Description

• 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

Epidemiology

• Most common electrolyte disorder seen in the general hospital population (1)
• Predominant age: All ages
• Predominant sex: Male = Female

Prevalence
2.5% of hospitalized patients (1)

Geriatric Considerations
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.

Risk Factors

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

General Prevention

Depends on underlying condition

Pathophysiology

• 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.

Etiology

• 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)
  • Diuretics
  • Osmotic diuresis
• Euvolemic hyponatremia (>30 mmol/L in urine):
  • Hypothyroidism
  • 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
  • Cirrhosis
  • Congestive heart failure (CHF)
  • Chronic renal failure
• Redistributive hyponatremia:
  • Hyperglycemia
  • Mannitol infusion
  • Hypertriglyceridemia
• Multiple myeloma

Commonly Associated Conditions

• Hypothyroidism
• Hypopituitarism
• 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