progressive muscular atrophyspinal muscular atrophy symptomsprogressive spinal muscular atrophyspinal muscular atrophy diagnosisperoneal muscular atrophyspinal and bulbar muscular atrophyspinal muscular atrophy curesma muscular atrophymuscle atrophy diseasesma spinal muscular atrophyatrophy and dystrophyspinal muscular atrophy gene therapy