[MIM*253400] the juvenile form, with onset in childhood or adolescence; characterized by progressive proximal muscular weakness and wasting, primarily in the pelvic girdle, followed by distal muscle involvement; caused by degeneration of motor neurons in the anterior horns of the spinal cord; autosomal recessive inheritance, caused by mutation in the SMN1 gene on 5q. SYN: juvenile muscular atrophy, juvenile spinal muscular atrophy, Kugelberg-Welander disease, Wohlfart-Kugelberg-Welander disease.