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.
Life expectancy varies.
The childhood SMAs are all autosomal recessive diseases. This means that they are familial and more than one case is likely to occur in the same generation of a family - in siblings or cousins. The parents are usually asymptomatic but carry the gene. Cousin marriages are more common in these families than in others. If one case has occurred in a family, antenatal diagnosis can be made in a subsequent pregnancy.