Characteristics of Syndrome of Inappropriate Antidiuretic Hormone (SIADH)
Euvolemia, not hypovolemia, is consistent with Syndrome of Inappropriate Antidiuretic Hormone (SIADH). 1
Diagnostic Criteria for SIADH
SIADH is characterized by the following features:
- Hyponatremia (serum sodium <134 mEq/L) 1
- Plasma hypoosmolality (<275 mOsm/kg), not hyperosmolarity 1
- Inappropriately high urine osmolality (>500 mOsm/kg) relative to plasma osmolality 1
- Elevated urinary sodium concentration (>20 mEq/L) 1
- Clinical euvolemia (not hypovolemia) 1
- Normal thyroid and adrenal function 1
What is NOT Consistent with SIADH
Looking at the options provided in the question:
Hypovolemia - Incorrect. SIADH patients are clinically euvolemic, not hypovolemic. Hypovolemia is characteristic of Cerebral Salt Wasting (CSW), which is an important differential diagnosis for SIADH. 1
Hypernatremia - Incorrect. SIADH is characterized by hyponatremia due to excessive ADH secretion causing water retention. 1, 2
Plasma hyperosmolarity - Incorrect. SIADH presents with plasma hypoosmolality (<275 mOsm/kg), not hyperosmolarity. 1, 2
Excessive diuresis - Incorrect. In SIADH, urine output is typically normal or low due to the antidiuretic effect of excessive ADH, which promotes water reabsorption in the kidneys. 1
Pathophysiology of SIADH
SIADH occurs due to persistent elevated plasma arginine vasopressin (ADH) concentrations despite hypoosmolality. This leads to:
- Water retention via V2 receptors in the kidney 3
- Decreased free water clearance 2
- Dilutional hyponatremia 2
- Failure of osmoregulated inhibition of thirst 2
Common Causes of SIADH
- Malignancies (especially small cell lung cancer, 10-45% of cases) 1
- CNS disorders (stroke, hemorrhage, trauma, infection) 1
- Pulmonary diseases (pneumonia, tuberculosis, asthma, COPD) 1
- Medications (antidepressants, antipsychotics, anticonvulsants, diuretics, NSAIDs, opioids) 1
- Post-surgical states 1
Clinical Presentation
Symptoms depend on severity and acuity of hyponatremia:
- General weakness
- Confusion
- Headache
- Nausea
- Seizures (in severe cases)
- Coma (in severe cases) 1
Key Differentiating Features
The key difference between SIADH and other hyponatremic states is volume status:
- SIADH: Euvolemic
- Cerebral Salt Wasting: Hypovolemic
- Heart failure or cirrhosis: Hypervolemic 1
Additionally, SIADH typically presents with:
- Low serum urea
- Low uric acid levels
- Lower anion gap with nearly normal total CO2 4