Definition and Pathophysiology

• Characterized by primary elevation in vasopressin (AVP/ADH) secretion or inappropriate activation of vasopressin V2 receptors.
• Impaired free water clearance leads to water retention and dilutional hyponatremia (serum sodium <135 mEq/L).
• Subsequent extracellular fluid expansion triggers compensatory mechanisms:
  • Suppression of renin-angiotensin-aldosterone system.
  • Elevation of atrial natriuretic peptide (ANP).
• Compensatory mechanisms induce marked natriuresis, resulting in normal-to-high urine sodium despite systemic hyponatremia.
• Net clinical state: Euvolemic or slightly hypervolemic hyponatremia with inappropriately concentrated urine.

Etiological Classification

Clinical Manifestations

• Presentation dictated by severity and rapidity of hyponatremia onset.
• Chronic/Mild: Often completely asymptomatic.
• Acute/Severe (Serum Na <120 mEq/L):
  • Water entry into cells causes cerebral edema/neuronal swelling.
  • Manifestations include lethargy, confusion, psychosis, generalized seizures, coma, and potential cerebral herniation.

Diagnostic Evaluation

%%{init: {"themeVariables": { "lineWidth": "3px", "lineColor": "#000000" } }}%%
graph TD
    A["Hyponatremia: Serum Na+ < 135 mEq/L"] --> B{Check Serum Osmolality}
    
    B -->|"Greater than 295 mOsm/kg"| C[Hypertonic/Isotonic Hyponatremia <br> e.g., Hyperglycemia, Pseudohyponatremia]
    B -->|"275 - 295 mOsm/kg"| C
    B -->|"Less than 275 mOsm/kg"| D[True Hypotonic Hyponatremia]
    
    D --> E{Assess Volume Status}
    
    E -->|Hypovolemic| F[Dehydration, Diuretics, GI losses]
    E -->|Hypervolemic| G[Heart Failure, Cirrhosis, Nephrotic Syndrome]
    E -->|Euvolemic| H{Check Urine Osmolality}
    
    H -->|"Less than 100 mOsm/kg"| I[Primary Polydipsia, Beer Potomania]
    H -->|"Greater than 100 mOsm/kg"| J{Check Urine Sodium & Endocrine Function}
    
    J -->|"U_Na Less than 30 mEq/L OR Abnormal Cortisol/TSH"| K[Adrenal Insufficiency, Hypothyroidism, or Diuretic use]
    J -->|"U_Na Greater than 30 mEq/L AND Normal Cortisol/TSH"| L[SIADH Confirmed]
    
    L --> M{Assess Symptom Severity}
    
    M -->|Severe: Seizures, Coma, Severe Confusion| N["Emergency Treatment: <br> 3% Hypertonic Saline bolus <br> Limit correction to < 8-10 mEq/L in 24h"]
    M -->|Mild to Moderate: Headache, Nausea, Mild Confusion| O["First-Line Treatment: <br> Fluid Restriction < 800-1000 mL/day <br> Address underlying cause"]
    
    O --> P["Second-Line if Fluid Restriction Fails: <br> Oral Salt Tablets + Loop Diuretics, or Vasopressin Antagonists /Vaptans/"]

    %% Class Definitions %%
    classDef assessment fill:#e3f2fd,stroke:#1565c0,stroke-width:2px,color:#1565c0;
    classDef decision fill:#fff3e0,stroke:#ef6c00,stroke-width:2px,color:#ef6c00;
    classDef outcome fill:#e0f2f1,stroke:#00695c,stroke-width:2px,color:#00695c;
    classDef standardTx fill:#f3e5f5,stroke:#6a1b9a,stroke-width:2px,color:#6a1b9a;
    classDef critical fill:#ffebee,stroke:#c62828,stroke-width:3px,color:#c62828;

    class A assessment;
    class B,E,H,J,M decision;
    class C,D,F,G,I,K,L outcome;
    class O,P standardTx;
    class N critical;

• Serum Chemistry:
  • Hyponatremia (Sodium <135 mEq/L).
  • Low effective serum osmolality (<270 mOsm/kg).
  • Low blood urea nitrogen (BUN).
  • Low serum uric acid (differentiates from hypovolemic hyponatremia where uric acid is high).
• Urine Chemistry:
  • Inappropriately concentrated urine (Osmolality >100 mOsm/kg, often >800 mOsm/kg).
  • High urine sodium (>30 mEq/L).
• Hormonal/Biomarker Profiling:
  • High vasopressin levels (except in NSIAD where levels are suppressed/undetectable).
  • Copeptin measurement (carboxy-terminus of AVP precursor) coupled with hypertonic saline infusion useful for subtype classification.
• Clinical Status:
  • Normal or high intravascular volume (euvolemia/hypervolemia).
  • Normal blood pressure; absence of orthostasis.
  • Absence of peripheral edema.
  • Normal adrenal and thyroid function (mandatory exclusion).

Differential Diagnosis

Management

Chronic, Euvolemic, or Mild SIADH

• Primary Therapy: Strict oral fluid restriction. Limit intake to 1000 mL/m2/day (covers obligate renal solute load and insensible losses).
• Pharmacologic Adjuncts (if fluid restriction compromises nutrition/growth):
  • Urea: Oral administration induces safe osmotic diuresis. Highly effective in pediatric SIADH and NSIAD.
  • Vaptans (Tolvaptan, Conivaptan): Non-peptide V2 receptor antagonists (aquaretics). Produce rapid free water excretion. Caveats: Not FDA approved in children. Risk of excessively rapid overcorrection, hepatotoxicity, and extreme thirst. Ineffective in NSIAD (activating V2 mutations).
  • Demeclocycline/Lithium: Induce nephrogenic DI. Historically used but limited in pediatrics due to significant renal and systemic toxicity.

Acute, Severe, or Symptomatic SIADH (Na <120 mEq/L with neurological compromise)

• Medical Emergency: Immediate intervention required to reverse cerebral edema.
• Hypertonic Saline: Administer 3% Sodium Chloride intravenously.
  • Standard guide: 12 mL/kg of 3% NaCl raises serum sodium by approximately 10 mEq/L.
• Correction Limits (Critical):
  • Raise serum sodium only high enough to resolve critical mental status changes.
  • Maximum correction rate: 0.5 mEq/L/hr or 12 mEq/L/24 hr.
  • Complication of rapid correction: Central Pontine Myelinolysis (Osmotic Demyelination Syndrome). Causes irreversible axonal demyelination and permanent brain damage within 24-48 hours.
• Contraindications: Avoid isotonic (0.9%) saline. Administering normal saline in SIADH frequently worsens hyponatremia because the sodium is rapidly excreted while the free water is retained.