Hyponatraemia Flashcards
what is hyponatraemia?
- occurs when there is a relative excess of water in the body compared to sodium
- is a serum sodium concentration <135mmol/L, with severe hyponatraemia being a serum sodium concentration <120mmol/L
- is further categorised into acute (<48h duration) or chronic hyponatraemia
what does urine sodium reveal about the kidneys in hypovolaemic hyponatraemia?
in a normally functioning kidney, sodium should be conserved in the urine if serum sodium levels are low:
- urinary sodium >20mmol/L suggests a renal cause of sodium loss (e.g. the kidney is not conserving sodium)
- urinary sodium <20mmol/L suggests a non-renal cause of sodium loss (e.g. the kidney is conserving sodium, but the sodium is being lost from elsewhere)
what are the renal causes of sodium loss (e.g. urine sodium >20mmol/L)?
- renal failure
- addison’s disease
- excess diuretic medications (e.g. thiazide-like diuretics)
- osmotic diuresis (e.g. hyperglycaemia)
what are the non-renal causes of sodium loss (e.g. urine sodium <20mmol/L)?
- gastrointestinal losses (e.g. vomiting, diarrhoea, SBO, fistulae)
- skin losses (e.g. sweating, burns)
what causes hyponatremia in a fluid-overloaded patient?
- fluid accumulates in the extracellular (‘third’) space
- this extra fluid causes a dilutional effect on serum sodium, causing hyponatraemia
what are the causes of hypervolaemic hyponatraemia?
- congestive cardiac failure
- liver cirrhosis
- ESRF
- nephrotic syndrome
what are the causes of euvolaemic hyponatraemia?
- SIADH
- primary polydipsia
- severe endocrine disturbances (e.g. hypothyroidism/cortisol deficiency)
what does a raised urine osmolality (>300 mOsm/kg) with low serum osmolality indicate?
- SIADH
- as the kidney is inappropriately producing concentrated urine despite low serum osmolality
what are the symptoms of hyponatraemia?
- anorexia
- headache
- nausea
- vomiting
- lethargy
- confusion
- ataxia
- seizures
- cerebral obtundation/coma
what are the signs associated with hypovolaemia?
- tachycardia
- hypotension
- dry mucous membranes
- reduced skin turgor
what are the signs associated with hypervolaemia?
- peripheral oedema
- raised JVP
- bibasal lung field crepitations (e.g. pulmonary rales)
what are the investigations for hyponatraemia?
- U&Es
- serum osmolality
- LFTs
- serum lipids
- serum glucose
- serum cortisol
- TFTs
- urine osmolality
- urine sodium
how is serum osmolality calculated?
serum osmolality = (2 x Na) + Glu + Urea (all in mmol/L)
what is pseudohyponatraemia?
hyperlipidaemia, hyperproteinaemia or hyperbilirubinaemia can interrupt some laboratory analysis methods of measuring serum sodium, leading to falsely low readings
how does hyperglycaemia cause hyponatraemia?
in patients with significant hyperglycaemia (e.g. DKA/HHS), the increase in serum glucose raises serum tonicity, pulling water out of cells and expanding the extracellular space, causing a dilutional effect on serum sodium concentrations
what is the management of acute severe hyponatraemia?
IV hypertonic saline bolus (100ml 3% NaCl)
what is the management of hyponatraemia without severe neurological symptoms?
the goal of treatment is to correct by no more than 6mmol/L in the first 6 hours and no more than 10mmol/L in the first 24 hours
what is the management of hypovolaemic hyponatraemia?
rehydration with intravenous 0.9% normal saline
what is the management of hypervolaemic hyponatraemia?
fluid restriction (<1.5L/24h)
what is the management of euvolaemic hyponatraemia?
- fluid restriction (1.5L/24h)
- oral salt tablets may be required if fluid restriction alone is ineffective
what are the acute complications of hyponatraemia?
- gait disturbance
- cerebral oedema
- osmotic demyelination syndrome
what is osmotic demyelination syndrome?
- severe and often irreversible neurological deficits secondary to intracerebral fluid shifts caused by the rapid correction of hyponatraemia
- classically occurs 2-4 days after the treatment
- typically presenting with quadriplegia and pseudobulbar palsy
