Alpha motor neuron diseases
are usually referred to as spinal atrophies. The latter term downplays
the fact that this group of diseases also affect the alpha motor neuron
in the cranial nerve motor nucleus. Alpha motor neuron diseases produce
hypotonia with decreased dynamic tone. Electromyogram may show fibrillations
and sometimes fasciculation potentials
and large-amplitude, long-duration polyphasic motor units.
Muscle biopsy is abnormal.
Sensory and motor nerve conductions are normal.
Alpha motor neuron diseases in the neonatal period may be caused by dysgenetic,
destructive, and degenerative processes.
Dysgenetic processes result from disruption in the number or migration
of the alpha motor neurons. They are nonprogressive and are usually not
associated with fasciculation potentials in the EMG.
processes are intrauterine hypoxia and infections (poliomyelitis). They
are nonprogressive and are usually associated with fasciculation potentials.
disorders include Werdnig-Hoffmann disease, Pompe disease (acid maltase
deficiency), neonatal adrenoleukodystrophy, and incontinentia pigmenti.
The most frequent alpha motor neuron disease in the United states is Werdnig-Hoffmann
disease. Neonatal poliomyelitis is very rare in the United States but
it occurs more frequently in other parts of the world. Degenerative processes
are progressive and may be associated with fasciculation potentials. Nevertheless,
progression in degenerative alpha motor neuron disorders does not always
occur in the neonatal period.
Salient features of alpha motor neuron disease. Arrow indicates the
anatomical location of the injury. The arrow pointing to the yellow
rectangle indicates involvement of the alpha motor neuron in the spinal
cord. The arrows pointing to the gray and yellow rectangles indicate
involvement of the motor neuron in the spinal cord and brainstem. DEG:
degenerative; DYS: dysgenesis; DES: destruction; Diag: diagnosis; D:
deficiency; *: tongue fasciculations may occur in degenerative and destructive
diseases. The illustrations below Werdnig Hoffmann disease indicate
a small tongue and a small heart. The illustrations below Pompe disease
indicate a large tongue and a large heart.