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.
Destructive processes are intrauterine hypoxia and infections (poliomyelitis). They are nonprogressive and are usually associated with fasciculation potentials.
Degenerative 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.