MODEL RELEASED. Woman with spinal muscular atrophy in a wheelchair. Spinal muscular atrophy (SMA) is a genetic disease which causes the degeneration of motor neurones in the anterior horn of the spinal cord. This deterioration gradually breaks the link between the brain and the muscles that this part of the spinal cord controls. As degeneration progresses, the muscles controlled by the spinal cord are used less and less, so they become weaker and shrink (atrophy). There is no known cure for SMA.