Hyponatraemia is when serum sodium is <135 mmol/L (although check the local cut off depending on your lab). It is one of the more common electrolyte abnormalities you will note on the ward round or during your on call, therefore a systematic approach is helpful.

Although severity does not affect your approach, it is important to recognise when hyponatraemia should be dealt with urgently.

Classification
  • Mild (Na 130-135): Usually asymptomatic, incidental finding 
    • This is quite common and rarely needs correcting in an otherwise well patient with normal observations. 
    • It might be a longstanding issue so check previous results. There may be no known cause.
    • If new, check recent introduction of medication (this could’ve caused it, particularly diuretics) and their blood glucose (this can cause pseudohyponatraemia)
    • It is worth applying the systematic approach described below to properly evaluate it to prevent it worsening
  • Moderate (Na 125-129): Patients can be asymptomatic or have non-specific symptoms 
    • Following evaluation of the potential causes, it is worth discussing this with seniors. 
  • Severe (Na <125): Possible symptoms include vomiting, headaches, drowsiness, seizures, coma and cardio-respiratory arrest
    • This warrants urgent discussion with seniors, particularly if symptomatic.
  • Duration: Acute (<48 hours onset) vs Chronic (>48 hours) – rapid onset can lead to the symptoms mentioned above, and chronic hyponatraemia can lead to gait instability, falls, concentration and cognitive impairment. Early escalation if rapid is key!
How to approach when you're called for a patient with hyponatraemia
1. Could this be a false hyponatraemia?
  • Bloods taken where intravenous fluids are running (and thus may be a mixed sample) should be hypernatraemic if 0.9% sodium chloride or hartmanns/plasma-lyte is going through it. However, 0.45% sodium chloride or dextrose could lead to inaccurate results
  • Regardless of how accurate the sample is, repeat with a venous blood gas, serum osmolality & lab urea & electrolytes
Normal plasma osmolality: 275-295 mOsmol/kg
In hyponatraemia, the main blood osmole sodium is low, therefore the serum osmolality should also be low. 
  • If not low consider pseudohyponatraemia, which is due to high lipids, proteins and glucose
    • It is high in hyperglycaemia but the VBG will give you the glucose. You can then calculate the corrected hyponatraemia
    • High serum lipids or abnormal proteins (e.g. myeloma) can cause lab artifact that results in pseudohyponatraemia & usually normal serum osmolality. In these cases, the VBG may give an accurate result as it is not usually susceptible to this
2. Take a history
  • Are they symptomatic? This requires urgent escalation
  • Have they had this before? Why?
  • Fluid intake & losses (e.g. diarrhoea/vomiting)
  • Symptoms of thyroid/adrenal disease
  • Symptoms of malignancy (particularly lung)
  • Oedema/ascites & organ failure (liver/kidney/heart)
  • Alcohol consumption
  • Medication history (diuretics, antidepressants, antipsychotics)
  • Recent surgery (large volume irrigation or surgery itself can cause transient hyponatraemia)
3. Examine the fluid status
  • Hypervolaemia is usually easier to identify (oedematous, known organ failure). Restrict fluid & sodium whilst treating the underlying cause with senior input
  • Hypovolaemia & euvolaemia are difficult to differentiate between although the former is much more common. History, fluid input/output, lying and standing postural BP drop can help identify hypovolaemia. Investigations can help guide also.
4. Investigations
  • If hypovolaemia, then 0.9% sodium chloride will help correct this. But beware of rapid correction causing dangerous fluid shifts - unless mild hyponatraemia do involve seniors to guide correction
  • If unsure of the fluid status then urinary osmolality & sodium (ensure it is paired with serum sodium!) can help and at the same time it is worth screening for endocrinological causes (TFTs, consider 9am cortisol) as these can cause euvolaemic or hypovolaemic hyponatraemia
  • With urine osmolality & urinary sodium, we are looking to see if the body is conserving fluid (high urine osmolality) & sodium  (low urine sodium) or losing it. We are asking "are the kidneys doing the right job or the wrong job and therefore they are in fact the problem?"
  • If the urine is dilute (osmolality <100 mOsm/kg), the kidneys are trying to remove excess water from possible excess water/alcohol intake. 
  • If they are hypovolaemic, then either they are losing sodium from the kidneys (high urinary sodium) or they are losing it to other places e.g. diarrhoea/vomiting or third spacing from pancreatitis (low urinary sodium as kidneys trying to conserve). As before, treatment is 0.9% sodium chloride with senior input
  • If euvolaemic, expect high urine sodium (>20 mmol/L and often >40), as something is impacting the kidney to perform normal sodium regulation such as an post op period, endocrinopathy or SIADH or medications. 0.9% sodium chloride can make things worse, therefore senior input is advised. 

Acute symptomatic hyponatraemia: High risk of cerebral oedema 
This is an emergency requiring consideration of HDU care for hourly monitoring of sodium correction to prevent cerebral oedema. Correction differs by trust but s about 1–2 mmol/h and 8 mmol/day with various concentrations of 0.9% sodium chloride. 
You should NEVER manage this on your own as an FY1. 

Further Reading/References
  • UpToDate on Hyponatraemia
  • BMJ Best Practice
  • Biswas M, Davies JS. Hyponatraemia in clinical practice. Postgrad Med J. 2007;83(980):373–378. doi:10.1136/pgmj.2006.056515
  • National Institute for Health and Care Excellence. Hyponatraemia. Retrieved from https://cks.nice.org.uk/hyponatraemia#!topicSummary 
  • Allen J, Newland-Jones P. How to manage adults with hyponatraemia. Clinical Pharmacist, Vol. 4, p262 | URI: 11108156
Written by Dr Angela Yan FY2 & Dr Akash Doshi CT2