Diabetes Insipidus Flashcards
diabetes insipidus
the passage of large volumes of dilute urine due to impaired water resorption by the kidney, because of reduced ADH secretion from the posterior pituitary (cranial DI) or impaired response of the kindey to ADH (nephrogenic DI)
role of ADH
facilitate concentrated urine and facultative water reabsorption
features of DI
polyuria
polydipsia - can be uncontrollable and all consuming
dehydration
symptoms of hypernatraemia
why does hypernatraemia occur
water loss in excess of sodium loss
causes of cranial DI
familial: defects in ADH gene and Wolfram syndrome
acquired: idiopathic (50%), trauma
rare: sarcoid, tumour, external irradiation, meningitis, haemorrhage
causes of nephrogenic DI
inherited - defects in ADH gene
drugs
metabolic: low potassium, high calcium
chronic renal disease
diagnosis
water deprivation test
- aims to test the ability of the kidneys to conentrate urine for the diagnosis of DI, and then to localise the cause
what must be established before the water deprivation test is performed
urine output >3L/day
outline water deprivation test
Stage 1:
- Fluid deprivation for 8 hours, start at 8am
- Empty bladder, then no drinks and dry food
- Weight hourly
- Collect urine every 2 hours
- Venous sample for osmolality every 4 hours
- Stop test after 8 hours
Stage 2: differentiate cranial DI from nephrogenic DI
- Proceed if urine still diluted e.g. urine osmolality <600 mOsmol/kg
- Give desmopressin. Water can be drunk now
- Measure urine osmolality for the next 4 hours
what is the expected outcome of the Water Deprivation Test
urine osmolality expected to be ≥2x that of serum osmolality
if not, there is a diagnosis of DI
how is osmolality calculated
2[Na + K] + glucose + urea
what is the normal osmolality range
275-295 mOsmol/kg
interpreting the Water Deprivation Test
Normal: urine osmolality ≥2x that of serum osmolality
Primary polydipsia: urine concentrates, but less than normal e.g. >400-600 mOsmol/kg
Cranial DI: urine osmolality increases to >600 mOsmol/kg after desmopressin
Nephrogenic DI: no increase in urine osmolality after desmopressin
management
desmospray
desmopressin oral tablets
desmopressin IM injection
