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Hyponatremia (British: hyponatraemia) is an electrolyte disturbance (a disturbance of the salts in the blood) in which the sodium (Natrium in Latin) concentration in the plasma is lower than normal (hypo in Greek; in this case, below 135 mmol/L).

Severe or rapidly progressing hyponatremia can result in swelling of the brain (cerebral edema), and the symptoms of hyponatremia are mainly neurological. Hyponatremia is most often a complication of other medical illnesses in which either fluids rich in sodium are lost (for example because of diarrhea or vomiting), or excess water accumulates in the body at a higher rate than it can be excreted (for example in polydipsia or syndrome of inappropriate antidiuretic hormone, SIADH). There may also be spurious hyponatremia (pseudohyponatremia or factitious hyponatremia) if other substances expand the serum and dilute the sodium (for example, high blood levels of fats in hypertriglyceridemia or high blood sugar in hyperglycemia).

Hyponatremia can also affect athletes who consume too much fluid during endurance events, people who fast on juice or water for extended periods and people whose dietary sodium intake is chronically insufficient.

The diagnosis of hyponatremia relies mainly on the medical history, clinical examination and blood and urine tests. Treatment can be directed at the cause (for example, corticosteroids in Addison's disease) or involve restriction of water intake, intravenous saline or drugs like diuretics, demeclocycline, urea or vaptans (antidiuretic hormone receptor antagonists). Correcting the salt and fluid balance needs to occur in a controlled fashion, as too rapid correction can lead to severe complications such as heart failure or a sometimes irreversible brain lesion known as central pontine myelinolysis.


Patients with low-level, chronic water intoxication are often asymptomatic, but may have symptoms related to the underlying cause.

Severe hyponatremia in acute or chronic form may cause osmotic shift of water from the plasma into the brain cells. Typical symptoms include nausea, vomiting, headache and malaise. As the hyponatremia worsens, confusion, diminished reflexes, convulsions, stupor or coma may occur. Since nausea is, itself, a stimulus for the release of ADH, which promotes the retention of water, a positive feedback loop may be created and the potential for a vicious cycle of hyponatremia and its symptoms exists.

A feedback loop can also be created by severe thirst, which is a symptom of some hyponatremic individuals. When these people consume large quantities of water without an adequate increase in sodium, the hyponatremic condition worsens.


One approach to determining causes of hyponatremia

An abnormally low plasma sodium level is best considered in conjunction with the person's plasma osmolality and extracellular fluid volume status.

Type Serum osmolality (mOsm/kg) Description
Hypotonic hyponatremia 280 When the plasma osmolality is low, the extracellular fluid volume status may be in one of three states: low volume, normal volume, or high volume.
Isotonic hyponatremia between 280 and 295 Certain conditions that interfere with laboratory tests of serum sodium concentration (such as extraordinarily high blood levels of lipid or protein) may lead to an erroneously low measurement of sodium. This is called pseudohyponatremia.
Hypertonic hyponatremia > 295 Hypertonic hyponatremia can be associated with shifts of fluid due to osmotic pressure.


The treatment of hyponatremia usually depends on the underlying cause. If a person has few symptoms, little treatment other than water restriction may be required. In the setting of volume depletion, intravenous administration of normal saline may be effective.

Over aggressive correction of hyponatremia may lead to a syndrome of central pontine myelinolysis. Thus, correction of serum sodium should not exceed 12 mEq/L per 24 hours nor 18 mEq/L per 48h.

Seizures associated with hyponatremia are typically treated with a 100 mL bolus of 3 % hypertonic saline.

Notable cases

  • Craig Barrett, a New Zealand athlete, collapsed during a 50 km walk, probably due to water intoxication.
  • Matthew Carrington, a student at California State Universitymarker in Chico, Californiamarker, died of hyponatremia in February 2005 during a fraternity hazing ritual.
  • James McBride, a police officer with the Metropolitan Police Department of the District of Columbia, died of hyponatremia on August 10, 2005. Officer McBride had been participating in a strenuous bicycle patrol training course. During a 12-mile (19 km) training ride on the second day of the course, Officer McBride drank as much as three gallons (11 liters) of water.
  • Leah Betts died on 16 November 1995 after taking an ecstasy tablet at her 18th birthday party and subsequently drinking too much water; the case received mass media coverage throughout the United Kingdom, which focused on the dangers of ecstasy.
  • Cynthia Lucero, who collapsed between miles 19 and 20 of the Boston Marathon in 2002 was the second person ever to die in the history of the race.
  • In January 2007, Jennifer Strange, a woman in Sacramento, Californiamarker, died following a water-drinking contest sponsored by a local radio station, Sacramento-based KDND-FM. The contest was called "Hold your wee for a Wii".
  • After completing the 2007 London Marathon, 22-year-old David Rogers collapsed and later died as a result of hyponatremia.
  • Professional wrestler Michelle McCool was hospitalized for 16 days in 2006 because of hyponatremia.


Sodium deficiency exists in grazing animals where soil sodium levels have been depleted by leaching. This is more common in mountainous regions. Agricultural science research conducted in the northern Thai highlands in the 1970s found that an endemic sodium deficiency masked all other nutrient deficiencies across all seasons and reduced productivity. Sodium supplementation increased liveweight gain by around 30% and also reproductive rates by around 30%. Simple salt supplementation is now recommended in this region and neighbouring mountains, as both a herd management tool and for increased productivity (see sources below).

See also


  1. Hyponatremia and athletes
  2. PestaƱa, C. (2000). Fluids and electrolytes in the surgical patient. Philadelphia, PA: Lippincott Williams and Wilkins. 75.
  4. Local report

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