spinal muscular atrophy, type III

spi·nal mus·cu·lar at·ro·phy, type III

(spī'năl mŭs'kyū-lăr at'rŏ-fē tīp) The juvenile form with onset in childhood or adolescence, characterized by progressive proximal muscular weakness and wasting, primarily in the legs, 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.