The syndrome of inappropriate antidiuretic hormone (SIADH) is a condition commonly found in the hospital population, especially in patients being hospitalized for central nervous system (CNS) injury. As the name states, this is a syndrome characterized by excessive release of antidiuretic hormone (ADH or vasopressin) from the posterior pituitary gland. The result is hyponatremia, and sometimes fluid overload.

Pathophysiology

The normal function of ADH is to control the amount of water retained by the kidneys. ADH acts in the distal portion of the renal tubule and causes the retention of water, but not solute. Hence, ADH activity promotes the dilution of sodium in the blood. Furthermore, it activates the brain's thirst mechanism, causing the individual to drink.

Through an unknown mechanism, the plasma ADH level increases such that water is inappropriately retained by the kidneys. This retention leads to a dilutional hyponatremia and all the consequences associated with that condition: headache, nausea, vomiting, and confusion. Severe hyponatremia may cause convulsions or coma.

Causes

Some common causes of SIADH include:

• Head injury
• Cancers
  • Lung cancer (especially small cell lung cancer, as well as other small-cell malignancies of other organs)
• Infections
  • Brain abscess
  • Pneumonia
  • Lung abscess
• Drugs
  • Chlorpropamide
  • Cyclophosphamide
  • Carbamazepine

Treatment

Treatment of SIADH includes:

• Fluid restriction
• Intravenous isotonic saline
• Drugs
  • Demeclocycline
  • Conivaptan (a new agent)

Care must be taken when correcting hyponatremia. A rapid rise in the sodium level may cause central pontine myelinolysis.

Differential diagnosis

Cerebral salt wasting syndrome also presents with hyponatremia, but is treated differently.

Reference

• Ashrafian H, Davey P. A review of the causes of central pontine myelinosis: yet another apoptotic illness? Eur J Neurol 2001;8:103-9. PMID 11430268.