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- Hyponatremia is a plasma sodium concentration of <135 mEq/L. Hyponatremia itself does not provide information about the “total body water” state of the patient. Patients with hyponatremia may be fluid overloaded, hypovolemic, or euvolemic.
- System(s) affected: Endocrine/Metabolic
- Most common electrolyte disorder seen in the general hospital population (1)
- Predominant age: All ages
- Predominant sex: Male = Female
2.5% of hospitalized patients (1)
The elderly have lower total body water, a decreased thirst mechanism, and decreased urinary concentrating ability; their kidneys are less responsive to ADH, and they show decreased renal mass, renal blood flow, and glomerular filtration rate, making them at higher risk for hyponatremia.
- Polymorphisms have been demonstrated.
- Mutations have been associated with nephrogenic syndrome of inappropriate antidiuresis (NSIAD, SIADH).
Depends on underlying condition
- Hypovolemic hyponatremia: Decrease in total body water and greater decrease in total body sodium; decreased extracellular fluid volume; orthostatic hypotension and other changes consistent with hypovolemia are present.
- Euvolemic hyponatremia: Increase in total body water with normal total body sodium; extracellular fluid volume is minimally to moderately increased but with no edema.
- Hypervolemic hyponatremia: Increase in total body sodium and greater increase in total body water; extracellular fluid increased markedly; edema is present.
- Redistributive hyponatremia: Shift of water from intracellular compartment to extracellular compartment with resulting dilution of sodium; total body water and total body sodium unchanged; occurs with hyperglycemia
- Pseudohyponatremia: Dilution of aqueous phase by excessive proteins, glucose, or lipids; total body water and total body sodium unchanged; occurs in hypertriglyceridemia or multiple myeloma
- Low sodium creates an osmotic gradient between plasma and cells and fluid shifts into cells, causing edema and increased intracranial pressure.
- Hypovolemic hyponatremia: Extrarenal loss of sodium (<30 mmol/L in urine):
- GI loss: Vomiting, diarrhea
- 3rd spacing: Peritonitis, pancreatitis, burns, rhabdomyolysis
- Skin loss: Burns, sweating, cystic fibrosis
- Heat-related illnesses
- Hypovolemic hyponatremia: Renal loss of sodium (>30 mmol/L in urine):
- Cerebral salt wasting syndrome
- Adrenal pathology (e.g., Addison disease, hemorrhage, tuberculosis)
- Osmotic diuresis
- Euvolemic hyponatremia (>30 mmol/L in urine):
- Hypopituitarism or other cause of glucocorticoid deficiency
- Medications (e.g., carbamazepine, clofibrate, cyclosporine, levetiracetam, opiates, oxcarbazepine, phenothiazines, tricyclic antidepressants, vincristine) (2).
- Primary polydipsia
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Iatrogenic (e.g., excess hypotonic IV fluids)
- Hypervolemic hyponatremia (<30 mmol/L in urine, except chronic renal failure):
- Nephrotic syndrome
- Congestive heart failure (CHF)
- Chronic renal failure
- Redistributive hyponatremia:
- Mannitol infusion
- Multiple myeloma
Commonly Associated Conditions
- Adrenocortical hormone deficiency
- HIV patients
- SIADH is associated with cancers, pneumonia, tuberculosis, encephalitis, meningitis, head trauma, cerebrovascular accident, HIV infection (3).
- Acute neurological patients, brain injury
- Marathon runners in hot environments