Hypertonic saline for symptomatic hyponatremia in SIADH

Electrolyte Disordersmedium

A 71-year-old man presents to the emergency department after a fall. His wife notes that he has become progressively more confused during the past 2 weeks, and his gait has become unsteady. On examination, BP is 112/70 mm Hg, with heart rate of 82/min without postural change. There are no demonstrable injuries related to the fall. Heart and lungs are normal, and there is no edema. Neurologic exam reveals confusion, decreased coordination, and inability to stand or walk independently. Laboratory data: serum sodium 117 mEq/L (reference 136–145); potassium 3.8 mEq/L (3.5–5.0); chloride 88 mEq/L (98–106); total CO₂ 24 mEq/L (23–28); BUN 12 mg/dL (8–20); creatinine 0.8 mg/dL (0.7–1.3); glucose 91 mg/dL (70–99, fasting); osmolality 253 mOsm/kg H₂O (275–295). Urine: sodium 52 mEq/L (varies with intake); creatinine 98 mg/dL (varies with intake); osmolality 493 mOsm/kg H₂O (varies with intake). Which ONE of the following is the next BEST STEP in management?

  1. A.Hypertonic salineCorrect
  2. B.Tolvaptan
  3. C.Fluid restriction
  4. D.Isotonic saline

Explanation

This patient with symptomatic hypo-osmolar hyponatremia should be treated with hypertonic saline. Hyponatremia with hypo-osmolality in euvolemic patients indicate a state of water overload. After excluding myxedema coma or adrenal insufficiency, the differential diagnosis includes the syndrome of inappropriate antidiuresis or water intoxication. This patient's inappropriately concentrated urine is consistent with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Treatment of symptomatic hyponatremia requires increasing the serum sodium by approximately 4–6 mEq/L within 6 hours to relieve symptoms. This can usually be accomplished empirically by administering a 100- or 150-mL bolus of hypertonic (3%) saline. Additional boluses may be needed based on follow-up laboratory data. The recommended goal of management is to increase the sodium by a cumulative total of 6–8 mEq in the first 24 hours, 12–14 mEq/L by 48 hours, and 14–16 mEq/L by 72 hours in patients with chronic hyponatremia to avoid overcorrection. Vasopressin receptor antagonists that block ADH, loop diuretics that decrease urine osmolality, and fluid restriction alone are all treatments for SIADH, but would be most appropriate for chronic management of patients without neurologic symptoms. Isotonic saline might be effective for hyponatremia caused by hypovolemia, but this patient shows no evidence of hypovolemia by physical examination, orthostatic vital signs, or urine chemistry. Normal saline administration would be expected to worsen hyponatremia caused by SIADH, so it would not be the next best step in therapy.

Why each option

A.
Correct. Moderately severe symptoms with sodium 117 mEq/L warrant 100 to 150 mL boluses of 3% saline to raise sodium 4 to 6 mEq/L.
B.
Tolvaptan is reserved for chronic SIADH without neurologic symptoms and carries overcorrection risk; it is not first-line for symptomatic hyponatremia.
C.
Fluid restriction corrects SIADH slowly over days and is for chronic management; confusion and inability to walk require prompt sodium elevation.
D.
With urine osmolality 493 and euvolemia, isotonic saline is excreted in concentrated urine, retaining free water and potentially worsening SIADH hyponatremia.

Reference: Verbalis JG, Am J Med 2013 (expert panel recommendations on diagnosis, evaluation, and treatment of hyponatremia); Spasovski G, Eur J Endocrinol 2014 (clinical practice guideline on hyponatraemia)

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