Hyponatremia
Basics
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
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
- Water shifts from intracellular fluid (ICF) to extracellular fluid (ECF), resulting in dilution.
- 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
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