APA Dictionary of Psychology

a hereditary (autosomal) motor neuron disease characterized by wasting (atrophy) of skeletal muscles associated with degeneration of nerve cells in the anterior horn of the spinal cord. There are three common types, based on age of onset and symptom severity. Type I (also called Werdnig–Hoffmann disease) is evident at birth or within the first few months of life and is the most severe, resulting in death usually prior to age 2. Type II has an onset of symptoms between 6 months and 2 years and results in delayed motor development, progressive loss of strength, and variable loss of ambulation. Type III (also called Kugelberg–Welander disease) is identified between ages 1 and 15 and is the least severe, associated with slower progression and lower incidence. There is also a Type IV (also called Aran–Duchenne disease), with an onset typically after age 30 and symptoms ranging from mild to severe. Also called progressive muscular atrophy; progressive spinal muscular atrophy.