Diabetes Insipidus (DI) is defined as the passage of large volumes (>3 litres in 24 hours) of dilute urine (< 300 mOsm/kg). It can arise as a result of:

  • Reduced pituitary/hypophyseal secretion of antidiuretic hormone ADH (Cranial Diabetes Insipidus). This occurs infrequently following traumatic brain injury.
  • Cranial Diabetes Insipidus can also occur in association with tumours in the suprasellar/intrasellar region (adjacent to the pituitary) or following intracranial vascular injury or inflammatory cerebral lesions.
  • Increased renal resistance to the action of ADH in the kidney (Nephrogenic Diabetes Insipidus). This can be caused by renal disorders or in response to specific medications.

Note: They can both lead to hypernatraemia.

Differential Diagnosis: Hypernatraemia can also be caused by:

  • Inadequate water intake.
  • Excessive water loss (e.g. diabetes mellitus, diuretic medication, renal disease).
  • Excessive extra-renal water loss (e.g. extreme sweating, diarrhoea, vomiting, intestinal fistulas, significant burns).
  • Excess solute intake/infusion.
  • Excessive sodium retention (hyperaldosteronism/Conn’s Syndrome).

Clinical Features of Hypernatraemia are non-specific and include:

  • Dehydration.
  • Confusion.
  • Lethargy.
  • Muscle jerks (myotonia).

NOTE: If untreated hypernatraemia can lead to seizures or coma.shutterstock_326807192

In addition Diabetes Insipidus is associated with the following specific clinical features:

  • Polyuria i.e. urine output of 3-20 litres per day.
  • Polydipsia (thirst).
  • Nocturia.

Diagnostic Investigations
The diagnosis of Diabetes Insipidus is based on measuring:

  • Increased 24-hour urine volume.
  • Reduced urine osmolarity.
  • Increased plasma osmolarity and plasma sodium.160261421-medical-history

The exclusion of Diabetes Mellitus as a cause of polyuria is also important in differential diagnosis.

To discriminate between cranial and nephrogenic Diabetes Insipidus measure:

  • Serum ADH.
  • Water deprivation test.

Treatment options for cranial Diabetes Insipidus include:

  • Increasing oral intake.
  • Intravenous hypoosmolar fluid.
  • Desmopressin(synthetic ADH) for cranial diabetes insipidus.

NOTE: You should seek senior medical advice or consult your employing health authority’s specific management protocols for investigation and management of this condition.