a hereditary motor neuron disease, characterized by wasting, atrophy, and of skeletal muscles associated with degeneration of nerve cells in the anterior section of the spinal cord. There are three common types, based on age of onset and symptom severity. Type 1 is seen at birth. Type 2 is seen between 6 months and 2 years. Type 3 is seen between ages 1 and 15 years.
SPINAL MUSCULAR ATROPHY (SMA): "Spinal muscular atrophy is seen in 3 types determined by age of onset."