The Approach Open Label Study: A Study of Volanesorsen (Formerly IONIS-APOCIIIRx) in Participants With Familial Chylomicronemia Syndrome

Inclusion Criteria: * Must give written informed consent to participate in the study (signed and dated) and any authorization required by law. * Able and willing to participate in a 65-week study. Group 1 and 2: * Satisfactory completion of ISIS 304801-CS6 (NCT02211209) or ISIS 304801-CS16 (NCT02300233) index studies with an acceptable safety profile, per Sponsor and Investigator judgment. Group 3: * Participants who did not participate in the CS6 or CS16 index studies and meet additional inclusion criteria of FCS may enroll in the study. * History of chylomicronemia. * A diagnosis of FCS (Type 1 Hyperlipoproteinemia.) * Fasting triglycerides greater than or equal to (≥)750 milligrams per deciliter \[mg/dL\] (8.4 millimoles per liter \[mmol/L\]) at Screening. Exclusion Criteria: * Unwilling to comply with lifestyle requirements for the duration of the study. Group 1 and 2: * Have any new condition or worsening of existing condition which in the opinion of the Investigator would make the participant unsuitable for enrollment, or could interfere with the participant participating in or completing the study. Group 3: * Diabetes mellitus if newly diagnosed or if hemoglobin A1c (HbA1c)≥ 9.0%. * Active pancreatitis within 4 weeks of screening. * Acute Coronary Syndrome within 6 months of screening. * Major surgery within 3 months of screening. * Treatment with Glybera therapy within 2 years of screening. * Have any other conditions in the opinion of the investigator which could interfere with the participant participating in or completing the study.