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Syndrome of Inappropriate Antidiuretic Hormone Secretion

Excessive release of antidiuretic hormone (ADH) disturbs fluid and electrolyte balance in syndrome of inappropriate antidiuretic hormone secretion (SIADH). The excessive ADH causes an inability to excrete dilute urine, retention of free water, expansion of extracellular fluid volume, and hyponatremia.

SIADH occurs secondary to diseases that affect the osmoreceptors (supraoptic nucleus) of the hypothalamus. The prognosis depends on the underlying disorder and response to treatment.

Causes

SIADH tends to occur in people with heart failure or people with a diseased hypothalamus (the part of the brain that works directly with the pituitary gland to produce hormones). In other cases, a certain cancer (elsewhere in the body) may produce the antidiuretic hormone, especially certain lung cancers. Other causes may include the following:

• meningitis - inflammation of the meninges, the membranes that cover the brain and spinal cord.
• encephalitis - inflammation of the brain.
• brain tumors
• psychosis
• lung diseases
• head trauma
• Guillain-Barré syndrome (GBS) - a reversible condition that affects the nerves in the body. GBS can result in muscle weakness, pain, and even temporary paralysis of the facial, chest, and leg muscles. Paralysis of the chest muscles can lead to breathing problems.
• certain medications
• damage to the hypothalamus or pituitary gland during surgery

Signs and symptoms

Diagnosis

A complete medical history revealing positive water balance may suggest SIADH. Serum osmolality less than 280 mOsm/kg of water and low serum sodium confirm it. Urine osmolality is greater than plasma osmolality.

Supportive laboratory values include high urine sodium secretion (more than 20 mEq/L) without diuretics. In addition, diagnostic studies show normal renal function and no evidence of dehydration.

Treatment

Symptomatic treatment begins with restricted water intake (500 to 1,000 ml/day). With severe water intoxication, administration of 200 to 300 ml of 3% saline solution may be necessary to raise the serum sodium level.

When possible, treatment should include correction of the underlying cause of SIADH. If SIADH results from cancer, success in alleviating water retention may be obtained by surgical resection, irradiation, or chemotherapy.

If fluid restriction is ineffective, demeclocycline may be helpful by blocking the renal response to ADH.

Prevention

Prompt treatment of causative conditions may be helpful.