Hyponatraemia is defined as serum sodium <135mmol and is the most common post-operative electrolyte abnormality.
Sodium is a large contributor to plasma osmolality, with derangements in osmolality causing fluid shifts across cell membranes. In hypo-osmotic hyponatraemia, low plasma osmolality can result in fluid shifts occurring intracellularly, causing cellular swelling and ‘oedema’.
Tissue oedema can impair the healing of tissues, and this can be particularly troublesome in surgical wounds or anastomoses. Furthermore, in the brain this can result in cerebral oedema and raised intra-cranial pressure. Thus, profound hyponatraemia may even result in significant cerebral dysfunction.
Aetiology
Causes of hypo-osmotic 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.
In fact, the first step when investigating hyponatraemia should be to measure serum osmolality, to see whether it is hypo-osmotic, iso-osmotic or hyper-osmotic. However, in clinical practice, most cases* of hyponatraemia encountered will be hypo-osmotic hyponatraemia, whereby there is either sodium depletion or water excess (or both).
*Iso-osmotic hyponatraemia can occur when there are extra-ordinarily high blood levels of lipids or protein; this increases the non-aqueous component of serum and reduces the relative aqueous component of serum (to which the sodium is confined), and is termed pseudo-hyponatraemia
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. For those who are symptomatic, symptoms are most commonly related to their volume status, depending on the aetiology of their hyponatraemia, rather than the hyponatraemia alone.
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.
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.
Intravenous fluids (such as 0.9% sodium chloride) 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.
Key Points
- Hyponatraemia is defined as serum sodium <135mmol/l
- Most cases of hyponatraemia are asymptomatic, however profound hyponatraemia may result in significant cerebral dysfunction
- Monitor fluid balance and replace sodium losses parenterally in severe cases, to ensure greater control