Diagnosis of cause of hyponatraemia

A definitive diagnosis of the underlying aetiology of hyponatraemia is not always possible at the time of initial presentation

• an accurate history should be taken
  • look for signs and symptoms of hyponatraemia
  • inquire about any medication (e.g. - diuretics, antidepressants), recently received intravenous fluids, vomiting, diarrhea and about any associated diseases (1)
• measure serum osmolality (2)
  • if serum osmolality is normal (275 – 295 mOsmol/l) or high (>295 mOsmol/l) consider causes such as hyperglycaemia, hyperlipidaemia, hyperprotinaemia and renal failure
  • in cases of low osmolality (<275 mOsmol/l) assess volume status of the patient,
    • hypovolaemia – tachycardia, postural hypotension, dry skin, reduced skin turgor, raised blood urea, raised plasma rennin
    • euvolaemia – blood urea normal or slightly reduced
    • hypervolaemia – oedema (peripheral, sacaral, pulmonary), ascites, increased JVP or CVP (3)
• the three major classifications of hypotonic hyponatraemia based on the patient?s ECF volume status:
  • hypovolaemic hyponatraemia
  • normovolaemic (euvolaemic) hyponatraemia
  • hypervolaemic hyponatraemia
• plasma glucose and lipids, renal function, thyroid function and adrenal function test should be done if clinically indicated (1,2)

Reference:

• (1) Guidelines and Audit Implementation Network (GAIN) 2010. Hyponatraemia in adults (on or after 16th birthday)
• (2) Reynolds RM, Padfield PL, Seckl JR. Disorders of sodium balance.BMJ.2006;332(7543):702-5
• (3) Thompson CJ, Crowley RK. Hyponatraemia. J R Coll Physicians Edinb2009; 39:154–7