Spinal muscular atrophy (SMA) is a genetic condition that causes muscle weakness and atrophy (when muscles get smaller). The disease often starts in the muscles of the back, making it hard for children to stand up and walk. It can begin to affect other parts of the body too, causing other muscles to grow weak and shrink. At Nemours, our neurologists and other specialists provide expert care for kids with spinal muscular atrophy.