Hyponatraemia

Question 1

Define hyponatraemia

Answer 1

Na< 135mmol/l

Question 2

What does low plasma sodium result in?

Answer 2

Reduced plasma osmolality
Water moves to ICF comparmtent
In the brain this can result in cerebral oedema and raised ICP so profound hyponatraemia can result in significant cerebral dysfunction.
Also tissue oedema that can impair healing of tissues.

Question 3

What are causes of hyponatraemia?

Answer 3

Hypovolaemia - vomiting, diarrhoea if urine Na < 20mmol. - diuretics if urine Na >20mmol

Euvolaemia
Acute fluid overload if urine Na <20mmol
SIADH if urine Na >20mmol

Hypervolaemia
CCF/liver cirrhosis if urine Na <20mmol (may be higher due to on diuretics)
Acute tubular necrosis if urine Na > 20mmol

Question 4

What causes post op hyponatraemia?

Answer 4

Fluid retention is part of stress response to surgery
Increase in hypothalamic pituitary hormone secretio - increased cortisol and ADH release
Resulting free water reabsorption in excess of sodium results in hyponatraemia.
Surgical patientis also receive lots of IV fluids during pre-op period. If the fluid is dextrose, this will cause dilution effect to the body’s serum sodium levels

Question 5

What are the clinical features of hyponatraemia?

Answer 5

Mostly symptomatic

Severe - neurological signs such as malaise, headache, confusion, reduced consciousness, seizures

Question 6

What can occur if there is rapid sodium correction in chronic hyponatraemia?

Answer 6

Central pontine myelinolysis

Question 7

What is central pontine myelinolysis

Answer 7

Neurological condition where a large change in extracellular osmolarity causes damage to myelin sheaths of the nerves of the brainstem. Patients can initially present with confusion and balance problems before developing pseudo bulbar palsy and quadriplegia.
Diagnosed via head MRI
No curative treatment - symptomatic management

Question 8

How is post-op hyponatraemia managed?

Answer 8

Fluid balance chart - catherisation if necessary.
IV fluids - 0.9% NaCl or Hartmann’s are generally advised over enteral hydration as they will provide greater control to the serum electrolyte levels.
Monitor renal function and electrolyte levels regularly.
If cause is unknown, measure urine osmolality and sodium concentration to inform diagnosis.