SIADH

Question 1

What is the pathophysiology of SIADH?

Answer 1

Excessive ADH secretion

Increased from hypothalamus + posterior pituitary
Ectopic (commonly from a small cell-lung cancer)

Increased water re-absorption in the collecting ducts, thus diluting the blood and reducing sodium concentration.

Not normally enough to cause fluid overload but can cause a euvolemic hyponatremia.

Question 2

What are the risk factors for SIADH?

Answer 2

Post operative – major surgery
Lung infection – atypical pneumonia and lung abscesses
Brain insults – trauma, stroke, infections
Medications – SSRIS and carbamezepine
Malignancy – ectopic ADH from small cell lung cancer
HIV

Question 3

What are the common signs and symptoms of SIADH?

Answer 3

Nausea and vomiting
Headache
Balance problems
Mental changes
Low blood sodium
Concentrated urine + small amounts
Muscle cramp or weakness

Question 4

What investigations should be done for SIADH?

Answer 4

Small volume of concentrated urine – high osmolarity

High urine sodium (as less water)

Lower serum sodium (due to fluid retention diluting the blood)

Question 5

What is the treatment for SIADH?

Answer 5

Vasopressin receptor antagonist (tolvaptan) block ADH receptors – require close monitoring due to rapid rise in sodium levels.

Fluid restriction
Demeclocycline - reduces responsiveness of the collecting tubule cells to ADH
Must be corrected slowly to avoid central pontine myelinolysis.