SIADH

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

Category	Specific Pathologies
Central Nervous System	Encephalitis, meningitis (tuberculous, bacterial), brain tumor (glioma, craniopharyngioma, germinoma), head trauma, brain malformations, hydrocephalus, Guillain-Barre syndrome, subarachnoid hemorrhage, postictal state.
Pulmonary Disorders	Pneumonia (viral/RSV, bacterial), tuberculosis, aspergillosis, asthma, cystic fibrosis.
Malignancy	Thymoma, lymphoma, Ewing sarcoma, leukemia.
Pharmacologic Agents	Carbamazepine, oxcarbazepine, chlorpropamide, cyclophosphamide, vinblastine, vincristine, cisplatin, tricyclic antidepressants (imipramine, amitriptyline), SSRIs (fluoxetine, sertraline), haloperidol.
Postoperative	Second phase of “triple-phase response” post-hypothalamic/pituitary surgery (caused by unregulated AVP release from dying neurons; lasts up to 10 days).
Genetic (NSIAD)	Nephrogenic Syndrome of Inappropriate Antidiuresis: Gain-of-function activating mutations in V2 receptor gene (AVPR2). X-linked. Features undetectable AVP levels.
Miscellaneous	Prolonged nausea, pain, AIDS, acute intermittent porphyria.

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

graph TD
     Confirm Hypoosmolar Hyponatremia
    CheckOsm --> ConfirmLowOsm{"Serum Osmolality < 275-280 mOsm/kg?"}
    ConfirmLowOsm -- No --> PseudoOrTrans["Suggests Pseudohyponatremia or Translocational Hyponatremia <br> (e.g., hyperglycemia)"]
    
     Evaluate Volume Status
    AppropUrineOsm -- Yes --> CheckVolumeStatus["Assess Volume Status"]
    CheckVolumeStatus --> DetermineStatus{"Status:"}
    
    DetermineStatus -- Hypovolemic --> LowVolume["Look for Extrarenal/Renal losses, Diuretics"]
    DetermineStatus -- Hypervolemic --> HighVolume["Consider Heart Failure, Cirrhosis, Nephrotic Syndrome"]
    DetermineStatus -- Euvolemic --> RuleOutConditions["Evaluate for essential SIADH criteria & rule out mimics"]
    
     Styling
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    classDef actionNode fill:#e1f5fe,stroke:#01579b,stroke-width:1px;
    classDef decisionNode fill:#fff9c4,stroke:#fbc02d,stroke-width:1px;
    classDef startNode fill:#e8f5e9,stroke:#2e7d32,stroke-width:2px,color:#000;
    classDef endNode fill:#ffebee,stroke:#c62828,stroke-width:2px,color:#000;
    
    class Start,NormNa startNode;
    class VerifyHypo,ConfirmLowOsm,AppropUrineOsm,DetermineStatus,AssessUrineNa,AssessAdrenal,AssessRenal decisionNode;
    class CheckOsm,CheckVolumeStatus,RuleOutConditions,RuleOutOtherCauses actionNode;
    class Diagnosed endNode;
    class PseudoOrTrans,WaterIntake,LowVolume,HighVolume,LowUNa,DiureticOrRenal,SpecificTreatment mainNode;

• 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

Feature	SIADH	Cerebral Salt Wasting (CSW)	Systemic Dehydration	Primary Polydipsia
Pathophysiology	Excess AVP action	Excess ANP/natriuretic peptides	Fluid/salt loss	Compulsive water intake
Intravascular Volume	Normal or High	Low (Hypovolemia)	Low	Normal or High
Blood Pressure	Normal	Decreased/Orthostatic	Decreased	Normal
Urine Sodium	High (>30 mEq/L)	Very High (>150 mEq/L)	Low (<20-30 mEq/L)	Normal
Serum Uric Acid	Low	Normal or High	High	Normal
BUN	Low	High	High	Low/Normal
Vasopressin Level	High	Low (Suppressed)	High	Low

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.