Diabetes insipidus

Question 1

Define

Answer 1

Passage of large volumes (>3L/day) of dilute urine due to impaired water resorption by the kidney, because of reduced ADH secretion from the posterior pituitary (cranial DI) or impaired response of the kidney to ADH (nephrogenic DI)

1) Central DI – no vasopressin
2) Nephrogenic DI – vasopressin present, but tissue

insensitivity

Associated with polyuria and polydipsia

Question 2

Causes

Answer 2

Cranial DI

• Idiopathic (50%)
• Congenital defect in ADH gene
• Tumour: pituitary tumour, craniopharyngiomas
• Trauma: temporary if distal to pituitary stalk as proximal

nerve endings grow out to find capillaries in scar tissue and

begin direct secretion again

• AI hypophysitis
• Infiltration: sarcoidosis
• Vascular: haemorrhage
• Infections: meningoencephalitis

Nephrogenic DI

• Inherited
• Metabolic: low K+, ↑Ca2+
• Drugs: lithium, demeclocycline
• Chronic renal disease
• Post-obstructive uropathy
• Pregnancy

Question 3

Epidemiology

Answer 3

medium age of onset is 24 years

Question 4

Symptoms

Answer 4

• Polyuria
• Nocturia
• Polydipsia
• Dehydration
• Hypernatremia symptoms

Question 5

Signs

Answer 5

Few signs if pt drinks adequate fluids Urine output is often >3L/24h

If fluid intake

Question 6

Investigations

Answer 6

Bloods

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

Water Deprivation Test

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

Results

• Normal - water restriction causes:
• Increased plasma osmolality
• Increased ADH secretion
• Increased water reabsorption
• Increase in urine osmolality (urine > 600 mosmol/kg)

Diabetes Insipidus

• Lack of ADH activity means that urine CANNOT be concentrated
• Urine osmolality is LOW (< 400 mosmol/kg)
• Cranial - urine osmolality rises > 50% following administration of desmopressin
• Nephrogenic - urine osmolality rises by < 45% following administration of desmopressin

Question 7

Management

Answer 7

Treat the CAUSE

Cranial DI

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

Nephrogenic DI

• Sodium and/or protein restriction helps with polyuria
• Thiazide diuretics

Question 8

Complications

Answer 8

Hypernatraemic dehydration

Excess desmopressin –> hyponatraemia

Question 9

Prognosis

Answer 9

Depends on CAUSE

Cranial DI may be transient following head trauma

It may be cured by removing the cause (e.g. drug discontinuation, tumour resection)