Diabetes insipidus

Question 1

Define diabetes insipidus

Answer 1

Disorder of inadequate secretion or of insensitivity to vasopressin (ADH) leading to hypotonic polyuria

Question 2

Aetiology of diabetes insipidus

1 + 2 + 1

Answer 2

Central DI = failure of ADH secretion by the posterior pituitary

Nephrogenic DI = insensitivity of collecting duct to ADH
Water channels fail to activate & luminal membrane of collecting duct remains impermeable to water

DI results in large volumes of hypotonic urine & polydipsia

Question 3

Causes of diabetes insipidus - central

6

Answer 3

Idiopathic
Tumours (e.g. pituitary tumour)
Infiltrative (e.g. sarcoidosis)
Infection (e.g. meningitis)
Vascular (e.g. aneurysms, Sheehan syndrome)
Trauma (e.g. head injury, neurosurgery)

Question 4

Causes of diabetes insipidus - nephrogenic

6

Answer 4

Idiopathic
Drugs (e.g. lithium)
Post obstructive uropathy
Pyelonephritis
Pregnancy
Osmotic diuresis (e.g. diabetes mellitus)

Question 5

Epidemiology of diabetes insipidus

age, general

Answer 5

Median onset 24 yrs

Depends on cause

Question 6

Presenting symptoms of diabetes insipidus

5

Answer 6

Polyuria
Nocturia
Polydipsia
In children - enuresis (bed wetting) & sleep disturbance
Other symptoms depend on aetiology

Question 7

Signs of diabetes insipidus on physical examination

4

Answer 7

Central DI has few signs if patient drinks sufficiently to maintain adequate fluid levels
Urine output >3 L/day
If fluid intake < output, sign of dehydration will be present (e.g. tachycardia, reduced tissue turgor, postural hypotension, dry mucous membranes)
Signs related to cause (e.g. visual defect due to pituitary tumour)

Question 8

Investigations for diabetes insipidus

2 groups

Answer 8

Bloods

Water deprivation test

Question 9

Investigations for diabetes insipidus - bloods

3

Answer 9

U&Es & Ca2+
Increased plasma osmolality
Decreased urine osmolality

Question 10

Investigations for diabetes insipidus - water deprivation test
(4 + results)

Answer 10

Water restricted for 8hrs
Plasma & urine osmolality are measured every hour for 8 hrs
Weigh patient hourly to monitor level of dehydration
STOP test if fall in body weight is > 3%
Desmopressin given after 8 hrs & urine osmolality measured

Normal result 
Increased plasma osmolality 
Increased ADH secretion
Increased water reabsorption
Increase in urine osmolality (urine >600 mosmol/kg)

DI
Lack of ADH activity means urine CANNOT be concentrated
Urine osmolality is LOW (<400 mosmol/kg)
Central - urine osmolality rises > 50% after desmopressin administration
Nephrogenic - urine osmolality rises < 45% after desmopressin

Question 11

Management of diabetes insipidus

general + 2 groups

Answer 11

Treat CAUSE

Central DI
Nephrogenic DI

Question 12

Management of diabetes insipidus - central

2

Answer 12

Give desmopressin (vasopressin analogue)
If mild - chlorpropamide or carbamazepine can be used to potentiate the residual effects of any residual vasopressin

Question 13

Management of diabetes insipidus - nephrogenic

2

Answer 13

Sodium &/or protein restriction helps polyuria

Thiazide diuretics

Question 14

Complications of diabetes insipidus

2

Answer 14

Hypernatraemic dehydration

Excess desmopressin —> hyponatraemia

Question 15

Prognosis of diabetes insipidus

3

Answer 15

Depends on cause
Cranial DI may be transient following head trauma
May be cured by removing cause (e.g. drug removal, tumour resection)