spinal muscular atrophy type 2sma spinal muscular atrophy type 2spinal atrophy type 2spinal muscular atrophy type iisystemic muscular atrophymuscular dystrophy atrophydiagnosis spinal muscular atrophyspinal muscular atrophy prognosisspinal muscular atrophy statisticsspinal muscular atrophy researchspinal muscular atrophy lifespanadult spinal muscular atrophy