The term hyponatremia refers to a condition where the blood sodium level is too low. A healthy sodium level is between 135 and 145 mmol/l and a person is considered to be hyponatremic if the level falls to below 135 mmol/l. The word hyponatremia is made up of two words – “hypo” meaning low and “natrium,” the Latin name for sodium.
Hyponatremia may be brought on by either a deficiency of sodium or an excess of water that dilutes the level of sodium. Most cases are caused by an excess of water, rather than by a loss of sodium.
Sodium helps to regulate the amount of water contained in and around the body’s cells. This helps to maintain normal blood pressure and support cell function. When the sodium level is low, the body’s water level rises causing the cells to swell, which can lead to a number of problems that range from mild to severe. Swelling in the brain is particularly dangerous, as the brain is confined to the skull and cannot swell without causing complications.
Types of hyponatremia
Hyponatremia is classified according to the extracellular fluid volume status, which can affect the balance of sodium and water in three main ways:
In hypovolemia, the levels of both water and sodium are too low. This can be caused by exercising in the heat without drinking enough water to replenish the electrolyte level. Excessive blood loss can also cause this form of hyponatremia.
Here, the blood water level is too high, which can be caused by drinking too much water or the presence of chronic health conditions such as cancer. The use of certain medications can also cause this form of hyponatremia.
Hypervolemic hyponatremia occurs when the level of water in the body is too high, which is often caused by kidney failure, liver failure or heart failure.
Some examples of medications that can lead to hyponatremia include:
- Diuretics such as thiazides, and thiazide-like diuretics
- Anti-epileptic drugs such as carbamazepine
- Blood sugar reducing medications such as sulphonylureas (but not gliclazide)
- Proton pump inhibitors such as omeprazole and rabeprazole
- Antidepressants, particularly selective serotnonin reuptake inhibitors (SSRIs) such as fluoxetine, paroxetine.
- ACE inhibitors such as enalapril
- Opiates such as morphine and codeine
Reviewed by Sally Robertson, BSc