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
what is the outcome of the water deprivation test in a patient with DI ?
plasma osmolality is high
urine osmolality is low
there is a failure to concentrate urine
after performing the water deprivation test what is the next best step in management ?
after the exclusion of primary polydipsia
give desmopressin subcutaneously
with central DI : the urine will concentrate and osmolality will rise by at least 50%
nephrogenic DI : no change in urine concentration
what is the treatment of central DI and nephrogenic DI ?
central DI : desmopressin ( nasal spray or tablets )
chlorpropamide for partial deficiency of ADH
nephrogenic DI : stop any offending drugs
correct electrolyte disturbance
thiazide diuretics plus a low solute diet
what is used for lithium toxicity in nephrogenic DI ?
amiloride
what is the difference between diabetes insipidus and SIADH ?
too much ADH in SIADH
not enough ADH in DI
what is the electrolyte abnormality in SIADH ?
euvolemic hyponatremia
what are the causes of SIADH ?
CNS disturbances
small cell carcinoma of the lung
chlopropamide
what are the effects of acute hyponatremia on the CNS ?
hyponatremic encephalopathy due to cerebral overhydration
what are the manifestations of chronic hyponatremia ?
asymptomatic
what are the consequences of rapid correction of hyponatremia ?
central pontine myelinolysis
osmotic demyelination syndrome
what is the treatment for SIADH ?
fluid restriction treat the underlying cause
ADH antagonist
IV saline
loop diuretics - furosemide
