Spinal Muscular Atrophy
The spinal muscular atrophies (SMAs) comprise a group of autosomal-recessive disorders characterized by progressive weakness of the lower motor neurons.
In the early 1980s, Werdnig and Hoffman described a disorder of progressive muscular weakness beginning in infancy that resulted in early death, though the age of death was variable. In pathologic terms, the disease was characterized by loss of anterior horn cells. The central role of lower motor neuron degeneration was confirmed in subsequent pathologic studies demonstrating a loss of anterior horn cells in the spinal cord and cranial nerve nuclei. [1 ]
Since then, several types of spinal muscular atrophies have been described based on age when accompanying clinical features appear. The most common types are acute infantile (SMA type I, or Werdnig-Hoffman disease), chronic infantile (SMA type II), chronic juvenile (SMA type III or Kugelberg-Welander disease), and adult onset (SMA type IV) forms.
The genetic defects associated with SMA types I-III are localized on chromosome 5q11.2-13.3. [2. 3. 4. 5 ]
Many classification systems have been proposed and include variants based on inheritance, clinical, and genetic criteria. Among these are the Emery [6 ]. Pearn [7 ]. and International SMA Consortium system [8 ]. The ISMAC system is most widely accepted and is used in this review.
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