Endocrinology: DI

Question 1

What are the features of diabetes insipidus?

Answer 1

Impaired ADH secretion (CRANIAL) OR impaired response to ADH (NEPHROGENIC)

ADH = insertion of aquaporin 2 channels for water reabsorption

Na of 150-170 mmol/L usually indicate vol depletion

Na >170 mmol/L are usually associated with DI (nephrogenic or cranial)

Na >190 mmol/L are usually a result of exogenous Na gain

Question 2

What are the 2 types of DI?

Answer 2

1) cranial DI = not enough ADH

2) nephrogenic DI = kidneys don’t respond to ADH

Question 3

What are the signs and symptoms of DI?

Answer 3

Excessive (5-20L) dilute urine

Polydipsia

Dehydration

Hypernatraemia = thirst, weakness, confusion, coma

Question 4

What are the causes of cranial DI?

Answer 4

Idiopathic - 50%

Congenital defect in ADH gene

Tumour = pituitary

Trauma

Vascular = haemorrhage

Infection

Question 5

What are the causes of nephrogenic DI?

Answer 5

Inherited

Drugs = lithium

Chronic renal disease

Post obstructive uropathy

Question 6

How should DI be investigated?

Answer 6

Bloods = glucose (rule out DM), U+Es, serum/urine osmolality

Water deprivation test = testing the ability of the kidneys to concentrate urine - do the kidneys produce dilute urine even when dehydrated

Diff between cranial and nephrogenic = desmopressin

• cranial = rise in urine osmolality >600
• nephrogenic = NO rise in urine osmolality

Question 7

How is serum osmolality calculated?

Answer 7

2Na + 2K + urea + glucose (all in mmol/L)

Question 8

How is cranial DI managed?

Answer 8

Find the cause - MRI head

Test anterior pituitary function

Give desmopressin (synthetic ADH analogue)

Question 9

How is nephrogenic DI managed?

Answer 9

Treat the causes

If it persists try bendroflumethazide and NSAIDs to lower urine vol and plasma Na