Diabetes Insipidus

Question 1

What is diabetes insipidus?

Answer 1

Diabetes insipidus is a lack of antidiuretic hormone (ADH) or a lack of response to ADH. This prevents the kidneys from being able to concentrate the urine leading to polyuria (excessive amounts of urine) and polydipsia (excessive thirst). It can be classified as nephrogenic or cranial.

Nephrogenic - kidneys don’t respond to ADH
Cranial - Hypothalamus doesn’t produce ADH

Question 2

What is primary polydypsia?

Answer 2

Primary polydipsia is when the patient has a normally functioning ADH system but they are drinking excessive quantities of water leading to excessive urine production. They don’t have diabetes insipidus.

Question 3

What can cause nephrogenic diabetes insipidus?

Answer 3

Nephrogenic diabetes insipidus is when the collecting ducts of the kidneys do not respond to ADH. It can also be caused by:

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 4

What can cause cranial diabetes insipidus?

Answer 4

Cranial diabetes insipidus is when the hypothalamus does not produce ADH for the pituitary gland to secrete. It can be idiopathic, without a clear cause or it can be caused by:

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

Question 5

How does diabetes insipidus present?

Answer 5

Polyuria (excessive urine production)
Polydipsia (excessive thirst)
Dehydration
Postural hypotension
Hypernatraemia

Question 6

Investigations for diabetes insipidus

Answer 6

Low urine osmolality
High serum osmolality
Water deprivation test

Question 7

What is the water deprivation test?

Answer 7

The water deprivation test is also known as the desmopressin stimulation test. This is the test of choice for diagnosing diabetes insipidus.

Question 8

ADH aka

Answer 8

Vasopressin

Question 9

Summary of the water deprivation test

Answer 9

Method

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.

Results

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.

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.

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 10

How is diabetes insipidus managed?

Answer 10

Desmopressin (synthetic ADH) can be used in:

Cranial diabetes insipidus to replace ADH

Nephrogenic diabetes insipidus in higher doses under close monitoring