Diabetes Insipidus

Question 1

What happens in diabetes insipidus?

Answer 1

Inadequate secretion of, or insensitivity to. vasopressin (ADH) resulting in hypotonic polyuria

Question 2

2 types of diabetes insipidus?

Answer 2

Cranial/central

Nephrogenic

Question 3

Mechanism of cranial/central diabetes insipidus and causes?

Answer 3

Posterior pituitary fails to secrete ADH

- pituitary tumour, infection (meningitis), sarcoidosis

Question 4

Mechanism of nephrogenic diabetes insipidus and causes?

Answer 4

Collecting ducts become insensitive to ADH

• hypercalcaemia
• hypokalaemia
• inherited (AVPV2 gene)
• idiopathic

Question 5

Presenting symptoms of diabetes insipidus?

Answer 5

Polyuria (urine output often >3L)
Polydipsia
Symptoms of hypernatraemia - lethargy, irritability, confusion

Question 6

Investigations in diabetes insipidus and which is diagnostic for it?

Answer 6

Bloods - U&Es (Ca, Na, K), glucose (to exclude DM)

Water deprivation test - diagnostic

Question 7

Process of water deprivation test?

Answer 7

• water restricted for 8 hours
• plasma + urine osmolality measured every hour
• after 8 hours, give desmopressin (synthetic ADH) and measure urine osmolality

Question 8

Results of water deprivation test in normal patient and why?

Answer 8

Water restriction causes plasma and urine osmolality to increase >600 (concentrates due to normal action/secretion of ADH in response to lack of water)
- thus adequate response so no desmopressin required

Question 9

Results of water deprivation test in cranial/central diabetes insipidus?

Answer 9

Water restriction - urine unable to concentrate, urine osm <400

Desmopressin administration - urine osm rises by >50%

• results due to a lack of ADH so replacing it results in normal osmolality maintenance

Question 10

Results of water deprivation test in nephrogenic diabetes insipidus?

Answer 10

Water restriction - urine unable to concentrate, urine osm <400

Desmopressin administration - urine osm rises by <45%

• results due to insensitivity to ADH so desmopressin has little/no effect

Question 11

Management of diabetes insipidus (cranial + nephrogenic)?

Answer 11

Treat the cause - if due to electrolytes or cancer

Cranial - intranasal desmopressin

Nephrogenic - constant access to water, thiazide diuretics or NSAIDs

Question 12

How do thiazide diuretics and NSAIDs treat nephrogenic diabetes insipidus?

Answer 12

Thiazide diuretics - creates mild hypovolaemia encouraging increased fluid and salt uptake in PCT

NSAIDs - reduce urine output and dehydration