Proximal spinal muscular atrophy type 3 (Orphanet_83419) (SMA3) is a relatively mild form of proximal spinal muscular atrophy (see this term) characterized by muscle weakness and hypotonia resulting from the degeneration and loss of the lower motor neurons in the spinal cord and the brain stem nuclei. Also known as: Juvenile spinal muscular atrophy, Kugelberg-Welander disease, SMA type 3, SMA-III, SMA3.