Syndrome of Inappropriate ADH

Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

• Definition: SIADH results from excessive ADH production or release, despite normal or low plasma osmolarity. ADH increases kidney water reabsorption.

• Pathophysiology: Increased ADH leads to fluid retention, decreased plasma osmolality, decreased sodium levels (dilutional hyponatremia), and concentrated urine.

• Etiology: Most commonly caused by cancer (especially small cell lung cancer), head trauma, or medications. Can also be caused by other conditions like CNS disorders and metabolic diseases.

Clinical Manifestations

• Early symptoms: Thirst, dyspnea, fatigue, muscle cramps, irritability, headache.

• Severe symptoms (lower sodium levels): Vomiting, abdominal cramps, muscle twitching, lethargy, confusion, seizures, coma, cerebral edema.

Diagnosis

• Diagnostic tests: Measurement of urine and serum osmolality. Key indicators include: serum sodium less than 135 mEq/L, serum osmolality less than 280 mOsm/kg, and urine specific gravity greater than 1.030.

Treatment

• Mild cases (serum sodium > 125 mEq/L): Fluid restriction of 800-1000 mL/day. Monitor for weight loss, and gradual rise in sodium and osmolality.

• More severe cases (severe hyponatremia): Fluid restriction of 500 mL/day. Careful slow correction of sodium levels (no more than 8-12 mEq/L/day) with possible IV hypertonic saline to avoid osmotic demyelination.

• Medications: Loop diuretics (like furosemide) can promote diuresis, but only if sodium levels are above 125mEq/L. Demeclocycline blocks ADH effects. Vasopressor receptor antagonists (conivaptan and tolvaptan) for hospitalized cases of euvolemic hyponatremia. Avoid these medications if liver disease is present

• Important considerations: Seizure precautions for patients with altered sensorium or seizures, keep the patient's head slightly elevated when possible(under 10 degrees). Monitor fluid intake and output, vital signs, electrolytes to avoid further complications, frequent position changes.

Chronic SIADH Management

• Fluid restriction: 800-1000 mL/day. Offer ice chips or sugar-free gum to manage thirst.

• Dietary supplements: Encourage dietary sodium and potassium.

• Monitoring: Daily weights are crucial for assessing fluid balance. Teach patients symptoms of fluid and electrolyte imbalances impacting sodium and potassium levels.