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Low salt or sodium in the blood (hyponatraemia)

What is hyponatraemia?

Although we're often urged to cut down on salt in our diet, salt or sodium is essential for many of the body's functions including fluid balance, blood pressure management and the nervous system.

Hyponatraemia is a low level of sodium or salt in the blood. Symptoms are not usually very specific and can include changes to a person's mental state, headache, nausea and vomiting, tiredness, muscle spasms and seizures.

Hyponatraemia may be caused by drinking too much water, for example during strenuous exercise, without adequate replacement of sodium.

It may also be due to a loss of sodium and body fluid.

Severe hyponatraemia can lead to coma and can be fatal.

Treatment of hyponatraemia involves intravenous fluid and electrolyte replacement, medicine to manage the symptoms of hyponatraemia, as well as any treatments for any underlying cause.

What causes hyponatraemia?

A low sodium level in the blood may result from excess water or fluid in the body, diluting the normal amount of sodium so that the concentration appears low. This type of hyponatraemia can be the result of chronic conditions such as kidney failure (when excess fluid cannot be efficiently excreted) and congestive heart failure, in which excess fluid accumulates in the body. SIADH (syndrome of inappropriate anti-diuretic hormone) is a disease whereby the body produces too much anti-diuretic hormone (ADH), resulting in retention of water in the body.

Hyponatraemia can also result when sodium is lost from the body or when both sodium and fluid are lost from the body - for example, during prolonged sweating and severe vomiting or diarrhoea.

Medical conditions that can sometimes be associated with hyponatraemia are adrenal insufficiency, hypothyroidism, and cirrhosis of the liver. The eating disorder anorexia can also cause a sodium imbalance.

Some medicines can lower blood sodium levels. Examples of these include diuretics (water tablets), desmopressin, and sulfonylureas.

What are the symptoms of hyponatremia (low blood sodium)?

When sodium levels in the body are low, water tends to enter cells, causing them to swell. When this occurs in the brain, it is referred to as cerebral oedema. Cerebral oedema is particularly dangerous because the brain is confined in the skull without room for expansion, and the swelling can lead to brain damage as the pressure increases within the skull.

In chronic hyponatraemia, in which the blood sodium levels drop gradually over time, symptoms are typically less severe than with acute hyponatraemia (a sudden drop in blood sodium level). Symptoms can be very nonspecific and can include:

  • Headache
  • Confusion or altered mental state
  • Seizures
  • Decreased consciousness which can proceed to coma and death.

Other possible symptoms include:

Nausea and vomiting may accompany any of the symptoms.

How is hyponatraemia diagnosed?

The symptoms of hyponatraemia are not specific, so a blood test measuring the sodium level is needed to confirm the diagnosis of hyponatraemia. Sometimes the medical history (such as prolonged vomiting or excessive sweating) will help a doctor with the diagnosis. In other cases, further blood tests, urine tests, and imaging studies may be needed in order to determine the exact cause of the hyponatraemia.

How is hyponatraemia treated?

Mild chronic hyponatraemia may not require treatment other than adjustments in diet, lifestyle or medication. For severe or acute hyponatraemia, treatment often involves intravenous (IV or drip) fluids and electrolytes. In this case medication is often needed to treat the underlying cause of the hyponatraemia as well as medication to manage the accompanying symptoms.

WebMD Medical Reference

Medically Reviewed by Dr Rob Hicks on May 14, 2014

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