Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Summary

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder of impaired water excretion due to the inability to suppress the secretion of antidiuretic hormone (ADH). The inappropriate secretion of ADH can be due to various causes, including increased production by the pituitary gland due to trauma, disease, or certain medications; the ectopic secretion of ADH by cancer; or hereditary causes (nephrogenic SIADH). Syndrome of inappropriate antidiuretic hormone secretion is characterized by impaired water excretion leading to dilutional hyponatremia , which is mainly asymptomatic but may cause neurologic symptoms. Syndrome of inappropriate antidiuretic hormone secretion should be suspected in any patient with hyponatremia , hypo-osmolality, and high urine osmolality.

Summary

• It is an endocrine disorder caused by increased ADH secretion in the pituitary gland (e.g., due to infection, drugs), ectopic production of ADH (e.g., small cell lung carcinoma), or enhanced stimulation of ADH in the kidneys as a result of a gene mutation.
• Excessive free water retention and impaired water excretion, leading to dilutional hyponatremia

Causes

• CNS disturbance (eg, stroke, hemorrhage, trauma)
• Medications (eg, carbamazepine, SSRIs, NSAIDs)
• Lung disease (eg, pneumonia)
• Ectopic ADH secretion (eg, small cell lung cancer)
• Pain &/or nausea

Clinical features

• Mild/moderate hyponatremia: nausea, forgetfulness, headache, muscle cramps
• Severe hyponatremia: seizures, coma, altered consciousness
• Euvolemia (eg, moist mucous membranes, no edema, no JVD)

Diagnosis

• Hyponatremia
• ↓ Serum osmolality <275 mOsm/kg H2O (hypotonic)
• Urine osmolality >100 mOsm/kg H2O
• Urine sodium >40 mEq/L
• Plasma ADH normal to elevated

Treatment

• Fluid restriction ± salt tablets
• Hypertonic (3%) saline for severe hyponatremia with ICU monitoring to prevent osmotic demyelination syndrome
• A loop diuretic (e.g., furosemide) can be added in more severe cases
• If initial measures fail: pharmacotherapy with vaptans (ADH antagonist)

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