What is Hyponatremia (Low Sodium)?
Sodium and water levels in the body are tightly regulated to keep it functioning normally. Sodium concentration is higher in the bloodstream than inside cells. Regulatory mechanisms help control and maintain sodium levels. The hormones aldosterone (made in the adrenal gland) and anti-diuretic hormone (ADH) or vasopressin (made in the pituitary) adjust the way the kidneys deal with water and sodium to maintain the appropriate total amount of sodium and water in the body.
Water in the body is closely linked to the location of sodium in the body. If the concentration of sodium is too high in the bloodstream, water will leak from cells into the blood stream to try to dilute and lower the sodium concentration. Conversely, if sodium levels in the bloodstream are too low, water will leave the blood and enter cells, causing them to swell.
Hyponatremia is the term used to describe low sodium levels in the bloodstream (hypo=low + natr=sodium + emia=blood). Acute hyponatremia describes the situation in which sodium levels drop quickly, while chronic hyponatremia describes situations with a gradual fall in the sodium concentrations over days or weeks. Chronic hyponatremia is often well tolerated since the body has a chance to adapt.
Neurologic changes are the most concerning consequence of hyponatremia. Cerebral edema (excess fluid in the brain, leading to swelling) may occur with severe or acute hyponatremia. Water enters the brain cells causing them to swell. Because the brain is enclosed in a bony skull that cannot expand, the brain is compressed since there is no room for swelling to occur. As a result, brain function may be compromised significantly.
What Causes Hyponatremia?
Hyponatremia occurs because of an imbalance of water and sodium. Most frequently it occurs when excessive water dilutes the amount of sodium in the body or when not enough total sodium is present in the body. A common classification of hyponatremia is based on the amount of total body water that is present.
Normal volume (euvolemic) hyponatremia
The amount of water in the body is normal, but an anti-diuretic hormone is being inappropriately secreted (SIADH =syndrome of inappropriate ADH secretion) from the pituitary gland. This may be seen in patients with pneumonia, small cell lung cancer, bleeding in the brain, or brain tumors
Excess volume (hypervolemic) hyponatremia
Too much total body water dilutes the amount of sodium contained in the body. This can be seen in heart failure, kidney failure, and liver diseases like cirrhosis. This situation is somewhat misnamed because while there is increased total body water, there may be a relative decrease of fluid within the bloodstream. Because of the underlying disease, fluid leaks into the space between tissues (called the third space) causing swelling of the extremities or ascites, fluid within the abdominal cavity.
Inadequate volume (hypovolemic) hyponatremia
The amount of water in the body is too low as can occur in dehydration. The anti-diuretic hormone is stimulated, causing the kidneys to make very concentrated urine and hold onto water. This may be seen with excessive sweating and exercising in a hot environment. It can also occur in patients with excess fluid loss due to vomiting and diarrhea, pancreatitis, and burns.
- Hyponatremia may be a side effect of medications, especially diuretics or water pills used to help control blood pressure. This class of drugs can cause excessive loss of sodium in the urine.
- Hormonal diseases such as Addison's disease or adrenal insufficiency and hypothyroidism may be associated with low sodium levels.
- Polydipsia, or excessive water intake, may cause "water intoxication," diluting sodium levels. This is occasionally associated withpsychiatric illness.
- In some people who exercise, their concern about the potential for dehydration causes them to drink more water than they lose by perspiration. This may cause significant hyponatremia and has been known to be fatal in marathon participants who drink too much fluid without replacing lost sodium, in excess of what their thirst mechanism dictates.
- In infants, hyponatremia may occur if the baby is fed tap water instead of formula or a balanced electrolyte solution like Pedialyte.
- Drug overdose with MDMA (ecstasy) is associated with hyponatremia.
Medically Reviewed by a Doctor on 10/19/2016
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