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

Incidence

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

Prevalence

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

Geriatric Considerations
Elderly patients have a decreased renal mass placing them at risk decreased urinary concentration and decreased response to antidiuretic hormone. Additionally, they have a lower TBW state, a decreased thirst mechanism, and decreased renal blood flow, which will also affect the glomerular filtration rate. 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

• Volume status and serum osmolality must be ascertained to determine etiology in order to direct management.
• Hypertonic hyponatremia: serum osmolarity (Osm) > 295 mOsmol/kg
  • 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
  • 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 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, medications (e.g., thiazide diuretics, loop diuretics, carbamazepine, clofibrate, cyclosporine, levetiracetam, oxcarbazepine, SSRIs, TCAs, vincristine).
• 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, and psychogenic polydipsia.

Genetics

• 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, and HIV infection.
• Traumatic brain injury
• Marathon runners in hot environments
• Beer potomania
• Tea-and-toast diet
• Ecstasy use