Hyponatremia is the low concentration of sodium in plasma, which is the fluid that surrounds the red blood cells (and all the other cells of the blood). It is a very common condition that affects sick people and is very commonly seen in hospitalized patients. There are various decrees of hyponatremia from very mild to very severe and can cause no symptoms or can cause death. When hyponatremia is diagnosed it is usually described as “low sodium in the blood”. As we are going to see, this can be misleading because hyponatremia, for the most part a disorder that reflects a water excess.
The normal concentration of sodium in the plasma ranges between 135 mEq/L and 145 mEq/L. Any value below the lower limit of that range is considered hyponatremia. There are multiple causes of hyponatremia (as we are going to see) and they usually reflect underlying processes occuring in the body. The regulation of the sodium concentration is managed by several hormones, neurotransmitters, the mechanism of thirst and molecular receptors in the kidneys.
There are many ways of treating hyponatremia. They can be as simple as controlling the amount of water that the patient drinks to the management of conditions such as cirrhosis of the liver and chronic heart failure. Several drugs can be used for the treatment of hyponatremia and we will mention them and explain their mechanism of action.
The importance of hyponatremia relies in how it affects the central nervous system. A hyponatremic state has to be regarded as a state of water excess. This water increases the size of the cells, and since the neurons of the brain exist in an encased environment by the cranium, their function is definitely affected by the increase in intracranial pressure.
Marco A. Ramos MD