Diabetes insipidus

Question 1

What is diabetes insipidus?

Answer 1

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

Question 2

Explain the aetiology/risk factors of diabetes insipidus

Answer 2

Central DI: failure of ADH secretion by the
posterior pituitary

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

Causes
Central
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)

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

Question 3

Summarise the epidemiology of diabetes insipidus

Answer 3

Median onset is 24 yrs

Depends on cause

Question 4

Recognise the presenting symptoms of diabetes insipidus

Answer 4

Polyuria
Nocturia
Polydipsia

In children:
Enuresis (bed-wetting)
Sleep disturbance

Other symptoms depend on aetiology

Question 5

Recognise the signs of diabetes insipidus on physical examination

Answer 5

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 6

Identify appropriate investigations for diabetes insipidus

Answer 6

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

Water deprivation test
Psychogenic polydipsia - Urine Osmolality rises straight away

Central DI - Urine Osmolality rise (>50%) after Desmopressin

Nephrogenic DI - Urine Osmolality does not rise (<45%) after Desmopressin

Question 7

Generate a management plan for diabetes insipidus

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 diuretic

Question 8

Identify possible complications of diabetes insipidus

Answer 8

Hypernatraemic dehydration

Excess desmopressin –> hyponatraemia

Question 9

Summarise the prognosis for patients with diabetes insipidus

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)