A rare condition characterized by progressive degeneration of the spinal and brainstem motor neurons. During fetal development excess primary neurons are formed. The body automatically destroys the extra primary neurons so that only some survive and mature into neurons. In spinal muscular dystrophy, the process that destroys the excess primary neurons doesn't switch off and continues destroying the neurons resulting in progressive motor problems. Various types of the condition range from mild to severe enough to cause death within a couple of years of birth. See also Spinal Muscular Atrophy