Diabetes Insipidus

Question 1

what is diabetes insipidus

Answer 1

↓ ADH or lack of response/insensitivity to ADH

Question 2

describe the types of diabetes insipidus

Answer 2

cranial DI
- ↓ADH prod from post. pit.

nephrogenic DI
- resistance to ADH in kidney

Question 3

what is ADH also know as

Answer 3

vasopressin

Question 4

what causes cranial DI

Answer 4

CIVIT

C - congenital defect in ADH gene
I - idiopathic
V - vascular
I - infection –> meningoencephalitis, meningitis
T - tumour (pituitary adenoma), TB, trauma

Question 5

what causes nephrogenic DI

Answer 5

DIMC

D - drugs e.g. lithium
I - inherited
M - metabolic; ↑Ca, ↓K
C - chronic renal disease

Question 6

what are some of the presentations of DI

Answer 6

• polyuria - ↑urine output over 24hrs (dilute urine)
• hypernatraemia
• polydipsia
• nocturia
• dehydration
• postural hypotension

Question 7

what investigations can be done in suspected DI

Answer 7

bloods:
- hypernatraemia is key result
- -> ↑serum osmolality
- -> ↓urine osmolality

MRI of pituitary/hypothalamus/pineal gland may help determine cause

water deprivation test:

• px deprived of water
• if no DI; ↓urine output, ↑urine osmolality
• if yes DI; ↑urine output, ↓urine osmolality

Question 8

how can you distinguish between cranial and nephrogenic DI

Answer 8

• px deprived of water
• given injection of desmopressin
• -> if urine osmolality ↑ = central DI
• -> if urine osmolality FAILS to ↑ = insensitive to ADH so nephrogenic DI

Question 9

what is desmopressin

Answer 9

synthetic analogue of ADH

Question 10

outline the management of DI

Answer 10

cranial:
- desmopressin replacement

nephrogenic:
- low Na diet/improved hydration
- high dose desmopressin (not always effective)
- thiazide diuretics
- prostaglandin synthase inhibitors
- correction of electrolyte imbalances

surgical –> excision of tumour if indicated

Question 11

what is the role of desmopressin

Answer 11

↑ number of aquaporin-2 channels in DCT and CD to ↑ water reabsorption

Question 12

how do thiazide diuretics help in nephrogenic DI

Answer 12

• despite being diuretics they actually ↑ fluid resorption at PCT
• helps fluid and electrolyte balance prod net benefit

(inhibit NaCl co-transporter, ^ Na excretion, lowers GFR, ^ PCT H20 reabsorption)

Question 13

what complications can arise from DI

Answer 13

• electrolyte imbalance

- dehydration