Syndrome of inappropriate secretion of ADH

Question 1

What is it

Answer 1

continued ADH secretion in spite of plasma hypotonicity and a normal or expanded plasma volume

(dont need to retain water)

Question 2

cause

Answer 2

disordered hypothalamic-pituitary secretion or ectopic production of ADH e.g. small-cell lung cancer

cancer - most common SCLC
neuro e.g. meningitis, cerebral abscess, stroke, SAH, SDH
infections /lung - pneumonia, tb
metabolic - porphyria, alcohol wd
drugs - opiates, carbamazepine, SSRIs, tricyclics, sulfonylureas

Question 3

clinical features

Answer 3

nausea, irritability and headache

N&V, tremors, muscle cramps

mild dilution hyponatraemia

fits + coma may occur with severe hyponatraemia (from cerebral oedema)

Question 4

Ix

Answer 4

low serum sodium
low plasma osmolality with ‘inappropriate’ urine osmolality
continued urinary sodium excretion
absence of hypokalaemia, hypotension and hypovolaemia
normal renal, adrenal and thyroid function

Question 5

how to distinguish SIADH from salt and water depletion, esp during illness in frail elderly patients

Answer 5

give a trial of saline

sodium depletion will respond whereas SIADH will not

Question 6

Mx

Answer 6

for symptomatic cases:

• water restriction
• high salt & high protein diet
• demeclocycline: inhibits the action of ADH on the kidney, may be useful if water restriction is poorly tolerated or ineffective
• tolvaptan: specific vasopressin receptor antagonist, used for hyponatraemia secondary to SIADH
• hypertonic saline, with furosemide to prevent circulatory overload, in severe cases

Question 7

Why must correction be done slowly?

Answer 7

to avoid precipitating central pontine myelinolysis