Hyponatraemia

Question 1

Hyponatraemia is defined as serum sodium

Answer 1

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

Question 2

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 … and ‘…’

Answer 2

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’.

Question 3

Tissue oedema can impair the … of tissues, and this can be particularly troublesome in surgical wounds or anastomoses. Furthermore, in the brain this can result in … and raised …. Thus, profound hyponatraemia may even result in significant cerebral dysfunction.

Answer 3

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.

Question 4

Profound hyponatraemia may even result in significant cerebral dysfunction - why?

Answer 4

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.

Question 5

Causes of hypo-osmotic hyponatraemia can be classified in terms of the patient’s extracellular fluid status and urine sodium concentration.

Answer 5

Question 6

Urine [Na+] <20mmol - Hypovolaemia causes

Answer 6

Vomiting or Diarrhoea

Question 7

Urine [Na+] >20mmol - Hypovolaemia causes

Answer 7

Diuretics

Question 8

Urine [Na+] >20mmol - Euvolaemia causes

Answer 8

SIADH

Question 9

Urine [Na+] <20mmol - Euvolaemia causes

Answer 9

Acute fluid overload

Question 10

Urine [Na+] <20mmol - Hypervolaemia causes

Answer 10

Congestive Cardiac Failure or Liver Cirrhosis*

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

Question 11

Urine [Na+] >20mmol - Hypervolaemia causes

Answer 11

Acute Tubular Necrosis

Question 12

The first step when investigating hyponatraemia should be to measure what?

Answer 12

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).

Question 13

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 …

Answer 13

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

Question 14

Post-Operative Hyponatraemia

Answer 14

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.

Question 15

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 … signs

Answer 15

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.

Question 16

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

Answer 16

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.

Question 17

Central Pontine Myelinolysis - what is this?

Answer 17

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.

Question 18

Post-operative hyponatraemia requires careful …

Answer 18

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.

Question 19

… are generally advised over … 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.

Answer 19

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.

Question 20

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

Answer 20

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

Question 21

Key Points - hyponatraemia

Answer 21

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