Both parents must pass on the defective gene in order for their children to inherit the disease. Occasionally, SMA is inherited as an autosomal dominant condition. In these cases, only one parent need have a genetic defect for a child to show the disease. 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.