Diabetes Insipidus

Question 1

Define Diabetes Insipidus

Answer 1

Metabolic disorder characterise by the inadequate secretion of / insensitivity to vasopressin (ADH) leading to hypotonic polyuria

Question 2

Aetiology of Diabetes Insipidus

Answer 2

Cranial or Nephrogenic

Cranial/central: failure of ADH secretion by the posterior pituitary
Idiopathic | Pituitary tumour | Sarcoidosis | meningitis | aneurysms, Sheehan’s syndrome | Head injury | neurosurgery | SAH | drugs e.g. phenytoin

Nephrogenic: insensitivity of the collecting duct to ADH
Idiopathic | drugs e.g. lithium | Post-obstructive uropathy | Pyelonephritis | pregnancy | osmotic diuresis e.g. DM | Wolfram’s syndrome

Question 3

Symptoms of Diabetes Insipidus

Answer 3

Polyuria (ranges from 3L - >20L a day)
Polydipsia
Nocturia
Skin rashes (papular rashes, ulcers, erythema nodosum)

Question 4

Signs of Diabetes Insipidus on examination

Answer 4

Volume dehydration: dry mucous membranes, poor skin turgor, tachycardia, hypotension
Hypernatraemia: irritability, restlessness, lethargy, spasticity, hyper-reflexia
Visual field defect (pituitary mass)
Focal motor deficits (intracranial pathology)
Sensorineural deafness (Wolfram’s syndrome)
Skin lesions e.g. papular rashes or ulcers
Erythema nodosum (sarcoidosis)

Question 5

Investigations for Diabetes Insipidus

Answer 5

Urine osmolality + dipstick: Reduced osmolality (<300) | dipstick -ve for glycosuria (DM), may be proteinuria (renal disease)
24h urine collection: >3L per 24 hours
Water deprivation test: Urine osmolality <300 + serum osmolality >290 (patient weighed to monitor dehydration - STOP test if weight >3% reduced)

Desmopressin stimulation test:
Central - reduction in urine output + increase urine osmolality
Nephrogenic - little/no reduction in urine output + increase in urine osmolality

Glucose: Normal (exclude DM)
U+Es: Sodium normal/elevated | potassium normal or low (low in nephrogenic) | Calcium normal or elevated (high in nephrogenic)

Question 6

Management for Diabetes Insipidus

Answer 6

Initial
Hypernatraemia => Oral/IV fluids (hypotonic)

Central
Acute => Desmopressin PO + oral/IV fluids (5% dextrose, 0.45% NaCl)
Chronic => Desmopressin

Nephrogenic

1. Maintain adequate fluid intake
2. Treat underlying cause
3. Consider hydrochlorothiazide orally once

Question 7

Complications of Diabetes Insipidus

Answer 7

Hypernatraemia
thrombosis
Bladder and renal dysfunction
Iatrogenic hyponatraemia

Question 8

Prognosis of Diabetes Insipidus

Answer 8

Cranial - after surgery will be transient, controlled on long acting desmopressin
Nephrogenic - commonly resolves on correction