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The classification of peripheral nerve disorders can be tricky, as with many major disease categories. Neuropathies may be classified by etiology (genetic versus acquired), physiology (demyelinating versus axonal), anatomy (focal versus generalized, sensory versus motor), and chronicity (acute versus chronic). This chapter focuses on the major categories of peripheral nerve disorders, including anterior horn cell diseases, using a combination of the above variables to classify them.

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Focal neuropathies, including brachial plexopathies, are relatively simple to localize on physical examination, especially when accompanied by a history of trauma or other clear cause. Generalized neuropathies, including anterior horn cell disorders, may be more difficult to localize neuroanatomically. A distal pattern of weakness is usually, but not always, consistent with a polyneuropathy. Distal muscle atrophy may be present. Anterior horn cell diseases are typically associated with proximal or generalized weakness. Deep tendon reflexes are diminished or absent in polyneuropathies and anterior horn cell disorders.

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The term anterior horn cell disease is synonymous with motor neuron disease, as the cell body of the motor neuron lies within the anterior horn of the spinal cord. In children, the dominant motor neuron diseases are spinal muscular atrophy (SMA) and poliomyelitis (in countries where vaccination is inconsistent), whereas in adults amyotrophic lateral sclerosis (Lou Gehrig disease) is most common. The latter will not be discussed, as the juvenile form of amyotrophic lateral sclerosis is extremely rare in childhood.

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SMA is an inherited disorder of the motor neuron in the spinal cord, which leads to progressive weakness of the skeletal muscles, affecting the proximal muscles in the lower extremities first, followed by spread to the distal muscles and upper extremities.1 The intercostal respiratory muscles are significantly involved, but the muscles of the heart and face (including eyes) are spared. Intelligence is normal. The carrier frequency is estimated to range from 1 in 40 to 1 in 50 in both genders and all known ethnic groups.2 Based on data from several countries, the incidence is estimated to range from 1 in 6000 to 1 in 10,000 live births.3-5

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Clinical Presentation

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The severity of SMA varies considerably, and most affected individuals can be classified phenotypically as having type 1, 2, or 3, with some patients having intermediate forms. The original term for SMA type 1 was Werdnig-Hoffman disease, but that term is less commonly used today. The original term for SMA type 3 was Kugelberg-Welander disease, but that term has also fallen into disuse. The classification is based on motor milestones achieved: type 1 patients never sit or walk, type 2 patients sit but never walk, and type 3 patients walk for at least some period of their lives. Type 1 patients tend to have onset before 6 months, type 2 between 6 and 18 months, and type 3 after 18 months. Some clinicians recognize a severe congenital variant of type 1 as type 0 and a mild ...

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