Hyponatraemia is defined as serum sodium <135mmol/l and is the most common post-operative electrolyte abnormality.

Sodium is a large contributor to plasma osmolality, with low plasma osmolality resulting in water moving intracellularly. In the brain, this can result in cerebral oedema and raised intra-cranial pressure. Thus, profound hyponatraemia may even result in significant cerebral dysfunction.

Low serum sodium can also result in significant tissue oedema that can impair the healing of tissue. This can be particularly troublesome in surgical wounds or anastomoses.


The causes of hyponatraemia can be classified in terms of the patient’s extracellular fluid status and urine sodium concentration.

Hypovolaemia Euvolaemia Hypervolaemia
Urine [Na+] <20mmol Vomiting or Diarrhoea Acute fluid overload Congestive Cardiac Failure or Liver Cirrhosis*
Urine [Na+] >20mmol Diuretics SIADH Acute Tubular Necrosis

*Beware many of these patients may also be on diuretics and consequently have a high urinary sodium concentration.

Post-Operative Hyponatraemia

Fluid retention is seen as part of stress response to surgery. There is an increase in hypothalamic-pituitary hormone secretion, resulting in increased cortisol and ADH release. The resulting free water reabsorption in excess of sodium results in a hyponatraemia.

In addition, surgical patients receive significant volumes of intravenous fluid during the perioperative period. If the fluid used is dextrose solution (especially if excessive or prolonged use), this will cause a dilutional effect to the body’s serum sodium levels.

Clinical Features

Most cases of hyponatraemia are asymptomatic. However, severe hyponatraemia can present with neurological signs, such as malaise, headache, and confusion, before progressing to reduced consciousness and seizures.

Central Pontine Myelinolysis

In individuals with a chronic hyponatraemia, rapid sodium correction can cause central pontine myelinolysis.

This is a neurological condition, whereby a large change in extracellular osmolarity causes damage to the myelin sheaths of the nerves of the brainstem. Patients can initially present with confusion and balance problems, before developing pseudobulbar palsy and quadriplegia.

It is mainly diagnosed via head MRI. There is no curative treatment therefore management is typically symptomatic. It can lead to serious long term disability.


Post-operative hyponatraemia requires careful fluid balance. Start close fluid monitoring, catheterising if necessary; this is particularly important during the intra-operative and post-operative period.

Intravenous fluid therapy (unless fluid overloaded) is generally advised over enteral hydration for the hyponatraemic patient, as they will provide a greater control to the serum electrolyte levels. Monitor renal function and electrolyte levels regularly, as potential derangement may occur during any fluid redistribution during your management.

If the cause is unknown or evidence of prolonged and marked hyponatraemia, urine osmolality and sodium concentration should be measured to inform additional diagnosis.

Key Points

  • Hyponatraemia is defined as serum sodium <135mmol/l
  • Most cases of hyponatraemia are symptomatic, however profound hyponatraemia may result in significant cerebral dysfunction
  • Monitor fluid balance and try to replace sodium losses parenterally, to ensure greater control


Question 1 / 5
Which of the following is a cause of hyponatraemia?


Question 2 / 5
Hyponatraemia is defined by a sodium level of...?


Question 3 / 5
Which neurological condition can occur as a complication of rapid correction of hyponatraemia?


Question 4 / 5
Administration of which fluid will cause a decrease in the serum sodium concentration?


Question 5 / 5
A patient has a serum sodium of 130mmol/L. He has signs of hypervolaemia. Which of the following causes is most likely?


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