Diabetes Insipidus

Question 1

Definition

Answer 1

· A disorder of inadequate secretion or of insensitivity to vasopressin (ADH) leading to hypotonic polyuria

Question 2

Aetiology

Answer 2

· Central DI: failure of ADH secretion by the posterior pituitary

· Nephrogenic DI: insensitivity of the collecting duct to ADH
o Water channels fail to activate and the luminal membrane of the collecting duct remains impermeable to water

· DI results in large volumes of hypotonic urine and polydipsia

Question 3

Causes (central)

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 (nephrogenic)

Answer 4

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

Question 5

Epidemiology

Answer 5

· Median onset is 24 yrs

· Depends on cause

Question 6

Presenting symptoms

Answer 6

· Polyuria
· Nocturia
· Polydipsia

· In children:
o Enuresis (bed-wetting)
o Sleep disturbance

· Other symptoms depend on aetiology

Question 7

Signs on physical examination

Answer 7

· Central DI has few signs if the patient drinks sufficiently to maintain adequate fluid levels

· Urine output > 3 L/day

· If fluid intake < fluid output, signs of dehydration will be present (e.g. tachycardia, reduced tissue turgor, postural hypotension, dry mucous membranes)

· Signs related to the cause (e.g. visual defect due to pituitary tumour)

Question 8

Investigations (bloods)

Answer 8

o U&Es and Ca2+

o Increased plasma osmolality

o Decreased urine osmolality

Question 9

Investigations (water deprivation test)

Answer 9

o Water is restricted for 8 hrs

o Plasma and urine osmolality are measured every hour for 8 hrs

o Weight the patient hourly to monitor level of dehydration

o STOP the test if the fall in body weight is > 3%

o Desmopressin is given after 8 hrs and urine osmolality is measured

Question 10

Water deprivation test results

Answer 10

· 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 11

Management plan

Answer 11

· Treat the CAUSE

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

· Nephrogenic DI
o Sodium and/or protein restriction helps with polyuria
o Thiazide diuretics

Question 12

Possible complications

Answer 12

· Hypernatraemic dehydration

· Excess desmopressin –> hyponatraemia

Question 13

Prognosis

Answer 13

· 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)