Sodium Disorders

Sodium is the primary extracellular cation and the principal determinant of serum osmolality (calculated: 2×Na + glucose/18 + BUN/2.8). Sodium disorders are fundamentally disorders of WATER balance, not sodium balance. Hyponatremia (Na <135 mEq/L): the most common electrolyte disorder in hospitalized patients (15-30%). Classified by serum osmolality: hypotonic (true hyponatremia — excess free water relative to sodium), isotonic (pseudohyponatremia from hyperlipidemia or hyperproteinemia), hypertonic (dilutional from hyperglycemia — Na decreases ~1.6-2.4 mEq/L for every 100 mg/dL glucose above normal). Hypotonic hyponatremia is classified by volume status: hypovolemic (Na and water lost, more Na than water — diuretics, vomiting, diarrhea), euvolemic (water excess with normal sodium — SIADH is the most common cause, hypothyroidism, adrenal insufficiency), hypervolemic (both Na and water increased, relatively more water — CHF, cirrhosis, nephrotic syndrome). SIADH: excessive ADH causes free water retention, diluting sodium; hallmark: concentrated urine (osmolality >100) in the setting of dilute serum. Hypernatremia (Na >145 mEq/L): always represents a free water deficit; classified as hypovolemic (water and Na lost, more water — osmotic diuresis, insensible losses), euvolemic (pure water loss — diabetes insipidus,...