Syndrome of inappropriate antidiuretic hormone
Syndrome of inappropriate antidiuretic hormone: Excerpt from Professional Guide to Diseases (Eighth Edition)
Syndrome of inappropriate antidiuretic hormone (SIADH), also known as dilutional hyponatremia, is marked by excessive release of antidiuretic hormone (ADH), which disturbs fluid and electrolyte balance. Such disturbances result from the inability to excrete dilute urine, free water retention, extracellular fluid volume expansion, 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
The most common cause of SIADH (80% of patients) is oat cell carcinoma of the lung, which secretes excessive ADH or vasopressor-like substances. Other neoplastic diseases, such as pancreatic and prostatic cancer, Hodgkin’s disease, and thymoma, may also trigger SIADH.
Less common causes include:
❑ central nervous system disorders: brain tumor or abscess, stroke, head injury, Guillain-Barré syndrome, and lupus erythematosus
❑ pulmonary disorders: pneumonia, tuberculosis, lung abscess, and positive-pressure ventilation
❑ drugs: chlorpropamide, vincristine, cyclophosphamide, carbamazepine, clofibrate, and morphine
❑ miscellaneous conditions: myxedema and psychosis.
Signs and symptoms
SIADH may produce weight gain despite anorexia, nausea, and vomiting; muscle weakness; restlessness; and, possibly, coma and seizures. Edema is rare unless water overload exceeds 4 L because much of the free water excess is within cellular boundaries.
Diagnosis
A complete medical history revealing positive water balance may suggest SIADH.
Confirming diagnosis Serum osmolality less than 280 mOsm/kg of water and a serum sodium level below 123 mEq/L confirm the diagnosis (normal urine osmolality is 1˝ times serum values).
Supportive laboratory values include high urine sodium secretion (more than 20 mEq/L) without diuretics and high urine osmolality. In addition, diagnostic studies show normal renal function and no evidence of dehydration.
Treatment
Treatment for SIADH is symptomatic and begins with restricted water intake (500 to 1,000 ml/day). Some patients who continue to have symptoms are given a high-salt, high-protein diet or urea supplements to enhance water excretion. They may also receive demeclocycline or lithium to help block the renal response to ADH. With severe water intoxication, administration of 200 to 300 ml of 5% saline may be necessary to raise the serum sodium level. When possible, treatment should include correction of the underlying cause of SIADH. If SIADH is due to cancer, success in alleviating water retention may be obtained by surgical resection, irradiation, or chemotherapy.
Special considerations
❑ Closely monitor and record intake and output, vital signs, and daily weight. Watch for hyponatremia.
❑ Observe for restlessness, irritability, seizures, heart failure, and unresponsiveness due to hyponatremia and water intoxication.
❑ To prevent water intoxication, explain to the patient and his family why he must restrict his intake.
Book Source Details
- Book Title: Professional Guide to Diseases (Eighth Edition)
- Author(s): Springhouse
- Year of Publication: 2005
- Copyright Details: Professional Guide to Diseases (Eighth Edition), Copyright © 2005 Lippincott Williams & Wilkins.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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