Hyponatraemia

Perioperative electrolyte imbalances are common and can have a significant effect on patient recovery. The most common post-operative electrolyte abnormality is hyponatraemia – defined as serum sodium of less than 135mmol/l.

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 can cause significant cerebral dysfunction.

In addition, low serum sodium can result in significant tissue oedema that can impair the healing of tissues. This can be particularly troublesome in surgical wounds or anastomoses.


Aetiology

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

Diarrhoea

Acute fluid overload

 

Congestive Cardiac Failure*

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

Mild hyponatraemia is commonly asymptomatic.

Severe hyponatraemia can present with predominantly neurological signs, such as malaise, headache, and confusion, before progressing to reduced consciousness and seizures.


Management

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

IV fluids (0.9% sodium chloride or Hartmann’s) are 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.

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.

Fig 1 - Axial fat-saturated T2-weighted image showing hyperintensity in the pons with sparing of the peripheral fibers, the patient was an alcoholic admitted with a serum Na of 101 treated with hypertonic saline, he was left with quadreparesis, dysarthria, and altered mental status.

Fig 1 – Axial MRI showing hyperintensity in the pons, in a patient with a serum [Na] of 101 and treated with hypertonic saline; the patient subsequently was left with quadreparesis and dysarthria

Quiz

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

Quiz

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

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

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

Quiz

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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