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A human body contains 50 to 70% water, of which about 2 thirds is located inside the cells, the other one third is in the extracellular fluid and blood plasma. Water can move freely between different compartments in the body, but its direction is determined by which compartment has more solutes, or higher osmolality. As a rule, water moves from the more diluted solution to the more concentrated solution – from lower to higher osmolality.
Sodium, being the major extracellular solute, is the principal determinant of plasma osmolality and the most important regulator of fluid balance. A normal blood sodium level is kept between 135 and 145 mmol/L. Hyponatremia occurs when blood sodium falls below 135, while hypernatremia is when it exceeds 145.
Clinical manifestations of sodium disorders reflect disturbances in water movement in the most sensitive organ of the body – the brain. In hypernatremia, high blood sodium levels draw water out of the brain cells, causing dehydration and shrinkage. Whereas in hyponatremia, low concentrations of plasma sodium drive water into brain cells, making them swell, causing edema. Both situations produce neurologic symptoms, which can range from headache, confusion, to seizures, coma or even death.
Hypernatremia most often occurs because of inadequate water intake, or excessive water loss or excretion. Water intake is regulated by thirst. When a decreased body fluid volume or an increased plasma osmolality is detected, the brain perceives it as thirst and produces water-seeking behavior. Impaired thirst mechanism is a common cause of hypernatremia in the elderly.
The body loses water primarily by excreting it in urine. Water excretion by the kidneys is mainly regulated by vasopressin, a hypothalamic hormone that causes the kidneys to retain water in response to low blood volume or high plasma osmolality. Impaired vasopressin release, renal dysfunction, and use of certain diuretics, are common causes of excessive water loss through the kidneys.
Fluid loss through the digestive tract is normally negligible, but can be substantial in vomiting or diarrhea. Sweat loss though skin can be significant in extreme heat or during excessive exercise.
Chronic hypernatremia is treated with oral hypotonic fluids, while acute or severe hypernatremia may require intravenous administration along with constant monitoring to avoid overcorrection. The underlying cause must also be addressed.
For hyponatremia, treatment depends on the body fluid volume:
– In low volume, or hypovolemic hyponatremia, both sodium and water levels decrease, but sodium loss is relatively greater. This commonly occurs due to loss of sodium-containing fluids, as in vomiting and diarrhea, especially when loses are replaced with plain water. This type is managed by rehydration with isotonic fluids.
– In high volume, or hypervolemic hyponatremia, both sodium and water levels increase, with a relatively greater increase in body water. This often results from fluid retention in conditions such as heart failure, liver cirrhosis, or kidney failure; and is usually treated with diuresis.
– In normal volume, or euvolemic hyponatremia, sodium level is normal, but there is an increase in total body water. This can be caused by excessive water intake combined with renal insufficiency, or syndrome of inappropriate ADH secretion, which causes the kidneys to retain more water. This type is managed by restricting free water intake and addressing the underlying cause.
Premenopausal women are more susceptible to acute hyponatremia with severe brain edema, perhaps because female hormones increase vasopressin level, and inhibit the brain sodium-potassium pump, which pumps sodium out of the cell and helps maintain normal brain volume.
Acute or symptomatic hyponatremia is an emergency and should be treated with intravenous hypertonic sodium chloride, but sodium levels must be closely monitored to avoid overly rapid correction.
