Hypovolaemic hyponatraemia

• presence of clinically detectable decreased extracellular fluid (ECF) volume generally reflects hypovolaemia from some degree of body solute depletion
• hyponatraemia with volume depletion can arise in a variety of settings
  • renal loss with water retention include:
    • diuretic therapy
    • mineralocorticoid deficiency
    • adrenal haemorrhage
    • cerebral salt wasting
    • adrenal enzyme deficiencies (congenital adrenal hyperplasia)
    • bicarbonaturia, glucosuria, ketonuria
  • extra renal loss with water retention include:
    • gastrointestinal losses - vomiting, diarrhoea
    • third space losses - bowel obstruction; pancreatitis; muscle trauma; burns
    • sweat losses e.g. endurance exercise
• volume depletion is generally diagnosed clinically from the history, physical examination, and laboratory results
• clinical signs of volume depletion include
  • orthostatic decreases in blood pressure and increases in pulse rate, dry mucus membranes, decreased skin turgor
  • if signs of volume depletion and hyponatraemia
    • then should be considered hypovolaemic hyponatraemia unless there are alternative explanations for these findings (1)
  • elevations of urea, creatinine, urea–creatinine ratio, and uric acid level indicate possible volume depletion
    • however these findings are neither sensitive nor specific, and they can be affected by other factors (eg, dietary protein intake, use of glucocorticoids).
  • urine sodium excretion is generally more helpful
    • spot urine [Na+] should be <30 mmol/L in patients with hypovolaemic hyponatraemia unless the kidney is the site of sodium loss
• if clinical assessment is equivocal
  • a trial of volume expansion can be a useful diagnostic tool (also will be therapeutic if volume depletion is the cause of the hyponatraemia)
    • a 0.5 to 1 L infusion of isotonic (0.9%) sodium chloride, patients with hypovolaemic hyponatraemia will begin to correct their hyponatraemia without developing signs of volume overload
    • in contrast, if SIADH
      • urine [Na+] will increase but the serum [Na+] will remain unchanged or decrease as the administered water is retained and the sodium load excreted in a smaller volume of concentrated urine (1)

Reference:

1. Adrogué HJ, Tucker BM, Madias NE. Diagnosis and management of hyponatremia: a review. JAMA. 2022 Jul 19;328(3):280-91