Definition - A genetic disease in which loss of nerve cells in the spinal cord called motor neurons affects the part of the nervous system that controls voluntary muscle movement.

Cause - A deficiency of a motor neuron protein called SMN, for survival of motor neurons, resulting from a faulty gene on chromosome 5. There is a rare X-linked form that affects male babies.

Onset - Before birth to 6 months.

Symptoms - Generalized muscle weakness, weak cry, trouble swallowing as well as sucking, and breathing distress. Cannot sit without support.

Progression - Can progress very rapidly with early childhood death. Most doctors, however, now consider SMN-related SMA to be a continuum of severity and prefer not to make rigid predictions about life expectancy or weakness.

Inheritance - Autosomal recessive; a faulty gene must be inherited from each parent. In rare cases, X-linked, meaning the disease affects males but is carried by females.

Facts About Spinal Muscular Atrophy