Hyponatremia

Essentials of Diagnosis

Thorough history and volume status examination are essential to determining etiology

Hyponatremia reflects excess water relative to sodium; total body sodium may be appropriate, low, or high

Hypotonic fluids commonly contribute to hyponatremia in hospitalized patients

General Considerations

• Most common electrolyte abnormality in hospitalized patients

• Defined as a serum sodium concentration < 135 mEq/L (135 mmol/L)

• Can be subclassified as

  • Mild (130–134 mEq/L)

  • Moderate (125–129 mEq/L)

  • Severe (< 125 mEq/L)

• A diagnostic algorithm using serum osmolality and volume status separates the causes of hyponatremia into therapeutically useful categories (Figure 21–1)

History and physical examination

• Obtain careful history, including

  • New medications

  • Changes in fluid and solute intake (polydipsia, anorexia, intravenous fluid rates, and composition)

  • Fluid output (vomiting, diarrhea, ostomy output, polyuria, oliguria, insensible losses)

• The physical examination should help categorize the patient’s volume status into

  • Hypovolemia

  • Euvolemia

  • Hypervolemia

Symptoms and Signs

• Mild hyponatremia (sodium concentrations of 130–135 mEq/L)

  • Usually asymptomatic

  • Gait disturbances, falls, and fractures may occur

• Nausea and malaise may progress to headache, lethargy, and disorientation as the sodium concentration drops

• Most serious symptoms are seizure and, very rarely, coma, brainstem herniation, and death

• Laboratory assessment should include

  • Serum electrolytes

  • Creatinine

  • Osmolality

  • Urine sodium, potassium, and osmolality

• Additional tests of thyroid and adrenal function may be necessary

• Hypovolemic hyponatremia

  • Patients require adequate fluid resuscitation, usually with isotonic fluid

  • Diuretics should be withheld in hypovolemic patients

  • Underlying cause of volume depletion should be addressed

  • Patients believed to have cerebral salt wasting may benefit from hypertonic saline (with close monitoring)

• Hypotonic hypervolemic hyponatremia

  • Loop diuretics are generally required

  • Dialysis may be needed in severe acute or chronic kidney injury

  • V₂ receptor antagonists (such as tolvaptan) may be helpful for severe hypervolemic hyponatremia from heart failure but are contraindicated in cirrhosis

• Hypotonic euvolemic hyponatremia

  • Should be treated based on the underlying etiology

  • Psychogenic polydipsia can correct quickly with water restriction alone

  • Mild to moderate hypotonic euvolemic hyponatremia

    • Often responds to fluid restriction and withholding offending medications or treating pain and nausea when due to secondary to SIADH (see SIADH)

    • Increased solute intake and loop diuretics can also be used to increase water clearance in refractory cases that are not severe

  • Acute severe symptomatic hyponatremia and chronic hyponatremia with critical symptoms (eg, seizures)

    • 100 mL of 3% hypertonic saline infused over 10 minutes (repeated twice as necessary)

    • Generally, the 24-hour correction goal rate of 4–6 mEq/L/h (for chronic hyponatremia) is still recommended, so after a brief period of rapid correction, no further correction may be needed until the next day

  • Chronic severe euvolemic hypotonic hyponatremia (but without critical symptoms of seizures or other immediate life-threatening symptoms)

    • 3% hypertonic saline at an initial rate of 0.5–2 mL/kg/h

    • Correction rate goal is 4–6 mEq/L/24 h

    • Close monitoring of serum sodium (initially every 1–2 hours) and urinary output to avoid overcorrection (less likely to occur if initial rate of infusion of 3% hypertonic saline is with lower end of recommended range)

    • High urinary output may be first sign that ADH secretion has stopped and patient is at even higher risk for overcorrection

  • Vasopressin 2 receptors:

    • Can be considered for patients with euvolemic hyponatremia (SIADH or heart failure)

Complications

• Cerebral osmotic demyelination syndrome from overly rapid correction of hypotonic hyponatremia

When to Refer

• Nephrology or endocrinology consultation should be considered in severe, symptomatic, refractory, or complicated cases of hyponatremia

• Aggressive therapies with hypertonic saline, V₂ antagonists, or dialysis mandate specialist consultation

• Consultation may be necessary with severe liver or heart disease

When to Admit

• For symptomatic or severe hyponatremia

• Patients requiring aggressive therapies for close monitoring and frequent laboratory testing