Diabetes Inspidus

Question 1

what is the equation for serum osmolality?

Answer 1

2 x ( Na + K) + urea + glucose

Question 2

Define diabetes insipidus?

Answer 2

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

Question 3

what are the risk factors for diabetes insipidus?

Answer 3

pituitary surgery

craniopharngioma

pituitary stalk lesions

traumatic brain injury

Question 4

outline the aetiology of central diabetes insipidus?

Answer 4

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 5

outline the aetiology of nephrogenic diabetes inspidus?

Answer 5

insensitivity of the collecting duct to ADH

Idiopathic

Drugs (e.g. lithium)

Post-obstructive uropathy

Pyelonephritis

Pregnancy

Osmotic diuresis (e.g. diabetes mellitus)

Question 6

summarise the epidemiology of diabetes insipidus?

Answer 6

Median onset is 24 yrs

Depends on cause

Question 7

what are the presenting symptoms of diabetes insipidus?

Answer 7

Polyuria

Nocturia

Polydipsia

In children:

• Enuresis (bed-wetting)
• Sleep disturbance

Other symptoms depend on aetiology

Question 8

what are the signs of diabetes insipidus on physical examination?

Answer 8

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 9

what are the appropriate investigations for diabetes insipidus?

Answer 9

U&Es – serum sodium (norm/high), calcium (norm/high), potassium (norm/low)

Increased plasma osmolality

Decreased urine osmolality

urine disptick- negative for glucose

Water deprivation test

Question 10

describe the water deprivation test?

Answer 10

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

Question 11

what are the results for the water deprivation test in a normal person?

Answer 11

Increased plasma osmolality

Increased ADH secretion

Increased water reabsorption

Increase in urine osmolality (urine > 600 mosmol/kg)

Question 12

what are the results for the water deprivation test in diabetes insipidus?

Answer 12

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 13

outline the management for diabetes insipidus?

Answer 13

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 or NSAIDs
• NSAIDs = inhibit prostaglandin synthase and prostaglandin inhibits ADH

Question 14

what are the possible complications of diabetes inspidus?

Answer 14

Hypernatraemic dehydration

Excess desmopressin –> hyponatraemia

Question 15

summarise the prognosis of patients with diabetes insipidus?

Answer 15

• 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)