Diabetes insipidus

Question 1

Define diabetes insipidus

Answer 1

Diabetes insipidus is a lack of antidiuretic hormone (ADH) or a lack of response to ADH.

Question 2

Is primary polydipsia a form of diabetes insipidus?

Answer 2

No, this is when a person is drinking excessive amounts of water leading to polydipsia, but they have normally functioning ADH.

Question 3

How can you classify diabetes insipidus?

Answer 3

Into nephrogenic or central/cranial

Question 4

What is Nephrogenic Diabetes Insipidus

Answer 4

Nephrogenic diabetes insipidus is when the collecting ducts of the kidneys do not respond to ADH

Question 5

What causes Nephrogenic Diabetes Insipidus

Answer 5

Drugs, particularly lithium used in bipolar affective disorder
Mutations in the AVPR2 gene on the X chromosome that codes for the ADH receptor
Intrinsic kidney disease
Electrolyte disturbance (hypokalaemia and hypercalcaemia)

Question 6

What is Central Diabetes Insipidus

Answer 6

Cranial diabetes insipidus is when the hypothalamus does not produce ADH for the pituitary gland to secrete

Question 7

What causes Central Diabetes Insipidus

Answer 7

Idiopathic
Brain tumours
Head injury
Brain malformations
Brain infections (meningitis, encephalitis and tuberculosis)
Brain surgery or radiotherapy

Question 8

How does Diabetes Insipidus present?

Answer 8

Polyuria 
Polydipsia 
Dehydration
Postural hypotension
Hypernatraemia

Question 9

What investigations can be done if diabetes insipidus is suspected?

Answer 9

Low urine osmolality
High serum osmolality
Water deprivation test

Question 10

Briefly describe the diagnosing test for diabetes insipidus

Answer 10

Water deprivation test:
Initially the patient should avoid taking in any fluids for 8 hours. This is referred to as fluid deprivation. Then, urine osmolality is measured and synthetic ADH (desmopressin) is administered. 8 hours later urine osmolality is measured again.

Question 11

What do these results of the water deprivation test indicate?
After deprivation: Low
After ADH: High

Answer 11

Cranial diabetes insipidus

Question 12

What do these results of the water deprivation test indicate?
After deprivation: Low
After ADH: Low

Answer 12

Nephrogenic diabetes insipidus

Question 13

What do these results of the water deprivation test indicate?
After deprivation: High
After ADH: High

Answer 13

Primary polydipsia

Question 14

Explain the results of the water deprivation test in central diabetes insipidus

Answer 14

In cranial diabetes insipidus the patient lacks ADH. The kidneys are still capable of responding to ADH. Therefore initially the urine osmolality remains low as it continues to be diluted by excessive water secretion in the kidneys. Then when synthetic ADH is given the kidneys respond by reabsorbing water and concentrating the urine so the urine osmolality will be high.

Question 15

Explain the results of the water deprivation test in nephrogenic diabetes insipidus

Answer 15

In nephrogenic diabetes insipidus the patient is unable to respond to ADH. They are diluting their urine with the excessive water secretion by the kidneys. Therefore the urine osmolality will be low initially and remain low even after the synthetic ADH is given.

Question 16

Explain the results of the water deprivation test in primary polydipsia

Answer 16

In primary polydipsia the 8 hours of water deprivation will cause the urine osmolality to be high even before the synthetic ADH is given. A high urine osmolality after 8 hours of water deprivation indicates no diabetes insipidus.

Question 17

Briefly describe how diabetes insipidus is managed

Answer 17

If possible, treat underlying cause. Mild cases can be managed conservatively without any intervention.

Desmopressin (synthetic ADH) can be used in:
Cranial diabetes insipidus to replace ADH
Nephrogenic diabetes insipidus in higher doses under close monitoring