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.

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.

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

Clinical Features

Most cases of hyponatraemia are symptomatic.

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.

By Jto410 [CC BY-SA 3.0], via Wikimedia Commons

Figure 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

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.