II. Definitions

  1. Isovolemic Hypoosmolar Hyponatremia
    1. Hyponatremia with low Serum Osmolality and normal Extracellular Volume (ECV)

III. Causes

  1. Urine Sodium < 10 (Urine Osmolality <100 mOsm/kg, maximally dilute urine)
    1. Water Intoxication (Psychogenic Polydipsia)
      1. Free water intake exceeds free water excretion
      2. Seen in Psychosis (Schizophrenia, Bipolar Disorder)
      3. Also seen in Exercise Associated Hyponatremia
    2. Excessive Alcohol intake (Beer Potomania Syndrome)
    3. Tea and Toast Syndrome (esp. elderly)
    4. Hypotonic Saline infusion
      1. Affects 4-5% of post-operative patients
    5. Methylenedioxymethamphetamine (MDMA, Ecstasy, Molly)
      1. May result in Water Intoxication to point of severe Hyponatremia (deaths have occurred)
  2. Urine Sodium > 20-30 (Urine Osmolality >100 mOsm/kg)
    1. Syndrome Inappropriate ADH Secretion (SIADH)
    2. Hypothyroidism
    3. Anterior Hypopituitarism
      1. Selective Glucocorticoid deficiency
      2. Incomplete ADH suppression
      3. Consider serum Aldosterone, plasma renin activity, morning or Free Urine Cortisol, ACTH
    4. Nausea, pain, emotional stress
    5. Potassium depletion and Diuretics
    6. Reset Osmostat (SIADH variant)
      1. Common in Pregnancy
      2. ADH released at lower levels of Serum Osmolality (lower threshold)
      3. Serum Sodium establishes a new lower steady state
      4. Normal fractional excretion of Uric Acid
    7. Medications
      1. See Medication Causes of SIADH

IV. Differential Diagnosis

  1. Often difficult to distinguish Iso- from Hypovolemic
  2. See Hypovolemic Hypoosmolar Hyponatremia

VI. Diagnostics (consider)

VII. Management

  1. See Hyponatremia Management
  2. See SIADH Management
  3. Restrict water intake to 1 liter per day
    1. Calculate excess Total Body Water to be excreted

VIII. References

  1. Kone in Tisher (1993) Nephrology, p. 87-100
  2. Levinsky in Wilson (1991) Harrison's IM, p. 281-84
  3. Rose (1989) Acid-Base and Electrolytes, p. 601-38
  4. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]

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