Definition and normal values
The term hypernatremia – or hypernatremia – identifies an increase in sodium concentrations in the blood, above what are considered normal values:
- normal sodium or natremia: 135-145 mmol/L*
- hyponatremia: < 136 mmol/L
- hypernatremia: > 146mmol/L
- mild hypernatremia < 155 mEq/L
- severe hypernatremia > 155 mEq/L
* millimoles of sodium per liter of blood, also expressed in mEq/L
To understand… role of sodium in the body
Sodium represents the main electrolyte of the extracellular fluid: 90% of the total body sodium is contained in the extracellular compartment, thanks to the action of the Na+ – K+ ATPase enzyme (which actively transports sodium out of the cell).
Sodium is an important regulator of plasma and extracellular fluid osmolarity. When the sodium concentration exceeds the normal range (excess sodium → hypernatremia) there is a more or less significant increase in the volume of blood and interstitial fluid, laying the foundations for creating edema and hypertension. At the same time, intracellular water is reduced and the cell “shrivels” (intracellular dehydration).
Furthermore, sodium is involved in the transmission of nervous impulses, in cellular exchange and in muscle contraction: from this, it is clear how a condition of hypernatremia can upset all these functions that the organism must fulfill.
- NOTE: Since sodium is freely distributed between plasma and interstitial fluid, the concentration of sodium in the blood is the same as that of extracellular fluids. In other words, if sodium in the blood increases, the concentration of sodium in the extracellular spaces also increases.
- Since the cell membrane is freely permeable to water, when the sodium concentration increases in the extracellular component, then water moves from the intracellular to the extracellular compartment to re-establish the osmotic balance.
- To stem the movement of water from the intra to the extracellular compartment it is necessary to increase the blood volume to dilute the sodium, and at the same time increase the urinary loss of the mineral.
- Ultimately, plasma sodium concentration is an indicator of intracellular volume status, therefore hyponatremia means cellular hyperhydration While hypernatremia means cellular dehydration.
Hypernatremia is a very frequent laboratory finding in clinical practice, although, fortunately, in the majority of cases it does not reach very high levels of hypernatremia. The latter, in fact, are particularly dangerous and lethal in a good percentage of cases.
Hypernatremia, in general, is not caused by an excess of sodium, but by a relative deficit of body water which leads to a water depletion of the blood with concentration of the mineral. In some cases of hypernatremia the amount of sodium in the blood is even lower than normal, but the blood volume is reduced to the point of creating hypernatremia.
Under normal conditions, even a modest increase in sodium levels above the basic threshold causes the stimulation of thirst; the consequent intake of water leads to the correction of sodium levels.
Hypernatremia is more common among children and the sick (who depend on others for water supplies), among the elderly (reduced effectiveness of the thirst mechanism), among subjects with an altered mental state and among those who do not care to the daily water intake by exaggerating the sodium intake. Hypernatremia is generally worsened by those pathologies that cause a loss of fluids, such as diarrhea or vomiting, and by infections in general.
An increase in sodium levels in the blood can therefore occur due to:
- true (absolute) increase in sodium:
- False (relative) increase in sodium due to decrease in body water:
- insufficient intake of pure water with the diet (common cause) → euvolemia or slight hypovolemia
- loss of water and hypotonic fluids (dehydration) → hypovolemia
In the first case, the absolute blood quantity of sodium increases (primary hypernatremia), while in the second the blood sodium increases only in relative terms (it is quantitatively the same or even lower, but as the blood volume and body water are lower, it is more concentrated).