Spinal Muscular Atrophy

The spinal muscular atrophies are a clinically and genetically heterogeneous group of neuromuscular disorders caused by degeneration of anterior horn cells. In proximal spinal muscular atrophy (SMA), the muscles of the extremities closest to the trunk are affected earlier and more severely. SMA is clinically classified into four types: acute or Werdnig-Hoffmann disease, intermediate, mild or Kugelberg-Welander disease, and adult (SMA types I, II, III, and IV, respectively) (1 ,2 ). The clinical criteria are summarized in Table 1 . With an estimated incidence of ∼1/10,000 (3 –7 ), and a carrier frequency of 1/40–1/60 (8 ,9 ), the SMAs are among the most frequent autosomal recessive hereditary disorders, and the incidence appears to be similar in different populations studied.

Clinical subtype	Age of Onset (in months)	Motor milestones	Death (in years)
SMA type I SMA type II SMA type III	<6 <18 >18	Never sit unaided Never walk unaided Stand unaided	<2 >2 adult