A Study to Find Out How Nusinersen is Processed in the Body When Given Through the ThecaFlex DRx™ System in Adult and Pediatric Participants With Spinal Muscular Atrophy (PIERRE-PK)

Key Inclusion Criteria: \- Participants must be screened in the PIERRE study to be eligible for enrollment in the PIERRE PK study. Participants in the 12 mg group * Participant must have completed all 4 required 12 mg loading doses as per the approved 12 mg dose regimen. * Participant is on regular LP maintenance dosing of nusinersen 12 mg every 4 months, with 120 days ± 14 days between the LP-delivered study dose and the last nusinersen dose prior to study enrollment. Participants in the 28 mg group * Participant must have completed 1 or 2 of the 50 mg loading doses as per the approved 50/28 mg dose regimen. * Participant is on regular LP maintenance dosing of nusinersen 28 mg every 4 months, with 120 days ± 14 days between the LP-delivered study dose and the last nusinersen dose prior to study enrollment. Key Exclusion Criteria: * Ongoing participation or participation within 6 months or 5 half-lives of the agent (whichever is longer) of enrollment in other interventional clinical trials for the treatment of SMA (except for the PIERRE study or interventional clinical trials of myostatin inhibitors \[e.g., apitegromab and taldefgrobep alfa\]). * Participant is naïve to nusinersen treatment. * Participant is receiving nusinersen at a dose other than 12 mg or 28 mg. * Participant has already undergone implantation of the ThecaFlex DRx system or any other implantable medical device for intrathecal (IT) administration. * Participant is pregnant, currently breastfeeding, or intending to become pregnant during the study. NOTE: Other protocol-defined Inclusion/Exclusion criteria may apply.