Diabetes Insipidous - Electrolytes

Diabetes Insipidous

·        Symptoms: polyuria, dilute urine despite dehydration, polydypsia

·        Central Diabetes Insipidous:

o   ¯Water resorption in kidney due to ¯ADH secretion from posterior pituitary ® low urine osmolality (eg 150 mosmol/kg) despite dehydration. > 5/l per day urine requires hypothalamic damage as well as posterior lobe

o   Causes of central DI: head injury, tumour, metastasis, sarcoidosis, vascular lesion, inherited, drugs (eg phenytoin), idiopathic (50%)

·        Nephrogenic DI:

o   Reduced response by kidney to ADH

o   Causes of nephrogenic DI: ¯K, ­Ca, drugs (lithium), pyelonephritis, congenital, loss of medullary hypertonicity (eg renal interstitial disease)

·        Tests:

o   U&E, Ca, plasma and urine osmolalities

o   Water deprivation test. Stop drinking then measure urine for 8 hours. If osmolality > 800 mosmol/kg then DI excluded. If diuresis continues, give nasal desmopresson and continue measuring