Hyponatremia

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

• Most common electrolyte disorder seen in the general hospital population
• Predominant age: all ages
• Predominant sex: male = female

• 2.5% of hospitalized patients
• 20–30% in hospitalized patients (1)
• 7.7% outpatients (1)

Geriatric Considerations
The elderly have lower TBW, a decreased thirst mechanism, and decreased urinary concentrating ability; their kidneys are less responsive to antidiuretic hormone (ADH), and they show decreased renal mass, renal blood flow, and glomerular filtration rate, putting them at higher risk for hyponatremia. Presenting symptoms may be frequent falls and gait disturbances.

Pediatric Considerations
Children <16 years of age have less intracranial space and are at increased risk of brain herniation from cerebral edema.

Etiology and Pathophysiology

• Assess serum osmolality and volume status to determine etiology. The etiology directs the management.
• Hypertonic hyponatremia: serum osmolarity (Osm) >295 mOsm
• Shift of water 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 mOsm
  • Excessive osmoles leading to dilution
  • Unchanged TBW and Na⁺
  • Causes: hyperlipidemia, hyperproteinemia (e.g., multiple myeloma), laboratory artifact, irrigant solutions
• Hypotonic hyponatremia: serum Osm <275 mOsm
  • Subdivided by volume status into hypovolemic, euvolemic, or hypervolemic
    • Hypovolemic hyponatremia: subtype of hypotonic hyponatremia with decreased TBW and Na⁺
  • Signs include orthostatic hypotension, decreased skin turgor, dry mucous membranes.
  • Urine Na⁺ <20 mmol/L; indicates extrarenal loss
    • Causes include GI loss (vomiting, diarrhea), third spacing (pancreatitis, peritonitis, burns, rhabdomyolysis), skin loss (burns, cystic fibrosis, sweating), and heat-related illnesses.
  • Urine Na⁺ >20 mmol/L; indicates renal loss
    • Causes include cerebral salt-wasting syndrome, adrenal insufficiency, diuretics, and osmotic diuresis.
• Euvolemic hyponatremia: most common subtype of hypotonic hyponatremia with increased TBW and normal Na⁺
  • Signs include a nonedematous state.
  • 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).
  • Urine Osm <100 mOsm/kg
    • Causes include primary polydipsia, beer potomania (massive consumption of beer, which is poor in solutes and electrolytes), and exercise-induced.
• Hypervolemic hyponatremia: subtype of hypotonic hyponatremia with increased TBW and Na⁺
  • Signs include edematous state.
  • Urine Na⁺ <20 mmol/L
    • Causes include congestive heart failure (CHF), cirrhosis, nephrotic syndrome, hypoalbuminemia, psychogenic polydipsia.
  • Urine Na⁺ >20 mmol/L
    • Causes include CHF, liver cirrhosis, nephrotic syndrome, and chronic renal failure.

• Polymorphisms have been demonstrated.
• Mutations have been associated with nephrogenic syndrome of inappropriate antidiuresis (NSIAD; SIADH).

General Prevention

Depends on underlying etiology

Commonly Associated Conditions

• Hypothyroidism
• Hypopituitarism
• Cirrhosis
• CHF
• Nephrotic syndrome
• Adrenocortical hormone deficiency
• HIV patients
• SIADH is associated with cancers, pneumonia, tuberculosis, encephalitis, meningitis, head trauma, cerebrovascular accident, HIV infection.
• Traumatic brain injury
• Marathon runners in hot environments
• Beer potomania
• Tea-and-toast diet
• Ecstasy use