1.4 Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

Describe the presentation of Syndrome of Inappropriate Antidiuretic Hormone (SIADH)?  What is the etiology?  How should it be treated?

  1. Hyponatremia (<135 mEg/L) and Hypo-osmolality (<280 mOsm/L); Urine osmolality is          greater than that of plasma.
     
  2. In extreme cases may present clinically with mental confusion, seizures and coma.
     
  3. Result of excessive release of antidiuretic hormone (ADH).

Etiology

  • The posterior pituitary gland is responsible for the release of the hormone arginine vasopressin (AVP) also known as the antidiuretic hormone (ADH).  An excessive release of this hormone often results in fluid overload or hyponatremia (low serum sodium)16

     

  • The development of the SIADH is a common complication of a brain injury17 and is often diagnosed several days post injury18.
     
  • The kidneys begin to conserve water19 in response to an excessive release of ADH hormone.

Clinical Presentation

  • SIADH should be suspected if a patient with TBI (who is not dehydrated or taking diuretics) presents with concentration18-20:
    • hyponatremia (<135 mEg/L),
       
    • hypo-osmolality (<280 mOsm/L),
       
    • a urine osmolality greater than that of plasma,
       
    • a urine sodium concentration that is usually above 25mEg/L,
       
    • normal acid-base and potassium balance,
       
    • and frequently a low plasma uric acid SIADH may present clinically with mental confusion, seizures and coma18

Treatment

  • The main treatment is fluid restriction. 
     
  • Sodium correction may include:
    • Demeclocycline(Declomycin), a tetracylcline antibiotic, 600-1,200 mg/day21
       
    • Furosemide (loop diuretic) which increases “renal loss of water in excess of sodium”22
       
    • Vasopressin receptor antagonists such as Pressyn which can be administered to patients with chronic hyponatremia21 but may bring about water intoxication
       
    • Osmotic diuretic (mannitol) increase excretion of free water22

For a more detailed discussion of SIADH and other Neuroendocrine Disorders Post ABI please see ERABI/Neuroendocrine Interventions Post ABI.