Hypernatremia

  Have you ever had a doctor say you need to watch your sodium intake and drink plenty of water? The ratio of sodium to water in your body is critical for many of life's processes. If the amount of sodium gets too high, a condition called hypernatremia can result and the consequences can be deadly. Before we can discuss what's so bad about having too much sodium, let's first explore why sodium is so important.

Sodium and Water:

  Like all the electrolytes in the body, sodium exists in its charged form, Na+. Sodium is the most abundant positively-charged electrolyte in the extracellular fluid (ECF), which surrounds our cells and tissues. This is because cells actively export Na+ into the ECF using a sodium-potassium pump (Na/K pump). The pump forces Na+ out of the cell and imports a K+, creating sodium and potassium gradients. These gradients are essential for nerve cells to transmit electrical signals. When the level of sodium changes in the ECF, nerve cells can't receive or relay their messages. The Na/K pump is so important to live that approximately 10% of all the calories you eat go to maintaining these pumps in your cells!

   Another important characteristic of sodium is that it controls the movement of water throughout your body. Water follows the movement of ions through the various compartments of the body (ECF, blood, inside cells, urine, etc.). Since there is so much sodium in our body, it controls the movement of water more than any other electrolyte. If there is too much sodium in the ECF, water will leave the cells, causing the cells to shrink. If sodium was Mary, then water would be her little lamb!

 Hypernatremia:

    Too Much Sodium and Not Enough Water

Ions and water move relatively freely from the ECF to the serum, the liquid part of the blood. So, the concentration of Na+ in the ECF and blood is approximately the same. The concentration of Na+ in our blood ranges from 135-145 mEq/L.

  Hypernatremia is an abnormally high sodium concentration in the ECF and serum. This is in contrast to hyponatremia, which is an abnormally low concentration of sodium. The Latin term for sodium is natrium, which is why the symbol for sodium on the periodic table is Na. So remember - the 'na' in hypernatremia stands for sodium. Hypernatremia occurs when serum values are greater than 157 mEq/L. To remember the difference between the prefixes hyper- and hypo-, visualize a hyper child that has had too much sugar. Another way is to remember the difference is that hypo- ends in an 'O' which looks a lot like a zero (0).

Under normal conditions (left) the concentration of sodium ions (green circles) is higher in the ECF than in the cytoplasm (CP), but the movement of water into the cell balances the movement out of the cell. In hypernatremia (right), the Na+ concentration in the ECF is even higher. This leads to an increased sodium gradient, which pulls water out of the cell causing it to shrink.

Causes of Hypernatremia:

 The main source of sodium is the food we eat and beverages we drink. The main way we lose sodium is through urine made by the kidneys.

A typical western diet contains 25 times more sodium than we actually need. Thank goodness it's really hard to develop hypernatremia just by eating too much salt! There have been cases where someone has died from drinking salt water or eating excessive amounts of salt, but it's very rare. This is because our kidneys are really good at removing extra sodium from the blood.

The body uses several hormones to regulate how the kidneys retain or excrete sodium and water. Although the details are beyond the scope of the lesson, it is important to know that if there is too much or too little of these hormones, it can lead to hyper- or hyponatremia.

Surprisingly, hypernatremia is not really a result of too much sodium. It's typically caused by not having enough water. Remember that the sodium concentration is a ratio of sodium to the water. So if the amount of Na+ goes up or the amount of water goes down, hypernatremia can occur. The most common causes are:

Signs and symptoms:

  The major symptom is thirst. The most important signs result from brain cell shrinkage and include confusion, muscle twitching or spasms. With severe elevations, seizures and comas may occur.

  Severe symptoms are usually due to acute elevation of the plasma sodium concentration to above 157 mmol/L (normal blood levels are generally about 135–145 mmol/L for adults and elderly). Values above 180 mmol/L are associated with a high mortality rate, particularly in adults. However, such high levels of sodium rarely occur without severe coexisting medical conditions. Serum sodium concentrations have ranged from 150–228 mmol/L in survivors of acute salt overdosage, while levels of 153–255 mmol/L have been observed in fatalities. Vitreous humor is considered to be a better postmortem specimen than postmortem serum for assessing sodium involvement in a death.

Cause:

Common causes of hypernatremia include:

Low volume;

In those with low volume or hypovolemia:

• Inadequate intake of free water associated with total body sodium depletion. Typically in elderly or otherwise disabled patients who are unable to take in water as their thirst dictates and also are sodium depleted. This is the most common cause of hypernatremia.
• Excessive losses of water from the urinary tract – which may be caused by glycosuria,  or other osmotic diuretics (e.g., mannitol) – leads to a combination of sodium and free water losses.
• Water losses associated with extreme sweating.
• Severe watery diarrhea (Osmotic diarrhea results in hypotonic (dilute) watery diarrhea resulting in significant loss of free water and a higher concentration of sodium in the blood; this type of water loss can also be seen with viral gastroenteritis).

Normal volume:

In those with normal volume or euvolemia:

• Excessive excretion of water from the kidneys caused by diabetes insipidus, which involves either inadequate production of the hormone vasopressin, from the pituitary gland or impaired responsiveness of the kidneys to vasopressin.

High volume:

In those with high volume or hypervolemia:

• Intake of a hypertonic fluid (a fluid with a higher concentration of solutes than the remainder of the body) with restricted free water intake. This is relatively uncommon, though it can occur after a vigorous resuscitation where a patient receives a large volume of a concentrated sodium bicarbonate solution. Ingesting seawater also causes hypernatremia because seawater is hypertonic and free water is not available. There are several recorded cases of forced ingestion of concentrated salt solution in exorcism rituals leading to death.
• Mineralocorticoid excess due to a disease state such as Conn's syndrome usually does not lead to hypernatremia unless free water intake is restricted.
• Salt poisoning is the most common cause in children. It has also been seen in a number of adults with mental health problems. Too much salt can also occur from drinking seawater or soy sauce.