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- 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
- Most common electrolyte disorder seen in the general hospital population
- Predominant age: all ages
- Predominant sex: male = female
2.5% of hospitalized patients
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.
Children <16 years of age have less intracranial volume 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) >285 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 280 to 285 mOsm
- Excessive osmoles leading to dilution
- Unchanged TBW and Na
- Causes: hyperlipidemia, hyperproteinemia (e.g., multiple myeloma)
- Hypotonic hyponatremia: serum Osm <280 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, osmotic diuresis.
- Euvolemic hyponatremia: 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., carbamazepine, clofibrate, cyclosporine, levetiracetam, opiates, oxcarbazepine, phenothiazines, SSRIs, TCAs, vincristine).
- Urine Osm <100 mOsm/kg
- Causes include primary polydipsia, beer potomania.
- 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.
- Urine Na >20 mmol/L
- Causes include renal failure.
- Polymorphisms have been demonstrated.
- Mutations have been associated with nephrogenic syndrome of inappropriate antidiuresis (NSAID; SIADH).
Depends on underlying etiology
Commonly Associated Conditions
- Adrenocortical hormone deficiency
- HIV patients
- SIADH is associated with cancers, pneumonia, tuberculosis, encephalitis, meningitis, head trauma, cerebrovascular accident, HIV infection.
- Acute neurologic patients, brain injury
- Marathon runners in hot environments