Spinal muscular atrophy (SMA) is a genetic disease that attacks nerve cells, called motor neurons, in your spinal cord. These neurons communicate with your voluntary muscles - the ones you can control, like in your arms and legs. As you lose the neurons, your muscles weaken. This can affect walking, crawling, breathing, swallowing and head and neck control.
There are many types of SMA, and some of them are fatal. Life expectancy depends on the type you have and how it affects your breathing.