(spī'năl mŭs'kyū-lăr at'rŏ-fē tīp) A form intermediate in severity between the infantile form (SMA type I) and the juvenile form (SMA type III); characterized by proximal muscle weakness with onset usually between 3-15 months and survival until adolescence; autosomal recessive inheritance, caused by mutation in the SMN1 gene on 5q.