Spinal muscular atrophy (SMA) is an inherited disease that causes progressive muscle weakness by affecting the nerve cells, known as motor neurons, that control voluntary muscle movements. The muscles' inability to respond to signals from nerve cells located in the spinal cord leads to their wasting away, causing muscle shrinkage and eventual disappearance, or atrophy.

This results in the patient's inability to control muscles in the arms, legs, head, and neck. Movement is affected, including walking, crawling, head and neck control, swallowing, and breathing. While SMA causes muscle loss and weakness, senses such as sight, hearing, taste, smell, and touch, as well as cognitive and emotional function, remain completely normal.