Spinal muscular atrophy (https://omim.org/entry/253300) refers to a group of autosomal recessive neuromuscular disorders characterized by degeneration of the anterior horn cells of the spinal cord, leading to symmetrical muscle weakness and atrophy (summary by Wirth, 2000).
Four types of SMA are recognized depending on the age of onset, the maximum muscular activity achieved, and survivorship: type I, severe infantile acute SMA, or Werdnig-Hoffman disease; type II (253550), or infantile chronic SMA; type III (253400), juvenile SMA, or Wohlfart-Kugelberg-Welander disease; and type IV (271150), or adult-onset SMA. All types are caused by recessive mutations in the SMN1 gene.
Lunn and Wang (2008) provided a detailed review of clinical features, molecular pathogenesis, and therapeutic strategies for SMA.