body water balance and diabetes insipidus B W7

Question 1

polyuria?

Answer 1

increased urinal output

Question 2

excess water loss features?

Answer 2

thirsty
if intake is inadequate then increased Na+, increased plasma osmolarity, sometimes BP drops. this can lead to collapse and confusion.
presents as polyuria

Question 3

excess water retention features?

Answer 3

usually little initial symptoms
later drop in Na+ and plasma osmolarity. confusion, drowsiness, nausea, fits
presents as unexplained confusion

Question 4

name for passing excessive urine volumes?

Answer 4

polyuria

Question 5

what can occur if there is ongoing polyuria? what is this?

Answer 5

polydipsia - high fluid intake

Question 6

4 main causes of polyuria?

Answer 6

diabetes insipidus
habitual/psychogenic
osmotic diuresis (due to increased levels of glucose or calcium - eg diabetes mellitus)
renal impairment (unusual)

Question 7

investigation of polyuria - 5 steps?

Answer 7

1- establish polyuria is present
2- check: glucose, Ca2+, urea, creatinine
steps 3-5 distinguish diabetes insipidus from habitual/psychogenic polydipsia:
3- check urine never normally concentrated
4- water deprivation test - show if unable to concentrate urine
5- if DI (diabetes insipidus), give DDAVP (no effect = nephrogenic DI, if osmolarity increases above 600mosmol/kg then cranial DI

Question 8

water deprivation test?

Answer 8

ask 12 hours before test to drink as little as you can
during test (up to 8 hrs) - no fluids, dry snacks.
hourly checks - weight, BP, urine sample
every 2 hours - blood tests

stop test if plasma osmolarity >600 or if danger (weight loss>3%, hypotensive + dizzy etc
when water is depleted give DDAVP if necessary

Question 9

hypertonic saline test for diabetes insipidus?

Answer 9

give hypertonic saline infusion. during this osmolarity rises and blood can be taken to measure vasopressin (and copeptin). normal response - AVP rises
diabetes insipidus response - AVP does not rise

Question 10

cranial diabetes insipidus - causes?

Answer 10

abnormality in hypothalamus or posterior pituitary. lesions of these areas eg neurosurgery, head injury, tumours, haemorrhage, genetic (isolated, DIDMOAD)

Question 11

what is DIDMOAD?

Answer 11

diabetes insipidus, diabetes mellitus, optic atrophy, deafness

Question 12

pregnancy (gestational DI)?

Answer 12

placenta has enzyme which breaks down vasopressin - if patient has eg subclinical DI this can develop into gestational DI which is resolved postpartum.

Question 13

nephrogenic DI?

Answer 13

resistance of kidneys to vasopressin due to:
hypercalcaemia/hypokalaemia
resolution after urinary tract obstructive
secondary effect of psychogenic polydipsia
lithium therapy effect
demeclocycline
inherited

Question 14

treatment of cranial diabetes insipidus?

Answer 14

treat other hypothalamo-pituitary deficiencies
replacement for vasopressin = desmopressin

Question 15

treatment of nephrogenic diabetes insipidus?

Answer 15

think of causes and treat if possible
some treatment of partial benefit (thiazide diuretics)
lower salt and protein diet (reduce osmotic load)