Description

• Hyponatremia is a plasma sodium (Na⁺) concentration of ≤135 mEq/L.
• Hyponatremia itself does not provide information about the total body water (TBW) state of the patient. Patients with hyponatremia may be hypervolemic, hypovolemic, or euvolemic.
• System(s) affected: endocrine/metabolic, renal, cardiovascular, central nervous system (CNS)

Epidemiology

Prevalence

• Most common electrolyte disorder seen in the general hospital population, affecting 2.5%
• 7.7% outpatients (1)

Geriatric Considerations
Elderly patients have a decreased renal mass placing them at risk for decreased urinary concentration and decreased response to antidiuretic hormone. 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

• Volume status and serum osmolality must be ascertained to determine etiology in order to direct management (2).
• Normal serum osmolality is 280 to 295 mOsmol/kg.
• Serum osmolality (Osm) (mOsmol/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.
    • Unchanged TBW and Na⁺
    • Causes: hyperglycemia, mannitol, sorbitol, radiologic contrast
• Isotonic hyponatremia (“pseudohyponatremia”): serum Osm 275 to 295 mOsmol/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), laboratory artifact, irrigant solutions, 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 moderated 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, 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, and renal failure.

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

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.
• Marathon runners in hot environments
• Beer potomania
• Tea-and-toast diet
• Ecstasy use