Syndrome of Inappropriate Secretion of Antidiuretic Hormone (SIADH):
The continuous, uncontrolled release of antidiuretic hormone (ADH), produced by tumor cells or by the abnormal stimulation of the hypothalmic–pituitary network, leads to increased extracellular fluid volume, water intoxication, hyponatremia, and increased excretion of urinary sodium.
As fluid volume increases, stretch receptors in the right atrium respond by releasing a second hormone, atrial naturetic factor (ANF). The release of ANF causes increased renal excretion of sodium, which worsens hyponatremia.
The most common cause of SIADH is cancer, especially small cell cancers of the lung. Antineoplastics— vinc ri stine, vinblastine, cisplatin, and cyclophosphamide and morphine also stimulate ADH secretion, which promotes conservation and reabsorption of water by the kidneys. As more fluid is absorbed, the circulatory volume increases, ANF is released, and sodium is actively excreted by the kidneys in compensation.
Serum sodium levels below 120 mEq/L (SI: 120 mmol/L): symptoms of hyponatremia including personality changes, irritability, nausea, anorexia, vomiting, weight gain, fatigue, muscular pain (myalgia), headache, lethargy, and confusion. Serum sodium levels below 110 mEq/L (SI: 110 mmol/L): seizure, abnormal reflexes, papilledema, coma, and death. Edema is rare.
• Decreased serum sodium level
• Increased urine osmolality
• Increased urinary sodium level
• Decreased BUN, creatinine, and serum albumin levels secondary to dilution
• Abnormal water load test results
Fluid intake range limited to 500 to 1,000 mL/day to increase the serum sodium level and decrease fluid overload. If water restriction alone is not effective in correcting or controlling serum sodium levels, demeclocycline is often prescribed to interfere with the antidiuretic action of ADH and ANF. When neurologic symptoms are severe, parenteral sodium replacement and diuretic therapy are indicated. Electrolyte levels are monitored carefully to detect secondary magnesium, potassium, and calcium imbalances. After the symptoms of SIADH are controlled, the underlying cancer is treated. If water excess continues despite treatment, pharmacologic intervention (urea and furosemide) may be indicated.
• Maintain intake and output measurements.
• Assess level of consciousness, lung and heart sounds, vital signs, daily weight, and urine specific gravity; also assess for nausea, vomiting, anorexia, edema, fatigue, and lethargy.
• Monitor laboratory test results, including serum electrolyte levels, osmolality, and blood urea nitrogen, creatinine, and urinary sodium levels.
• Minimize the patient’s activity; provide appropriate oral hygiene; maintain environmental safety; and restrict fluid intake if necessary.
• Reorient the patient and provide instruction and encouragement as needed.
• As adjuncts to water restriction, demeclocycline may be ordered to inhibit the renal response to ADH in patients with lung malignancies.
• Avoid hypotonic enemas to treat constipation because water intoxication can be potentiated.
• Institute pressure ulcer prevention strategies.