diabetes insipidus and SIADH Flashcards
where is ADH synthetized ?
in the hypothalamus and secreted by the posterior pituitary gland
main mechanism of action of ADH ?
increases water reabsorption through the collecting duct in the nephron
act on V1 and V2 receptors
V1: control water excretion
V2: responsible for vasoconstriction
what are the triggers for the release of ADH ?
hyperosmolar state ( increased sodium concentration )
depletion of circulating volume
what is diabetes insipidus ?
disorder where there is an excretion of a large volume of dilute urine due to decreased water reabsorption
when is it called polyuria ?
24 hour urine output more than 30 to 50 ml/kg
what are the types of diabetes insipidus ?
central DI - complete or partial deficiency of ADH secretion from the posterior pituitary gland
nephrogenic DI - caused by end-organ unresponsiveness of the kidney to ADH
what are the different causes of central DI ?
1- post-surgical ( develops 1-6 days after surgery and often disappears, recurs or becomes chronic )
2- head trauma
3- tumors
what are the causes of nephrogenic DI ?
congenital or acquired
what drug is known to cause nephrogenic DI ?
lithium
what is the clinical presentation of DI ?
large volumes of urine exceeding 3 litres a day
urinary frequency and nocturia
excessive thirst
craving for cold liquids
what test is used to test for diabetes insipidus ?
water deprivation test
what is the purpose of the water deprivation test ?
differentiate between primary polydipsia
central polydipsia
nephrogenic polydipsia
how is the water deprivation test done ?
1- water intake of the patient is restricted after 10 pm
2- body weight, plasma osmolality and serum sodium, urine volume and urine osmolality
what are the indications to stop the water deprivation test ?
if the body weight decreases by 3%
what is the outcome of the water deprivation test in a healthy person ?
in a healthy individual : with dehydration , urine osmolality should increase due to increased ADH secretion
and so DI can be excluded
