Hyponatremia

Key Features

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

• Hyponatremia in hospitalized patients commonly caused by administration of hypotonic fluids

General Considerations

• Most common electrolyte abnormality in hospitalized patients

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

• Whether hyponatremia is symptomatic depends on both severity and acuity

• Severity can be subclassified as

  • Mild (130–134 mEq/L)

  • Moderate (125–129 mEq/L)

  • Severe (< 125 mEq/L)

• Acuity

  • Acute hyponatremia defined as lasting < 48 hours

  • Chronic hyponatremia defined as lasting > 48 hours (often diagnosed on routine electrolyte panels)

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

• SIADH is a clinical diagnosis characterized by

  • Hyponatremia

  • Hypoosmolality (< 280 mOsm/kg)

  • Absence of heart, lung, or kidney disease

  • Normal thyroid and adrenal function

  • Urine Na usually > 20 mEq/L

  • BUN may be < 10 mg/dL [3.6 mmol/L] and serum uric acid < 4 mg/dL [238 mcmol/L]

  • Low BUN and low serum uric acid are not only due to dilution but to increased urea and uric acid clearances in response to volume expansion

Clinical Findings

History and physical examination

• Obtain 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

Diagnosis

• Laboratory assessment should include

  • Serum electrolytes

  • Creatinine

  • Osmolality

  • Urine sodium, potassium, and osmolality

• Additional tests of thyroid and adrenal function may be necessary

Treatment

• Documented acute hyponatremia < 48 hours

  • Sodium can be corrected at the rate at which it fell

• Most hyponatremia is chronic > 48 hours

  • In general, sodium needs to be corrected more slowly to minimize risk of osmotic demyelination

• Pseudohyponatremia from hypertriglyceridemia or hyperproteinemia

  • No therapy required except confirmation with the clinical laboratory

• Translocational hyponatremia

  • From hyperglycemia or mannitol

  • Can be managed with glucose correction or mannitol discontinuation ...