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, sacral, pulmonary), ascites, increased JVP or CVP (3)
• the three major classifications of hypotonic hyponatraemia based on the patients 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. Adrogué HJ, Tucker BM, Madias NE. Diagnosis and management of hyponatremia: a review. JAMA. 2022 Jul 19;328(3):280-91.
2. Spasovski G, Vanholder R, Allolio B, et al; Hyponatraemia Guideline Development Group. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014 Feb 25;170(3):G1-47.