Some doctors use the term SMA type 4 for adult-onset SMA, regardless of the inheritance pattern. They differ mostly in their time of onset and rate of progression. All forms affect the skeletal muscles of the trunk and limbs. In general, those muscles closer to the center of the body are more affected than those farther away. SMA type 1 and SBMA affect the neurons controlling the mouth and throat muscles more and therefore involve more problems with chewing and swallowing.
Respiratory muscles are involved to varying degrees in all forms of the disease.
In SMA type 1, the most severe form, the onset of the disease is noted within the first 6 months of the child's life. Children with SMA type 1 are never able to sit without support, and death usually occurs before age 2. SMA type 2 is an intermediate form of the disease. Onset is between 7 and 18 months. The child's survival depends in large part on the degree of respiratory and swallowing difficulties.