The primary objective is to evaluate the long-term safety and tolerability of nusinersen (ISIS 396443) administered by intrathecal (IT) injection to participants with Spinal Muscular Atrophy (SMA) who previously participated in investigational studies of nusinersen. The secondary objective is to examine the long-term efficacy of nusinersen administered by IT injection to participants with SMA who previously participated in investigational studies of nusinersen.