Study of Nusinersen (BIIB058) in Participants With Spinal Muscular Atrophy

The primary objectives of this study are to examine the clinical efficacy of nusinersen administered intrathecally at higher doses to participants with spinal muscular atrophy (SMA), as measured by change in Children\'s Hospital of Philadelphia-Infant Test of Neuromuscular Disorders (CHOP-INTEND) total score (Part B); to examine the safety and tolerability of nusinersen administered intrathecally at higher doses to participants with SMA (Parts A and C). The secondary objectives of this study are to examine the clinical efficacy of nusinersen administered intrathecally at higher doses to participants with SMA (Parts A, B and C); to examine the effect of nusinersen administered intrathecally at higher doses to participants with SMA (Parts A and C); to examine the safety and tolerability of nusinersen administered intrathecally at higher doses to participants with SMA, to examine the effect of nusinersen administered intrathecally at higher doses compared to the currently approved dose in participants with SMA (Part B).