Diabetes Insipidus Flashcards
What is the definition of diabetes insipidus? [2]
condition in which there is passage of large volumes (>3L/day) of dilute urine / due to impaired water reabsorption by the kidney
What are the causes of diabetes insipidus? [2]
- cranial DI - reduced ADH secretion from the posterior pituitary
- nephrogenic DI - impaired response of the kidney to ADH
What is the aetiology of cranial DI? [5]
- idiopathic (<50%)
- congenital defects in ADH gene
- disease of the hypothalamus
- tumour - craniopharyngioma, metastases, posterior pituitary tumour
- trauma - neurosurgery
- infiltrative disease - sarcoidosis
What is the aetiology of nephrogenic DI? [6]
- inherited - mutation of ADH receptor
- hypokalaemia
- hypercalcaemia
- chronic renal disease
- drugs - lithium chloride
- prolonged polyuria
What is the pathophysiology of diabetes insipidus? [5]
reduced ADH secretion or impaired response to ADH / results in significant water losses / as there is impaired water reabsorption in the kidney (from DCT and collecting ducts) / resulting in large volumes of dilute urine
What are the symptoms of diabetes insipidus? [4]
- polyuria
- polydipsia
- symptoms of hypernatraemia - lethargy, thirst, weakness, irritability, confusion, coma and fits
- dehydration - can be severe if thirst mechanism or consciousness impaired/no access to fluid
What 3 tests can be carried out to diagnose diabetes insipidus? [3]
- measure urine volume to confirm polyuria - DI unlikely if urine volume < 3L/day
- check blood glucose to exclude diabetes mellitus
- water deprivation test
What does the water deprivation test involve? [3]
- aims to determine if the kidneys continue to produce dilute urine despite dehydration
- serum and urine osmolality, and urine volume measured every hour for 8 hours during fasting and without fluids
- in DI, serum osmolality rises without adequate concentration of urine
How can cranial DI by differentiated from nephrogenic DI in the water deprivation test? [3]
- give IM desmopressin (synthetic analogue of vasopressin)
- in nephrogenic DI, urine will not be concentration (as impaired response of cells to ADH)
- in cranial DI, urine will be concentrated (as counteracting reduced ADH secretion)
What is the treatment of cranial DI? [2]
- find the underlying cause - MRI of the head and test anterior pituitary function to look for a tumour that could affect the posterior pituitary
- give synthetic analogue of ADH - oral desmopressin / long duration of action and has no vasoconstrictive effects
What is the treatment of nephrogenic DI? [3]
- treat the underlying cause
- thiazide diuretics - e.g. oral bendroflumethiazide, to encourage kidneys to take up more Na+ and water
- NSAIDs - e.g. ibuprofen, which lower urine volume and plasma Na+ / by inhibiting prostaglandin synthase, as prostaglandins locally inhibit the action of ADH
