Hyponatremia

BASICS

BASICS

BASICS

Think about hyponatremia as a water problem, rather than a true sodium (Na+) problem.

DESCRIPTION

DESCRIPTION

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

EPIDEMIOLOGY

EPIDEMIOLOGY

Prevalence

Prevalence

Prevalence

  • Most common electrolyte disorder seen in the general hospital population, affecting 2%.
  • 7.7% outpatients (1)

Geriatric Considerations
Elderly patients have decreased renal mass placing them at risk for decreased urinary concentration and decreased response to ADH. Presenting symptoms may be frequent falls and gait disturbances. Clinicians should also consider the impact of comorbidities and acute disease.Pediatric Considerations
Children are at increased risk of brain herniation from cerebral edema.

ETIOLOGY AND PATHOPHYSIOLOGY

ETIOLOGY AND PATHOPHYSIOLOGY

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.

Genetics

Genetics

Genetics

Mutations have been associated with nephrogenic syndrome of inappropriate antidiuresis (NSAID; SIADH).

GENERAL PREVENTION

GENERAL PREVENTION

GENERAL PREVENTION

Maintain appropriate hydration.

COMMONLY ASSOCIATED CONDITIONS

COMMONLY ASSOCIATED CONDITIONS

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

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