Diabetes insipidus

Question 1

how does ADH get from hypothalamus to the posterior pituitary?

Answer 1

migrates in neurosecretory granules along axonal pathways

Question 2

what is secretion of ADH determined by?

Answer 2

**plasma osmolality

also large falls in blood pressure or volume

Question 3

site of action of ADH

Answer 3

at collecting tubule of the kidney to cause water reabsorption (aquaporins)

at high levels ADH also causes vasoconstriction

Question 4

Symptoms of DI

Answer 4

polyuria, nocturia + compensatory polydipsia

Question 5

how is polyuria defined

Answer 5

dilute urine in excess of 3L/24 hrs

Question 6

What is CDI?

Answer 6

cranial diabetes insipidus

- impaired ADH secretion

Question 7

What is NDI?

Answer 7

nephrogenic diabetes insipidus

- renal resistance to the action of ADH

Question 8

what must DI be distinguished from?

Answer 8

primary polydipsia (psychiactric condition with excessive thirst) and other causes of polyuria and polydipsia e.g. hyperglycaemia

Question 9

Cause of CDI

Answer 9

disease of hypothalamus: neurosurgery, trauma, tumours, infiltrate disease (sarcoidosis, histiocytosis) and idiopathic

(if hypothalamus is intact but pp isn’t, or tract between the 2 isn’t, ADH can still leak out from damaged end of neutron)

Question 10

Cause of NDI

Answer 10

hypokalameia, hypercalcaemia, drugs(demeclocycline, lithium), renal tubular acidosis, sickle cell disease, prolonged polyuria of any cause, familial
tubulo-interstitial disease: obstruction, sickle-cell, pyelonephritis

Question 11

Ix

Answer 11

urine volume - to confirm polyuria
plasma biochem - high or high-normal sodium conc + osmolality
urine vol is inappropriately low for the high plasma osmolality
water deprivation test
MRI of hypothalamus in cases of CDI

high plasma osmolality, low urine osmolality
a urine osmolality of >700 mOsm/kg excludes diabetes insipidus

Question 12

Mx for CDI

Answer 12

Tx of underlying condition seldom improves established CDI

- *desmopressin (oral, nasal or IM)

Question 13

Mx NDI

Answer 13

Tx of the cause will usually improve NDI

- thiazide diuretics, carbamazepine + Chlorpropamide can sensitize the renal tubules to endogenous vasopressin

Question 14

carbamazepine, what class of drugs is it?

Answer 14

anti-convulsant

has been shown to have anti-diuretic activity

Question 15

Chlorpropamide what class is it

Answer 15

sulfonylurea class

used in T2DM

Question 16

example of a thiazide diuretic + MOA

Answer 16

Hydrochlorothiazide

inhibit the Na/Cl cotransporter in the distal convoluted tubule of the nephron

Question 17

water deprivation test

Answer 17

If there is no change in the water loss despite fluid deprivation, desmopressin may be administered to distinguish between the two types of diabetes insipidus which are central & nephrogenic diabetes insipidus.