Study of the Safety and Effectiveness of SAMSCA® (Tolvaptan) in Children and Adolescents With Euvolemic or Hypervolemic Hyponatremia

Inclusion: * Male and female participants ≥4 weeks (or ≥44 weeks adjusted gestational age) to \<18 years old * Participants hospitalized with euvolemic or hypervolemic hyponatremia resistant to initial standard background therapy * Persistent euvolemic or hypervolemic hyponatremia defined as being documented as \<130 milliequivalent (mEq)/L and present for at least 48 hours, evidenced by at least 2 serum sodium assessments (12 hours apart) * Ability to maintain adequate fluid intake (orally or intravenously) * Ability to take oral medications * Ability to comply with all requirements of the trial * Completion of the trial-specific informed consent/assent as age appropriate * Ability to commit to remain fully abstinent or practice double-barrier birth control as required by the trial Exclusion: * Evidence of hypovolemia or intravascular volume depletion * Serum sodium \<120 mEq/L * Use of potent cytochrome P450 3A4 (CYP3A4) inhibitors in participants \<12 kilogram (kg) or moderate CYP3A4 inhibitors in participants \<6 kg * Lacks free access to water (inability to respond to thirst) or without intensive care unit level fluid monitoring and management * History or current diagnosis of nephrotic syndrome * Transient hyponatremia likely to resolve * Hyperkalemia * Estimated glomerular filtration rate \<30 milliliters/minute/1.73 meters squared * Acute kidney injury * Severe or acute neurological symptoms requiring other intervention * Prior treatment for hyponatremia with hyper…