Diabetes Insipidus Flashcards
Define Diabetes Insipidus
Characterised by either a decrease secretion of ADH from the pituitary (cranial cause) or an insensitivity to ADH (nephrogenic cause)
The consequence of inadequate ADH secretion or response results in an inability to concentrate urine
Name the two classifications of diabetes insipidus
Cranial diabetes insipidus
Nephrogenic diabetes insipidus
Define Cranial diabetes insipidus
Caused by reduced secretion of ADH
Define Nephrogenic diabetes insipidus
Caused by insensitivity of the collecting ducts of the kidneys to ADH
Where is ADH synthesised
In the hypothalamus
What releases ADH
Released by the posterior pituitary
What triggers the release of ADH
Triggered by a rise in plasma osmolality
What is the function of ADH
Action is to reabsorb water in the kidneys
What is the most common cause of central diabetes insipidus
Idiopathic i.e. cause unknown
Give examples of causes of central diabetes insipidus
Idiopathic (most common)
Secondary to tumours, hypopituitarism or following surgery
Give examples of causes of nephrogenic diabetes insipidus
Inherited defect in the gene for the ADH receptor on the X-chromosome
Acquired causes e.g. chronic lithium use and hypercalcaemia
What are the two key clinical features of diabetes insipidus
Polyuria – increase in urination
Polydipsia – thirst
Plasma osmolality is ____ in diabetes insipidus
a) High
b) Low
a) High
Urine osmolality is ____ in diabetes insipidus
a) High
b) Low
b) Low
What investigation is used to confirm a diagnosis of diabetes insipidus
Water deprivation test
How long are patients deprived of water for the water deprivation test
Patients are deprived of fluids for 8 hours
Cranial Diabetes Insipidus (in water deprivation test):
After deprivation = ____ urine osmolality?
After deprivation = Low urine osmolality
Cranial Diabetes Insipidus (in water deprivation test):
After ADH = ____ urine osmolality?
After ADH = High urine osmolality
After water deprivation the urine osmolality will be low in diabetes insipidus.
Why?
Unable to concentrate the urine (as reduced release or unresponsive to ADH) thus the urine continues to be diluted by excess water secretion in the kidneys
After ADH, urine osmolality will be high in patients with cranial diabetes insipidus in water deprivation test.
Why is this?
When synthetic ADH is given the kidneys respond by reabsorbing water and concentrating the urine so the urine osmolality will be high
Nephrogenic Diabetes Insipidus (in water deprivation test):
After deprivation = ____ urine osmolality?
After deprivation = Low urine osmolality
Nephrogenic Diabetes Insipidus (in water deprivation test):
After ADH = ____ urine osmolality?
After ADH = Low urine osmolality
After ADH, urine osmolality will be low in patients with nephrogenic diabetes insipidus in water deprivation test.
Why is this?
The issue is with the kidneys being unresponsive and thus when synthetic ADH is given the kidneys do not respond and therefore does not reabsorb the water
What would be the water deprivation test results for central diabetes insipidus
What would be the water deprivation test results for nephrogenic diabetes insipidus
What would be the water deprivation test results for primary polydipsia
Define primary polydipsia
Patient has normally functioning ADH system but they drink excess quantities of water leading to excess urine production
What does a high urine osmolality after 8 hours of water deprivation indicate
No diabetes insipidus
How is cranial diabetes insipidus managed
Desmopressin – synthetic ADH. As it is caused by reduced
Sodium should be monitored routinely due to the risk of hyponatraemia.
How is nephrogenic diabetes insipidus managed
Correcting any metabolic abnormality and stopping any offending drugs
Thiazides diuretic
