Syndrome of inappropriate secretion of ADH (SIADH)

Question 1

What is the definition of SIADH? [3]

Answer 1

body continues to secrete excess ADH / despite low plasma osmolality / leading to excess retention of water and hyponatraemia

Question 2

What is the aetiology of SIADH? [3]

Answer 2

1. tumours - small-cell carcinoma of the lung, pancreas, prostate, thymus, lymphoma
2. CNS disorder - meningitis, abscess, stroke, haemorrhage
3. pulmonary lesions - pneumonia, TB, asthma, cystic fibrosis, lung abscess

Question 3

What is the pathophysiology of SIADH? [4]

Answer 3

excess secretion of ADH / increased insertion of aquaporin 2 channels into apical membrane of DCT/collecting ducts / excess water retention which will lower plasma osmolality / resulting in hyponatraemia

Question 4

What are the symptoms of SIADH? [5]

Answer 4

symptoms of hyponatraemia -

1. anorexia, nausea and malaise
2. irritability
3. confusion
4. weakness and aches
5. fits and coma

Question 5

How is SIADH diagnosed? [3]

Answer 5

1. concentrated urine with Na+ >20mmol/L
2. and urine osmolality >100mOsmol/kg
3. in the presence of hyponatraemia and low plasma osmolality

Question 6

What is the treatment of SIADH? [4]

Answer 6

1. treat the underlying cause
2. restrict fluid intake to 500-1000mL daily
3. hypertonic saline in emergencies to prevent brain swelling
4. oral demeclocycline - induces nephrogenic DI by inhibiting the action of ADH on the kidney