Hyponatraemia
Contents
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 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