Diabetes Insipidus

Question 1

What is diabetes insipidus?

Answer 1

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

Question 2

What is central diabetes insipidus? List 6 causes

Answer 2

failure of ADH secretion by the posterior pituitary
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 3

What is nephrogenic diabetes insipidus? List 6 causes

Answer 3

insensitivity of the collecting duct to ADH
Idiopathic
Drugs (e.g. lithium)
Post-obstructive uropathy
Pyelonephritis
Pregnancy
Osmotic diuresis (e.g. diabetes mellitus)

Question 4

Describe the epidemiology of diabetes inspidius

Answer 4

Median onset is 24 yrs

Depends on cause

Question 5

List 5 symptoms of diabetes insipidus

Answer 5

Polyuria 
Nocturia 
Polydipsia 
In children: Enuresis (bed-wetting) + Sleep disturbance 
Other symptoms depend on aetiology

Question 6

Describe signs of diabetes insipidus

Answer 6

Cranial: few signs if pt 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 7

What bloods should be taken in diabetes insipidus? Describe the urine and plasma osmolality

Answer 7

U+Es
Ca2+
Increased plasma osmolality
Decreased urine osmolality

Question 8

What special test should be performed if diabetes insipidus is suspected ?

Answer 8

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

Question 9

Describe the results of a water deprivation test in a normal patient and a patient diabetes insipidus

Answer 9

Normal:
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 CANT be concentrated. Urine osmolality is LOW (< 400 mosmol/kg)

Question 10

How is cranial diabetes insipidus treated?

Answer 10

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

Question 11

How is nephrogenic diabetes insipidus treated?

Answer 11

Sodium +/or protein restriction helps with polyuria

Thiazide diuretics

Question 12

What complications can arise from diabetes insipidus?

Answer 12

Hypernatraemic dehydration

Excess desmopressin –> hyponatraemia

Question 13

Describe the prognosis in diabetes insipidus

Answer 13

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

Question 14

How do cranial and nephrogenic diabetes inspidius differ in the water deprivation test?

Answer 14

Cranial: urine osmolality rises > 50% following administration of desmopressin
Nephrogenic: urine osmolality rises by < 45% following administration of desmopressin

Question 15

What is the aetiology of diabetes inspidius?

Answer 15

Water channels fail to activate + the luminal membrane of the CD remains impermeable to water
Results in large volumes of hypotonic urine + polydipsia