The disorder causes weakness and atrophy of the voluntary muscles.
Weakness is often more severe in the legs than in the arms.
SMA type I , also called Werdnig-Hoffmann disease , is evident in utero or within the first few months of life. There may be a paucity of fetal movement in the final months of pregnancy. Affected children never sit or stand and usually die before the age of 2. II usually begin between 3 and 15 months of age. These children may learn to sit but do not stand.