An Overview of Hyponatremia

Cause, Symptoms, and Treatment of Abnormally Low Sodium

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Sodium is an important electrolyte in the body that transmits electrical signals between cells and also helps keep fluid within cells. Too little sodium in the body—an electrolyte disorder called hyponatremia—usually occurs when the body fails to remove water normally. In other words, the body either loses or retains too much water, which ultimately affects the body's sodium content.

Symptoms of hyponatremia, or low sodium in the body, may include nausea and vomiting, confusion, weakness, and in severe cases, seizures, coma, and even death.

In order to diagnose hyponatremia, a doctor will perform a medical history, physical examination, and order laboratory tests. Treatment involves addressing the underlying cause—such as administering a salt-water solution through the vein, or alternatively, restricting water and salt intake.

A normal sodium level is between 135 and 145 milliequivalents per liter (mEq/L). Hyponatremia is defined as a sodium level of less than 135 mEq/L.

Causes

The causes of hyponatremia are typically classified by the amount of fluid in the body and broken down into three categories:

  • Hypovolemic hyponatremia (low volume)
  • Euvolemic hyponatremia (normal volume)
  • Hypervolemic hyponatremia (high volume)

Hypovolemic Hyponatremia

Hypovolemic hyponatremia occurs when the body loses too much water with an even greater decrease in the sodium level. Usually, the fluid loss is from the kidneys (for example, the overuse of diuretics) or the digestive tract (such as severe or persistent vomiting or diarrhea).

Hypovolemic hyponatremia is common at endurance sporting events, affecting as many as 6% of endurance cyclists, 8% of marathon runners, 11% of Ironman competitors, and 67% of ultramarathon contestants.

Primary adrenal insufficiency, known as Addison's disease, is another common cause of hypovolemic hyponatremia.

Euvolemic Hyponatremia

Euvolemic hyponatremia occurs when the total amount of water in the body is increased, but the sodium level remains normal. This may happen with athletes who engage in intense exercises, like a marathon or triathlon, and then drink too much water. It is also seen in people who consume excess amounts of alcohol or use the drug ecstasy.

Malnutrition, severe hypothyroidism, and a water-retaining condition called syndrome of inappropriate antidiuretic hormone secretion (SIADH) can also cause euvolemic hyponatremia.

Hypervolemic Hyponatremia

Hypervolemic hyponatremia occurs when the body holds on to too much water, relative to its sodium content. Medical conditions that may cause this type of hyponatremia, include:

Symptoms

The symptoms of hyponatremia depend on the rate at which the sodium level dropped and the severity of the drop. In fact, many people with hyponatremia have no symptoms—instead, the electrolyte abnormality is found incidentally through a blood test drawn for other purposes.

Hyponatremia may cause a variety of potential symptoms including:

  • Weakness
  • Headache
  • Nausea and vomiting
  • Muscle cramps
  • Restlessness or irritability
  • Slurred speech
  • Confusion

Signs of volume overload (too much water) can manifest with edema (swelling of the limbs) or ascites (accumulation of fluids in the abdomen).

By contrast, people with hypovolemic hyponatremia will often have signs of dehydration from volume loss (including dry mouth, decreased skin elasticity, and orthostatic hypotension).

In severe cases where sodium levels drop below 120 mEq/L, seizure and coma may occur along with acute encephalitis (brain swelling), brain damage, and even death.

Diagnosis

The diagnosis of hyponatremia involves a medical history, physical examination, and various laboratory tests.

Physical Examination

In addition to reviewing your symptoms and medical history, your doctor will perform a physical exam to check for dehydration and edema. A simple, in-office neurological exam may also be performed.

Laboratory Tests

The sodium level can be measured within a simple blood test, called a basic or complete metabolic panel. In addition, the following tests may be ordered to make a diagnosis of hyponatremia:

Other laboratory tests may be ordered to determine the underlying cause of hyponatremia. This may include a thyroid-stimulating hormone (TSH) test, cortisol test, or adrenocorticotropic hormone (ACTH) stim test.

Treatment

The treatment of hyponatremia is complex and varies based on the underlying cause and other factors, such as:

  • The presence or absence of symptoms
  • The severity of the sodium loss
  • Whether hyponatremia is acute or chronic
  • Your volume status

For example, for hypervolemic hyponatremia caused by heart failure or cirrhosis, treatment may involve the restriction of salt and water as well as the use of diuretics to remove excess fluid. On the other hand, for hypovolemic hyponatremia from severe vomiting or diarrhea, a saline solution may be delivered intravenously (into a vein).

Other therapies may include:

  • Taking salt tablets or increasing salt intake
  • Discontinuing or cutting back on medications that cause hyponatremia
  • Taking medications called vasopressin antagonists (vaptans) that help treat hyponatremia in people with heart failure, cirrhosis or SIADH

It is important to treat hyponatremia promptly and appropriately. People hospitalized for hyponatremia are at an increased risk of severe illness and death. Hyponatremia also affects between 10% and 30% of people admitted to hospital, according to a 2016 review in the European Journal of Internal Medicine.

A Word From Verywell

Hyponatremia is a common electrolyte condition that varies considerably in severity and etiology. Being knowledgeable of the potential symptoms and causes is a key first step to understanding this disorder. If you are concerned about your sodium level. be sure to discuss it with your doctor.

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