatomic localization:

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  1. 1. Neurogenic disorders, including motor neuron diseases

    2. Disorders of the neuromuscular junction

    3. Myopathies, including muscular dystrophies

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History and physical examination remain critical in focusing the differential diagnosis of neuromuscular diseases (see also Chapter 569). Polyneuropathies typically present with distal weakness and atrophy, loss of deep tendon reflexes, and sensory deficits. Children with chronic polyneuropathies, especially Charcot-Marie-Tooth disease, may be unaware of sensory loss. Footdrop may develop in advanced cases. Motor neuron disease such as spinal muscular atrophy will cause proximal or generalized weakness. Disorders of the neuromuscular junction are difficult to diagnose in children. Infant botulism typically begins with constipation, followed by a descending pattern of weakness. Juvenile myasthenia gravis may present with a variety of symptoms, including fatigue, but this disorder is almost always accompanied by ocular or bulbar deficits. Myopathies tend to cause proximal or generalized weakness. Neither neuromuscular junction disorders nor myopathies are accompanied by sensory loss.

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During the last 20 years, the field of neuromuscular disease has changed drastically, with advancements in the molecular pathogenesis of myopathies and other neuromuscular disorders.1 These breakthroughs have allowed improved and new diagnostic tests that can confirm clinical diagnoses very accurately and less invasively. In this chapter, we will describe the diagnostic tests currently used in clinical practice.

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Measurements of serum muscle enzyme levels may be quite useful in the initial evaluation of children with suspected neuromuscular disease. The most well-known serum marker of neuromuscular disease is creatine kinase (CK), also known as creatine phosphokinase (CPK). This muscle enzyme is normally found in small amounts in serum (up to 150 or 175 U/L in many diagnostic laboratories) but may be released in large quantities when muscle membrane breakdown occurs in muscular dystrophy and other myopathies.2,3 Mild elevations in CK levels up to 300 U/L are nonspecific and may be a normal finding, but values up to 500 U/L are sometimes found in neuromuscular diseases such as spinal muscular atrophy or congenital myopathies. Moderate (501 to 1000 U/L) to severe (above 1000 U/L) elevations in CK levels are usually associated with primary muscle disease, which can be inherited, as in muscular dystrophy, or acquired, as in acute rhabdomyolysis. CK levels are generally not useful in the evaluation of possible neuropathies and disorders of the neuromuscular junction.

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Other enzymes are also released by injured muscle tissue and can be measured in serum. Among these, aldolase may also be elevated in muscle disease.4 Normal aldolase levels typically range up to 12 U/L. Three liver enzymes are also found in muscle tissue and may be mildly increased in muscle disease: alanine aminotransferase, aspartate aminotransferase, and lactate dehydrogenase. Patients who have unexplained mild elevations of liver enzymes should have CK and aldolase levels checked before undergoing invasive testing such as liver ...

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