Diabetes Insipidus

Question 1

What is the physiology of diabetes insipidus?

Answer 1

• Passage of large volumes of urine (>3L a day) due to impaired water resorption by the kidney
• Due to reduced ADH secretion from the posterior pituitary gland (cranial DI)
• Due to impaired response of the kidney to ADH (nephrogenic DI)

Question 2

What are the symptoms of diabetes insipidus?

Answer 2

• Polyuria
• Polydipsia
• Dehydration
• Symptoms of hypernatraemia

Question 3

What are potential causes of cranial DI?

Answer 3

• Idiopathic
• Congenital - Defects in ADH gene
• Tumour - Presents with hypopituitarism too
• Trauma - Temporary if distal to pituitary stalk as proximal nerve endings grow out to find capillaries in scar tissue and reconnect
• Hypophysectomy
• Autoimmune
• Infiltration - sarcoidosis, histiocytosis
• Vascular - Haemorrhage
• Infection - Meningoencephalitis

Question 4

What are potential causes of nephrogenic DI?

Answer 4

• Inherited
• Metabolic - Low potassium, high calcium
• Drugs - lithium, demeclocycline
• Chronic renal disease
• Post obstructive uropathy

Question 5

What investigations would you do?

Answer 5

1. Bloods
   - U+E
   - Calcium
   - Serum osmolality
2. Bedside
   - Glucose to exclude DM
   - Urine sample - osmolality

Question 6

What is serum osmolality actually measuring?

Answer 6

The concentration of ions in the plasma

It equals 2(Na+K)+urea+glucose in mmol/L`

Question 7

What is normal serum osmolality?

Answer 7

285-295mmol/L

Question 8

What urine:plasma osmolality ratio excludes DI?

Answer 8

Over 2:1, urine should be dilute.

This is only is plasma osmolality is normal (less than 295)

Question 9

What will osmolality results in blood and urine show in DI?

Answer 9

• Raised plasma osmolality
• Reduced urine osmolality
• U:P ratio <2

Question 10

What test is required for diagnosis?

How do you do it?

Answer 10

The 8hr water fast
Stage 1 -
- Stop drinking water for 8hrs and weigh hourly, collect urine every 2hr, measure volume and osmolality
- Measure serum osmolality every 4hr
Stage 2 -
- Differentiate cranial from nephrogenic DI by giving desmopressin

Question 11

What is a normal result of the 8hr water fast?

Answer 11

• Urine osmolality >600mOsm in stage 1
• U:P ratio >2
• Kidneys have ability to concentrate urine in dehydration

Question 12

What is the result of the 8hr water fast showing primary polydipsia?

Answer 12

• Urine concentrates but less than normal eg >400-600mOsm

- This is due to wash out of the normal concentration of the renal medulla

Question 13

What is the result of the 8hr water fast showing cranial DI?

Answer 13

• Urine osmolality increases to >600mOsm AFTER desmopressin given (synthetic ADH)

Question 14

What is the result of the 8hr water fast showing nephrogenic DI?

Answer 14

• No increase in urine osmolality after desmopressin

Question 15

What is the treatment for cranial DI?

Answer 15

• Head MRI
• Test anterior pituitary function
• Give desmopressin

Question 16

What is the treatment of nephrogenic DI?

Answer 16

• Treat the underlying cause
• Try bendroflumethiazide if it persists
• NSAID’s lower urine volume and plasma Na+ by inhibiting prostaglandin synthase, prostaglandins usually inhibit the action of ADH

Question 17

What is the emergency treatment for DI?

Answer 17

1. Do plasma U+E, serum and urine osmolalities
2. Monitor urine output
3. IVI to keep up with urine output
4. Desmopressin IM and reduce hypernatraemia slowly with 0.9% saline