What is Hyponatreamia?

Hyponatremia is a medical condition where the concentration of serum sodium is lower than 135 mmol/l. Serum sodium values are normally between 135 and 145 mmol/l.


A. Hyponatremia is classified based on the fluid balance in the body:

  1. Reduced fluid volume as a result of fluid loss via the kidneys, for example through diuretic use or gastrointestinal loss such as vomiting/diarrhoea. There is a combined loss of salt and water in this state, although salt depletion is much greater.
  2. Increased fluid volume due to heart failure, liver failure or renal failure. The amount of water in the body (extracellular) is significantly increased in relation to the amount of salt, which results in a relative hyponatremia.
  3. Neither clinical signs of reduced nor increased fluid volume, but a slightly increased water volume in the body (extracellular volume) without salt depletion. Often caused by SIADH, (refer to section “What is SIADH”), severe thyroid hormone or adrenal insufficiency.

B. Hyponatremia is also classified based on rate at which it develops: acute (in less than 48 hours) and chronic (in more than 48 hours).

Consequences of hyponatremia:

Hyponatremia is the most common electrolyte disorder amongst hospitalised patients, and is associated with a worsened prognosis. Severe hyponatremia (S-Na < 125 mmol/l) is associated with a prolonged hospital stay.


The degree of hyponatremia and how quickly the condition develops is decisive for prognosis and symptoms. Severe cases of hyponatremia develop acutely, and can cause brain oedema which leads to neurological symptoms such as convulsions, coma and can in some cases death. These patients require acute corrective treatment. The more common chronic hyponatremia is in most cases mild and can lead to symptoms such as memory disturbances and difficulty concentrating.

Type of Disorder Symtoms

Muscle convulsions
Loss of appetite

Mild symptoms

Motor disorder

Moderate symptoms
Epileptic seizures
Brain injury
Respiratory arrest
Severe symptoms

Adapted from the 2012 Swedish health care program for hyponatremia.


Acute hyponatremia occurs amongst hospitalised patients, and the most common cause can be provision of salt depleted fluids in connection with surgery.

Common causes of chronic hyponatremia are thiazide diuretics and SIADH (refer to section “What is SIADH”).

A combination of causes is more common amongst elderly patients with an increased number of underlying conditions, where drug treatments are often contributors.

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  2. http://www3.svls.se/sektioner/endokrin/documents/ Vardprogram_hyponatremi_110913.pdf
    Svenskt vårdprogram för hyponatremi 2012
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