Syndrome of inappropriate secretion of ADH (SIADH)

SIADH is defined as continued secretion of ADH despite a low serum osmolality. The diagonsis of SIADH can only be made when there is a normal state of hydration; normal renal, thyroid, and adrenal function; and when the patient is not taking diuretics. In all instances of excess ADH secretion, patients have abnormally low concentrations of sodium ions in their blood (a condition known as hyponatremia) and have urine that is less than maximally dilute. Possible causes of SIADH include ADH secretion from malignant systemic tumors (for example, small cell lung carcinoma, lymphoma, and pancreatic tumors) or chronic obstructive pulmonary disease. SIADH can also be induced by drugs such as phenothiazine, tricyclic antidepressants, carbamazepine, and lithium. Central nervous system disorders can be associated with SIADH, perhaps through loss of chronic inhibition of the brain on the hypothalamic neurons. SIADH occurs after pituitary surgery in approximately 10% of patients. This disorder is more common in patients with Cushing’s disease after surgery.

Symptoms

Symptoms of SIADH depend on the onset, but can include confusion, stupor, coma, and seizures. Generally, symptoms do not develop in a normal individual until sodium levels fall below 125 mmol/L of serum.

Treatment

Treatment of SIADH commonly involves free water restriction of 600-800 ml per day, resulting in a gradual rise in sodium over five to ten days. Treatment with conivaptan may hasten the recovery.