Type 0: most severe form, begins before birth, limited movement in fetus, and little movement/ swallowing/ breathing after birth.
Type I: Werdnig- Hoffman disease, symptoms may be present a birth, have difficulty breathing/ swallowing, and unable to sit without support.
Type II: develop muscle weakness between 6 and 12 months. They cannot stand or walk without help.
Type III: Kugelberg- Welander disease, milder form of SMA, symptoms occur older than age 1. They are able to stand and walk without help, but usually lose the ability later in life.