Causes of increased sodium in the blood (hypernatremia)

Hypernatremia is always associated with hyperosmolarity. When plasma osmolarity becomes higher than 290 mOsm/l, an increase in the secretion of antidiuretic hormone by the posterior pituitary gland is observed. A decrease in the volume of extracellular fluid enhances this reaction, while an increase can weaken it. The kidneys' reaction to antidiuretic hormone is aimed at preserving free water in the body and consists of a decrease in diuresis.

Causes of hypernatremia (serum sodium concentration above 150 mmol/L):

• dehydration due to water exhaustion (increased water loss through the respiratory tract during shortness of breath, fever, tracheostomy, artificial ventilation of the lungs under conditions of insufficient humidification of the breathing mixture, use of non-humidified oxygen, open treatment of burns, prolonged sweating without appropriate water compensation); it is generally accepted that an excess of every 3 mmol/l of sodium in the serum over 145 mmol/l means a deficit of 1 liter of extracellular water;
• salt overload of the body (tube feeding with concentrated mixtures without the appropriate introduction of water during prolonged unconsciousness, after brain surgery, due to esophageal obstruction, when feeding through a gastrostomy);
• diabetes insipidus (decreased sensitivity of kidney receptors to antidiuretic hormone);
• renal diseases accompanied by oliguria;
• hyperaldosteronism (excessive secretion of aldosterone by an adenoma or tumor of the adrenal glands).

Preferential losses of water compared to sodium lead to an increase in plasma osmolarity and sodium concentration; due to a decrease in circulating blood volume, renal blood flow decreases and the formation of aldosterone is stimulated, which leads to sodium retention in the body. At the same time, hyperosmolarity stimulates the secretion of antidiuretic hormone and reduces the excretion of water in the urine. Depletion of water reserves is quickly restored if the body receives a sufficient amount of water.

Depending on the water balance disturbances that always accompany hypernatremia, the following forms are distinguished:

• hypovolemic hypernatremia;
• euvolemic (normovolemic) hypernatremia;
• hypervolemic hypernatremia.

Hypovolemic hypernatremia may result from water loss that exceeds sodium loss. Sodium loss in any body fluid other than intestinal and pancreatic juice results in hypernatremia (total body sodium decreases). Consequences of hypotonic fluid loss include hypovolemia (caused by sodium loss) and increased osmotic pressure of body fluids (due to loss of free fluid). Hypovolemia is a serious complication that can lead to hypovolemic shock.

Euvolemic hypernatremia occurs in diabetes insipidus and water losses through the skin and respiratory tract. Water losses without sodium losses do not lead to a decrease in intravascular fluid volume. In addition, hypernatremia does not develop unless the patient's water intake is reduced.

There are two main types of excessive water diuresis (euvolemic hypernatremia): central diabetes insipidus and nephrogenic diabetes insipidus.

Most patients with progressive chronic kidney disease gradually lose their ability to concentrate urine. In chronic renal failure of any etiology, decreased sensitivity to antidiuretic hormone may develop, which is manifested by the excretion of hypotonic urine. When treating such patients who can still "form" urine, it is very important to remember that consuming a certain amount of fluid is necessary for them, since it allows them to influence daily osmotic clearance in a non-invasive way. Limiting fluid intake in such patients can lead to the development of hypovolemia.

Hypervolemic hypernatremia usually develops as a result of the administration of hypertonic solutions (eg, 3% sodium chloride solution), as well as correction of metabolic acidosis with intravenous infusions of sodium bicarbonate.

Clinical manifestations of hypernatremia as such are thirst, tremors, irritability, ataxia, muscle twitching, confusion, seizures, and coma. Symptoms are most pronounced when the serum sodium concentration rises sharply.