Children with the intermediate form of SMA usually sit unsupported. Weakness of the legs and trunk usually, but not always, holds the child back from standing and walking alone. Sometimes the muscle weakness can seem to be non-progressive, but in most cases weakness and disability will increase over many years. Severe illness with prolonged periods of relative immobility, putting on excessive weight or growth spurts may contribute to deterioration in function. If the scoliosis becomes severe it can cause discomfort and can interfere with breathing function as well.
Surgery can straighten the spine and slow down any further spinal deterioration.
A person's prognosis depends mainly on the severity of weakness of the muscles of the chest wall and on the development of scoliosis. Lifespan is always difficult to predict. Severely affected children may die, due to pneumonia and other chest problems, before or in their teens. Mildly affected children are increasingly living into their adult years and go on to lead full lives. Every tissue in the body is made up of cells.