Infantile spinal muscular atrophy, Type I

**Definition:** In SMA type 1, onset of weakness may be prenatal (decreased fetal movements toward the end of pregnancy) or within the first six months of life. Infants demonstrate a characteristic frog position with the thighs externally rotated and abducted and the knees flexed (floppy infant). Bulbar weakness causes feeding difficulty. Children are never able to sit without support, and the average survival is 9 months; survival beyond 2 years is rare.