Syndrome of inappropriate ADH secretion

Question 1

what is syndrome of inappropriate ADH secretion (SIADH)

Answer 1

characterised by dilutionnal hyponatraemia secondary to the excessive water retention - involves an excessive release of antidiuretic hormone (ADH)

Question 2

ADH is produced by the hypothalamus and stored in

Answer 2

posterior pituitary gland

Question 3

In SIADH this increase in body fluid volume does not lead to

Answer 3

expected signs of fluid overload, such as oedema or hypertension, because the excess fluid is uniformly distributed throughout all body fluid compartments

Question 4

malignancy causes of SIADH ?

Answer 4

small cell lung cancer
also: pancreas, prostate

Question 5

neurological causes of SIADH ?

Answer 5

stroke
subarachnoid haemorrhage
subdural haemorrhage
meningitis/encephalitis/abscess

Question 6

infective causes of SIADH ?

Answer 6

tuberculosis
pneumonia

Question 7

drug causes of SIADH ?

Answer 7

sulfonylureas
SSRIs,
tricyclics antidepressants - lofepramine
carbamazepine
vincristine
cyclophosphamide

Question 8

other causes of SIADH ?

Answer 8

positive end-expiratory pressure (PEEP)

porphyrias

Question 9

investigations to order for SIADH ?

Answer 9

Urine osmolality: Urine osmolality is inappropriately high (>100 mOsm/kg) in relation to serum osmolality, as the kidneys should normally dilute urine in the setting of low serum osmolality.

Urine sodium concentration: Urine sodium concentration is typically high (>40 mmol/L) due to the action of ADH on the renal tubules.

Question 10

Mx of SIADH ?

Answer 10

fluid restriction

demeclocycline

ADH (vasopressin) receptor antagonists

Question 11

how does demeclocycline work?

Answer 11

reduces the responsiveness of the collecting tubule cells to ADH