An Extension Follow-up Trial to Evaluate the Long-term Safety of Children and Adolescent Participants With Euvolemic or Hypervolemic Hyponatremia

Core Safety Follow-up Component Inclusion Criteria: Participation in a prior pediatric trial with tolvaptan for euvolemic or hypervolemic hyponatremia. Exclusion Criteria: None Optional Tolvaptan Treatment Component (per treatment cycle) Eligibility Criteria: 1. Male and female participants ≥ 4 years of age (or per local Health Authority age restrictions) and ≥ 10 kilograms (kg). 2. Participant must have been off treatment with the investigational medicinal product for at least 7 days following the end of treatment in the previous tolvaptan trial for hyponatremia (euvolemic or hypervolemic). 3. Persistent dilutional (euvolemic or hypervolemic) hyponatremia defined as being documented as present for at least 48 hours, evidenced by at least 2 serum sodium assessments \< 130 milliequivalents (mEq)/liter (L) (millimole \[mmol\]/L) drawn at least 12 hours apart (these values can be documented using historical values previously obtained per standard of care); a third (STAT) serum sodium assessment \< 130 mEq/L (mmol/L), which will serve as the baseline value for efficacy endpoints, had to be obtained within 2 to 4 hours prior to the first dose of tolvaptan. 4. Ability to swallow tablets. 5. Ability to maintain adequate fluid intake whether orally or via intravenous support with adequate monitoring. 6. Ability to comply with all requirements of the trial. 7. Trial-specific written informed consent/assent obtained from a parent/legal guardian or legally acceptable representative, as applicable per age of participant or local laws, prior to the initiation of any protocol-required procedures. In addition, the participant as required by local laws must provide informed assent at the pretreatment baseline for this trial and must be able to understand that he or she can withdraw from the trial at any time. All informed consent/assent procedures must be in accordance with the trial center's Institutional Review Board/Independent Ethics Committee and local regulatory requirements. 8. Ability to commit to remain fully abstinent (periodic abstinence \[for example, calendar, ovulation, symptothermal, post-ovulation methods\] or withdrawal are not acceptable methods of contraception) or practice double-barrier birth control during the trial and for 30 days following the last dose of IMP for sexually active females of childbearing potential. Ineligibility Criteria: 1. Had evidence of hypovolemia or intravascular volume depletion (for example, hypotension, clinical evidence of volume depletion, response to saline challenge); if the participant had systolic blood pressure or heart rate outside of the normal range for that age, then volume status was to be specifically clinically assessed to rule out volume depletion. 2. Had serum sodium \< 120 mEq/L (mmol/L), with or without associated neurologic impairment (that is, symptoms such as apathy, confusion, or seizures). 3. Use of potent CYP3A4 inhibitors in participants ≤ 50 kg or moderate CYP3A4 inhibitors in participants \< 20 kg. 4. Lacked free access to water (inability to respond to thirst) or without ICU-level fluid monitoring and management. 5. Had a history or current diagnosis of nephrotic syndrome. 6. Had transient hyponatremia likely to resolve (for example, head trauma or post-operative state). 7. Had hyperkalemia defined as serum potassium above the upper limit of normal (ULN) for the appropriate pediatric age range. 8. Had estimated glomerular filtration rate (eGFR) \< 30 milliliters (mL)/minute (min)/1.73 meters squared (m\^2) calculated by the following equation: eGFR (mL/min/1.73 m\^2) = 0.413 x height (centimeter \[cm\])/serum creatinine (mg/deciliter \[dL\]). 9. Had acute kidney injury defined as: Increase in serum creatinine by ≥ 0.3 mg/dL (≥ 26.5 micromoles \[μmol\]/L) within 48 hours; or increase in serum creatinine to ≥ 1.5 times baseline, which was known or presumed to have occurred within the prior 7 days; or urine volume \< 0.5 mL/kg/hour for 6 hours. 10. Had severe or acute neurological symptoms requiring other intervention (for example, hyperemesis, obtundation, seizures). 11. Had had treatment for hyponatremia with: * Hypertonic saline (including normal saline challenge) within 8 hours of qualifying sodium assessments; * Urea, lithium, demeclocycline, conivaptan, or tolvaptan within 4 days of qualifying serum sodium assessments; * Other treatment for the purpose of increasing serum sodium concurrent with dosing of trial medication 12. Had anuria or urinary outflow obstruction, unless the participant was, or could be, catheterized during the trial. 13. Had a history of drug or medication abuse within 3 months prior to the pretreatment visit or current alcohol abuse. 14. Had a history of hypersensitivity and/or idiosyncratic reaction to benzazepine or benzazepine derivatives (such as benazepril). 15. Had psychogenic polydipsia (participants with other psychiatric illness may be included per medical monitor approval). 16. Had uncontrolled diabetes mellitus, defined as fasting glucose \> 300 mg/dL (16.7 mEq/L \[mmol/L\]). 17. Had screening liver function values \> 3 x ULN. 18. Had cirrhosis and met any of the following conditions: a major gastrointestinal bleed within the past 6 months, evidence of active bleeding (for example, epistaxis, petechiae/purpura, hematuria, or hematochezia), platelet count \< 50,000/μL, or use of concomitant medications known to increase bleeding risk. 19. Had hyponatremia due to the result of any medication that can safely be withdrawn (for example, thiazide diuretics). 20. Had hyponatremia (for example, hyponatremia in the setting of adrenal insufficiency, untreated hypothyroidism, or hypotonic fluid administration) that is most appropriately corrected by alternative therapies. 21. Was currently pregnant or breastfeeding. 22. Had any medical condition that, in the opinion of the investigator, could interfere with evaluation of the trial objectives or safety of the participants. 23. Was deemed unsuitable for trial participation in the opinion of the investigator. 24. Participation in another investigational drug trial within the past 30 days, without prior approval from the sponsor medical monitor. 25. Participants \< 4 years of age (or per local Health Authority age restrictions), weight \< 10 kg, or who were unable to swallow tablets.