SIADH: Water Balance & The Neurohypophyseal Axis

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SIADH & Water Balance Masterclass

01. Pathophysiology

What is SIADH?

SIADH (Syndrome of Inappropriate Antidiuretic Hormone) is a disorder of impaired water excretion caused by the inability to suppress the secretion of antidiuretic hormone (ADH/Vasopressin).

The Logic: ADH acts on the V2 receptors in the kidneys to "save water." In SIADH, the body saves water even when the blood is already too dilute. It is a dilutional hyponatremia, not a sodium deficiency.

Relationship: Higher ADH (X) leads to maximally concentrated urine (Y).

02. Common Etiologies

CNS

Trauma, Stroke, Infection (Meningitis), and Rathke's Cleft Cyst rupture.

Malignancy

Small Cell Lung Cancer (Ectopic ADH), Pancreatic, Olfactory Neuroblastoma.

Pulmonary

Pneumonia, TB, and Positive Pressure Ventilation.

Drugs (The 3 S's)

SSRIs, Seizure meds (Carbamazepine), Sytotoxic (Cyclophosphamide).

03. The Rathke's Cleft Cyst Phenomenon

Rupture & The Triphasic Response

A Rathke's Cleft Cyst is a benign fluid-filled mass in the pituitary. If it ruptures, it leaks irritating fluid that causes inflammation of the pituitary stalk. This triggers a specific, three-phase sequence of water balance disorders:

01

Initial Phase: Central DI (Days 1-2)

"Axonal Shock" prevents the release of ADH. The patient experiences polyuria and hypernatremia.

02

The SIADH Phase (Days 3-10)

As the damaged pituitary neurons die/necrose, they leak all stored ADH at once. This causes a sudden, severe SIADH with dangerous hyponatremia.

03

Final Phase: Permanent DI

Once the stores are depleted and the neurons are gone, the patient reverts to a permanent state of Central Diabetes Insipidus.

04. Clinical Presentation

Severity Spectrum

  • Mild: Malaise, headache, nausea.
  • Moderate: Confusion, disorientation, gait instability.
  • Severe: Seizures, coma, brainstem herniation.

The "Hidden" Sign

The most critical sign for exams is Clinical Euvolemia. Despite retaining water, patients do NOT have edema, JVD, or ascites because the body compensates via "Pressure Natriuresis" (dumping sodium to keep volume stable).

05. Schwartz-Bartter Logic Engine

Patient Labs

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Analyzing Data

06. The 45-Second Exam Algorithm

Rapid Diagnostic Flow (Step 2 CK Trick)

Step 1: Volume
Is the patient dehydrated?
• No → Think SIADH
• Yes → Think DI or hypovolemic hyponatremia
Step 2: Sodium
What is the sodium?
• Low Sodium → SIADH or hypovolemic hyponatremia
• High Sodium → Diabetes Insipidus (DI)
Step 3: Urine
What does the urine look like?
• Concentrated urine → ADH is working (SIADH or hypovolemia)
• Dilute urine → ADH isn't working (DI)

High-Yield Memory Aid

  • SIADH = Soaked in water → Low Na, concentrated urine, euvolemic
  • DI = Dry → High Na, dilute urine, polyuria
  • Hypovolemic hyponatremia = Dehydrated + low Na → kidneys save both to fix volume

07. Differential Diagnosis Matrix

Condition Volume Status Urine Na+ Urine Osm
SIADH Euvolemic > 40 > 100 (Inappropriate)
Cerebral Salt Wasting Hypovolemic > 40 > 100
Hypovolemia (Dehydration) Hypovolemic < 20 > 400 (Appropriate)

08. Management & Safety

Sodium Correction (Adrogué-Madias)

Predicted ΔNa per 1L
+ 0.00
Hierarchy of Care
  1. Fluid Restriction: First-line (< 800mL/day).
  2. Oral Salt Tablets: For chronic outpatient management.
  3. Vaptans (Tolvaptan): V2 receptor antagonist.
    Indications: Clinically significant hypervolemic or euvolemic hyponatremia (Na+ < 125 mEq/L) resistant to fluid restriction, or highly symptomatic cases (e.g., heart failure-associated).
    Caution: Risk of rapid overcorrection and hepatotoxicity. Do not use for > 30 days.
  4. 3% Hypertonic Saline: Emergencies only.
Safety Limit: ODS Risk

Limit correction to < 8 mEq/L in 24 hours. Rapid shifts cause Osmotic Demyelination Syndrome (Central Pontine Myelinolysis).

Medical Study Tool • Focused on Physiology • v3.0

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