spinal muscular atrophy symptomsinfantile spinal muscular atrophyspinal muscular atrophy prognosisprogressive spinal muscular atrophyspinal and bulbar muscular atrophydistal spinal muscular atrophyspinal atrophyspinal muscular atrophy curespinal muscular diseasemuscular atrophy diseasesma spinal muscular atrophyspinal atrophy type 1