Hyponatremia (Low Blood Sodium)

What is hyponatremia?

Hyponatremia is a condition where the sodium (Na) levels in the blood are lower than normal. Sodium, often referred to a salt and associated with table salt (sodium chloride), is an important electrolyte in the body. It is not the only electrolyte, and therefore not the only salt. Low blood sodium is a fairly common abnormality which can easily be corrected but is dangerous in severe cases. Acute hyponatremia may be seen with a range of conditions, but is more common as a consequence of persistent vomiting and diarrhea as part of dehydration  It is when hyponatremia arises with chronic conditions and the long term use of certain medication that there is a greater risk of serious complications and even death.

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Hyponatremia Pathophysiology

Substances that dissolve in fluid are known as solutes. Sodium is one of the many solutes in the human body. This important electrolyte has several functions in the body, including its role in maintaining blood volume, blood pressure, activity of nerves and muscle action. There are two ways in which the blood sodium levels can be lower than normal :

  • if the fluid volume increases while the sodium levels stay within the normal quantity.
  • if the sodium levels drop while the fluid volume remains within the normal range.
  • if there is an increase of both sodium and water, but the sodium rise is not as much as the increase in water.

Sometimes the shift in water from within the blood vessel into the tissue spaces also causes hyponatremia although the total body water (TBW) level is normal. In an attempt to restore the sodium levels within the normal range, the water output may increase or salts from inside the cell are gradually lost to the surrounding tissue fluid.

Hyponatremia Sodium Levels

The sodium levels in the tissue fluid around cells and in the bloodstream range between 135 to 145 mEq/L (milliequivalents per liter).

  • Hyponatremia = blood sodium levels below 135 mEq/L.
  • Severe hyponatremia = blood sodium level below 125 mEq/L.

Sometimes hyponatremia may arise with the body excreting too much sodium. Therefore the sodium levels in the urine may be higher than 20 to 40 mEq/L.

Hyponatremia Types

There are several types of hyponatremia which is classified according the the differences in the solute concentration in the fluid outside of cells. This is known as osmolality. It is clinically useful in identifying the most likely causes. Sometimes the sodium levels are normal but other chemical factors alter the blood serum readings.

Hypertonic hyponatremia

Hypertonic hyponatremia is where certain substances in the blood causes water to be drawn out of the cells thereby raising the extracellular fluid volume. This may be seen with high levels of glucose and mannitol in the blood.

Normotonic hyponatremia

Also known as pseudohyponatremia, this type of hyponatremia occurs when there is a drop in the water content in the blood due to the presence of fats and proteins.

Hypotonic hyponatremia

This is the more common type of hyponatremia. There several different forms such as :

  • Hypovolemic hypotonic hyponatremia where there is loss of water and sodium although more sodium is lost than water.
  • Hypervolemic hypotonic hyponatremia where the total body water volume is high and so is the sodium levels but not elevated to the degree of TBW.
  • Normovolemic (euvolemic) hypotonic hyponatremia where the water volume is high but sodium levels are at near-normal levels.

Hyponatremia Causes

Hypovolemic hypotonic hyponatremia

  • Burns
  • Diarrhea
  • Diuretics
  • High blood levels of glucose, urea and mannitol
  • Kidney diseases such as interstitial nephritis, partial urinary tract obstruction and polycystic kidney disease
  • Mineralocorticoid deficiency
  • Pancreatitis
  • Peritonitis
  • Rhabdomyolysis
  • Small intestine obstruction
  • Vomiting

Hypervolemic hypotonic hyponatremia

  • Acute kidney dysfunction
  • Cirrhosis
  • Chronic kidney disease
  • Heart failure
  • Nephrotic syndrome

Euvolemic hypotonic hyponatremia

  • Adrenal insufficiency as in Addison’s disease
  • Emotional stress
  • Hypothyroidism
  • Medication such as diuretics, barbiturates, opioids, NSAIDs, oxytocin
  • Pain
  • Postoperative states
  • Primary polydipsia
  • Syndrome of inappropriate ADH secretion (SIADH)

Hyponatremia Symptoms

The symptoms depends on the type of hyponatremia. Often in mild hyponatremia it is the symptoms of the underlying causes that may be more prominent. Some of the symptoms that may be seen, particularly with severe hyponatremia, include :

  • Changes in personality
  • Confusion
  • Coma
  • Headaches
  • Lethargy
  • Nausea
  • Muscle weakness
  • Seizures
  • Stupor
  • Twitching and cramps
  • Vomiting (sometimes)

Hyponatremia Diagnosis

The symptoms of hyponatremia are not specific for the condition and often occurs in some of the causative conditions. Blood and urine tests are therefore essential to make the diagnosis of hyponatremia. These tests include :

  • Blood osmolality
  • Urine osmolality and sodium concentration

Other tests are helpful to determine the cause of hyponatremia but not confirm the diagnosis of the condition. This includes :

  • Thyroid profile (thyroid hormones and thyroid-stimulating hormone)
  • Blood albumin and triglyceride levels
  • Blood uric acid levels
  • Chest x-ray
  • Computed tomography (CT) scan

Hyponatremia Treatment

Conservative measures may sometimes be sufficient without the need for any therapeutic measures. In cases of hyponatremia induced by medication, simply stopping the drug may see the hyponatremia resolve within a few days.

Fluids

The treatment options may vary depending on the type of hyponatremia. Intravenous (IV) fluid or fluid restriction may be necessary.

  • Hypervolemic hyponatremia – salt and fluid restriction.
  • Hypovolemic hyponatermia –  IV saline.

Fluid restriction may also be advisable for euvolemic hyponatremia but the abnormality is usually unlikely to resolve without other treatment measures. Furthermore patients do not often adhere to fluid restriction.

Medication

Drugs may be needed to treat the symptoms associated with hyponatremia until the blood sodium levels return to normal.

  • Loop diuretics aid with free water loss in patients with increased total body water (TBW).
  • Arginine-vasopressin blockers (antagonists) prevents the body for conserving water.
  • Certain antibiotics may be able to disrupt vasopressin and allow for more water to be passed out.

Hyponatremia Diet

Fluid restriction may be necessary but this should first be discussed with a doctor after a complete assessment. Attempting to increase the salt intake in the diet suddenly to treat hyponatremia is not recommended. Hyponatremia needs to be treated medically by a doctor.However, patients with various diseases as mentioned under the causes should be aware of the risk of developing hyponatremia and try to prevent it before it arises.

Hyponatremia Prevention

Preventative measures are more important for patients with diseases that may lead to hyponatremia.

  • Avoid the consumption of excess amounts of water on its own. Oral rehydration solutions are preferable to prevent dehydration in instances of diarrhea and vomiting.
  • Moderate the fluid intake and opt for sports drinks or other fluids with electrolytes during strenuous activity and in very hot weather. Oral rehydrating solutions are often the better choice although it may not be as palatable as sports drinks.
  • Never use diuretics (“water pills”) unless it is prescribed by a medical doctor. Use the medication as prescribed and ensure adequate water intake as instructed by the doctor.

References :

http://emedicine.medscape.com/article/242166-overview

http://www.mayoclinic.com/health/hyponatremia/DS00974

http://www.merckmanuals.com/professional/endocrine_and_metabolic_disorders/electrolyte_disorders/hyponatremia.html

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