The major management issue is the prevention and early treatment of respiratory infections. Lifespan generally unaffected. Individual symptoms, course and prognosis will vary. The disorder causes weakness and atrophy of the voluntary muscles.
Weakness occurs more often in the legs than in the arms.
Most types, however, are extremely rare, occurring in only one or two families. Some of the more common types are briefly described below. SMA type I, also called Werdnig-Hoffmann disease, generally has onset in utero or within the first few months of life. There may be lack of fetal movement in the final months of pregnancy. Onset of SMA type II usually occurs between 3 and 15 months of age.