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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 ...