Endocrinology - Diabetes insipidus

Question 1

What is diabetes insipidus?

Answer 1

Lack of ADH or lack of response to ADH

This prevents the kidneys from being able to concentrate the urine leading to polyuria and polydipsia

Question 2

Classes of diabetes insipidus

Answer 2

Nephrogenic

Central

Primary polydipsia - normal ADH system but patient is drinking excessive quantities of water leading to polyuria

Question 3

What is nephrogenic DI?

Answer 3

When the collecting ducts of the kidneys don’t respond to ADH

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 is central DI?

Answer 4

When the hypothalamus doesn’t produce enough ADH for the pituitary to secrete

Can be idiopathic or caused by:

• Brain tumours
• Head injury
• Brain malformations
• Brain infections
• Brain surgery/radiotherapy

Question 5

Presentation of diabetes insipidus

Answer 5

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

Question 6

Investigations to do in suspected DI

Answer 6

Urine osmolality - low
Serum osmolality - high
Water deprivation test

Question 7

What is the water deprivation test?

Answer 7

Diagnostic test for DI

1. Avoid fluids for 8h
2. Then, urine osmolality is measured and synthetic ADH (desmopressin) is administered.
3. 8 hours later urine osmolality is measured again.

Question 8

What are the meaning of the results of water deprivation test?

Answer 8

Cranial DI -

• Urine osmolality is low after fluid deprivation
• Urine osmolality is high after the desmopressin
• As the kidneys still respond to ADH and reabsorb water, concentrating the urine

Nephrogenic DI:

• Urine osmolality is low after fluid deprivation
• Urine osmolality is still low after the desmopressin
• Kidneys don’t respond to ADH and don’t reabsorb water, so urine remains dilute

In primary polydipsia the urine osmolality will be high both after fluid deprivation and after desmopressin

Question 9

Management of DI

Answer 9

Treat the underlying cause

Desmopressin can be used in cranial DI and in nephrogenic DI (in higher doses under close monitoring)