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-Hoffman 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.