There are five types of spinal muscular atrophy (SMA): Types 0, 1, 2, 3, and 4. The type of SMA is based on the age that symptoms begin, and the highest physical milestone achieved. Even within each type, abilities can vary from person-to-person. In addition, individuals with SMA can lose function over time if muscles continue to weaken.
With treatment, individuals may gain more physical milestones than they would have otherwise. And, as newborn screening for SMA becomes more common, infants can receive treatment even before symptoms begin. Because of these factors, doctors believe that we may soon stop describing SMA as specific “types” and instead focus on the highest motor milestone achieved: non-sitter, sitter, and walker. But even then, there will still be a wide range of severity associated with SMA.