Diabetes Insipidus

Question 1

Define Diabetes Insipidus

Answer 1

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

Question 2

Name the two classifications of diabetes insipidus

Answer 2

Cranial diabetes insipidus

Nephrogenic diabetes insipidus

Question 3

Define Cranial diabetes insipidus

Answer 3

Caused by reduced secretion of ADH

Question 4

Define Nephrogenic diabetes insipidus

Answer 4

Caused by insensitivity of the collecting ducts of the kidneys to ADH

Question 5

Where is ADH synthesised

Answer 5

In the hypothalamus

Question 6

What releases ADH

Answer 6

Released by the posterior pituitary

Question 7

What triggers the release of ADH

Answer 7

Triggered by a rise in plasma osmolality

Question 8

What is the function of ADH

Answer 8

Action is to reabsorb water in the kidneys

Question 9

What is the most common cause of central diabetes insipidus

Answer 9

Idiopathic i.e. cause unknown

Question 10

Give examples of causes of central diabetes insipidus

Answer 10

Idiopathic (most common)

Secondary to tumours, hypopituitarism or following surgery

Question 11

Give examples of causes of nephrogenic diabetes insipidus

Answer 11

Inherited defect in the gene for the ADH receptor on the X-chromosome

Acquired causes e.g. chronic lithium use and hypercalcaemia

Question 12

What are the two key clinical features of diabetes insipidus

Answer 12

Polyuria – increase in urination

Polydipsia – thirst

Question 13

Plasma osmolality is ____ in diabetes insipidus

a) High
b) Low

Answer 13

a) High

Question 14

Urine osmolality is ____ in diabetes insipidus

a) High
b) Low

Answer 14

b) Low

Question 15

What investigation is used to confirm a diagnosis of diabetes insipidus

Answer 15

Water deprivation test

Question 16

How long are patients deprived of water for the water deprivation test

Answer 16

Patients are deprived of fluids for 8 hours

Question 17

Cranial Diabetes Insipidus (in water deprivation test):

After deprivation = ____ urine osmolality?

Answer 17

After deprivation = Low urine osmolality

Question 18

Cranial Diabetes Insipidus (in water deprivation test):

After ADH = ____ urine osmolality?

Answer 18

After ADH = High urine osmolality

Question 19

After water deprivation the urine osmolality will be low in diabetes insipidus.

Why?

Answer 19

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

Question 20

After ADH, urine osmolality will be high in patients with cranial diabetes insipidus in water deprivation test.

Why is this?

Answer 20

When synthetic ADH is given the kidneys respond by reabsorbing water and concentrating the urine so the urine osmolality will be high

Question 21

Nephrogenic Diabetes Insipidus (in water deprivation test):

After deprivation = ____ urine osmolality?

Answer 21

After deprivation = Low urine osmolality

Question 22

Nephrogenic Diabetes Insipidus (in water deprivation test):

After ADH = ____ urine osmolality?

Answer 22

After ADH = Low urine osmolality

Question 23

After ADH, urine osmolality will be low in patients with nephrogenic diabetes insipidus in water deprivation test.

Why is this?

Answer 23

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

Question 24

What would be the water deprivation test results for central diabetes insipidus

Answer 24

Question 25

What would be the water deprivation test results for nephrogenic diabetes insipidus

Answer 25

Question 26

What would be the water deprivation test results for primary polydipsia

Answer 26

Question 27

Define primary polydipsia

Answer 27

Patient has normally functioning ADH system but they drink excess quantities of water leading to excess urine production

Question 28

What does a high urine osmolality after 8 hours of water deprivation indicate

Answer 28

No diabetes insipidus

Question 29

How is cranial diabetes insipidus managed

Answer 29

Desmopressin – synthetic ADH. As it is caused by reduced

Sodium should be monitored routinely due to the risk of hyponatraemia.

Question 30

How is nephrogenic diabetes insipidus managed

Answer 30

Correcting any metabolic abnormality and stopping any offending drugs

Thiazides diuretic