Hyponatremia

DESCRIPTION

• Hyponatremia is a plasma Na⁺ concentration of <135 mEq/L.
• In healthy individuals, the excess water is renally excreted in due to suppression of antidiuretic hormone (ADH) to maintain homeostatic state.

EPIDEMIOLOGY

ETIOLOGY AND PATHOPHYSIOLOGY

• Volume status and serum osmolality must be ascertained to determine etiology in order to direct management.
• Normal serum osmolality is 280 to 295 mOsm/kg.
• Serum osmolality (Osm) (mmol/kg) = (2 × serum [Na]) + (serum [glucose]/18) + (blood urea nitrogen (BUN)/2.8)
• Hypertonic hyponatremia: serum Osm >295 mOsmol/kg
  • Accumulation of solutes that are osmotically active, causing water shifts from intracellular fluid (ICF) to extracellular fluid (ECF), resulting in dilution.
    • Causes: hyperglycemia, mannitol, sorbitol, radiologic contrast
• Isotonic hyponatremia (“pseudohyponatremia”): serum Osm 275 to 295 mOsm/kg
  • Falsely low levels of sodium; actual levels are normal.
  • Osmolality is normal; usually euvolemic
  • Unchanged TBW and Na⁺
  • Causes: hyperlipidemia, hyperproteinemia (e.g., multiple myeloma), jaundice (caused by elevated lipoprotein X levels), laboratory artifact, hyperglycemia
• Hypotonic hyponatremia: serum Osm <275 mOsmol/kg
  • Subdivided by volume status into hypovolemic, euvolemic, or hypervolemic
  • Hypovolemic hyponatremia: low TBW and low Na⁺
    • Signs include orthostatic hypotension, decreased skin turgor, dry mucous membranes.
    • If urine Na⁺ <30 mmol/L, it indicates extrarenal loss such as GI loss (vomiting, diarrhea), third-spacing (pancreatitis, burns), skin loss (burns, cystic fibrosis, sweating), and heat-related illnesses.
    • If Urine Na⁺ >30 mmol/L, it indicates renal loss such as cerebral salt wasting, adrenal insufficiency, diuretics, and osmotic diuresis.
• Euvolemic hyponatremia: mild to moderate increase in TBW, normal Na⁺ (most common subtype)
  • Signs include a nonedematous state.
  • If urine Osm >100 mOsm/kg, causes include syndrome of inappropriate antidiuretic hormone (SIADH), hypothyroidism, adrenal insufficiency, and medications (e.g., thiazide diuretics, loop diuretics, carbamazepine, clofibrate, cyclosporine, levetiracetam, oxcarbazepine, SSRIs, TCAs, vincristine, barbiturates, chlorpropamide, opioids).
  • If urine Osm <100 mOsm/kg, causes include primary polydipsia, beer potomania, and exercise-induced hyponatremia.
• Hypervolemic hyponatremia: increased TBW and Na⁺
  • Signs include edematous state.
  • Urine Na⁺ <30 mmol/L
  • Causes include congestive heart failure (CHF), cirrhosis, nephrotic syndrome, hypoalbuminemia, psychogenic polydipsia, renal failure.

GENERAL PREVENTION

Maintain appropriate hydration.

COMMONLY ASSOCIATED CONDITIONS

• Hypothyroidism, hypopituitarism
• Cirrhosis, CHF, nephrotic syndrome
• Adrenocortical hormone deficiency
• SIADH is associated with cancers, pneumonia, tuberculosis, encephalitis, meningitis, head trauma, cerebrovascular accident, and HIV infection.
• Medications: thiazide diuretics, SSRIs, carbamazepine
• Marathon runners in hot environments
• Beer potomania
• Tea-and-toast diet
• Ecstasy use