A Study to Assess the Efficacy, Safety, Tolerability, and Pharmacokinetics of Multiple Doses of ISIS 396443 Delivered Intrathecally to Patients With Infantile-Onset Spinal Muscular Atrophy

The primary objective is to examine the clinical efficacy of multiple doses of nusinersen (ISIS 396443) administered intrathecally to participants with Infantile-Onset Spinal Muscular Atrophy (SMA). The secondary objectives are to examine the safety and tolerability of multiple doses of nusinersen administered intrathecally to participants with infantile-onset SMA and to examine the cerebral spinal fluid (CSF) and plasma Pharmacokinetics (PK) of multiple doses of nusinersen administered intrathecally to participants with infantile-onset SMA.