SMA Type 3, also known as Mild SMA, begins after 18 months, or after the child has taken 5 walking steps by themselves. People with mild SMA live for a long time and can have professional careers decorated with accomplishments. People with SMA type 3 are usually dependent upon assistive devices, and throughout their lives need to continually monitor where they are at with regards to their respiratory
and spinal curvature
risks. They tend to stop walking sometime in their lives. When they stop walking varies between adolescence and their 40s.
While children with type 3 SMA can move and walk, there is muscle weakness and wasting of proximal muscles, i.e. those closest to the spine.
There is a 4th type of SMA - adult onset SMA. Type 4 generally shows up when the person is in her or his 30s. As you may have guessed, SMA type 4 is the mildest form on the continuum of severity of this disease. Symptoms of type 4 are very similar to those of type 3.