The human body keeps blood sodium concentration in a tight range of 135 to 145 meq/L. According to Harrison's Principles of Internal Medicine, low blood sodium, or hyponatremia, is defined as a sodium level less than 135 meq/L. Mild hyponatremia is less than 125 meq/L; severe hyponatremia occurs when sodium decreases to less than 110 meq/L, which may cause coma and death.
Hyponatremia has many causes, which can be divided into the broad categories of sodium loss, water gain or edema.
Sodium Loss
Sodium may be lost from the skin by sweating or burns; the gastrointestinal tract by vomiting or diarrhea; or the kidneys by thiazide diuretics. Hyponatremia results when losses are replaced by ingesting plain water or liquids low in sodium.
According to Merck.com, when tissues are burned, fluid leaks into them from blood vessels, causing swelling.
Water Gain
According to The Washington Manual of Medical Therapeutics, people with psychiatric illnesses who drink water compulsively may exceed the 12 liter per day excretory capacity of the kidneys causing hyponatremia.
People who drink large amounts of beer can develop hyponatremia due to low-solute and protein intake, whch decreases the amount of urine that can be generated.
High levels of antidiuretic hormone, or vasopressin released by the pituitary gland in the brain cause water retention, which results in hyponatremia, according to MayoClinic.com. The major stimulus for antidiuretic hormone secretion is increased solute concentration, primarily sodium. Some disorders can cause inappropriate antidiuretic hormone secretion, for example: psychiatric and pulmonary diseases, tumors, postoperative pain and medications. This is a very common cause of hyponatremia in hospitalized patients.
Adrenal insufficiency and an underactive thyroid, causing hyothyroidism, can produce hyponatremia; the former by decreasing aldosterone synthesis, a hormone needed to conserve sodium in the kidneys and the latter by decreasing cardiac output and the filtration rate of the kidneys.
Edema
Hyponatremia may be due to heart failure. According to Braunwald's Heart Disease, the degree of hyponatremia is a marker for the severity of heart failure. Hyponatremia may also be due to cirrhosis of the liver or the nephrotic syndrome, which is characterized by greater than 3.5 grams per day of protein in the urine, and edema. All three diseases cause decreased arterial volume, which leads to increased thirst and edema.
A complication of decompensated cirrhosis is the abnormal accumulation of fluid in the peritoneal cavity, or ascites.
The loss of protein and albumin in urine in the nephrotic syndrome causes edema and volume overload.
References
- "Harrison's Principles of Internal Medicine"; Dennis Kasper, M.D.; 2005
- Mayo Clinic: Hyponatremia
- "Braunwald's Heart Disease"; Peter Libby, M.D.; 2008
- Merck: Burns
- "The Washington Manual of Medical Therapeutics"; Gopa Green, M.D.; 2004
