Last reviewed 03/2023

Hyponatraemia is a low plasma sodium, defined as sodium less than 135 mmol/l.

  • it is the most common disorder of electrolytes encountered in clinical practice:
  • occurs in up to 15% to 30% of both acutely and chronically hospitalised patients
  • more common in
    • acute neurological admissions
    • 50% of patients with subarachnoid haemorrhage
    • elderly
    • in patients with respiratory tract infection, with a history of alcohol excess and who are treated with thiazide diuretics (2)
  • most cases are mild and relatively asymptomatic - however:
    • acute severe hyponatraemia can cause substantial morbidity and mortality
    • mortality is higher in patients with hyponatraemia who have a wide range of underlying diseases
    • overly rapid correction of chronic hyponatraemia can cause severe neurologic deficits and death

Presence of significant hypoosmolality (in hyponatraemia) indicates excess water relative to solute in the extracellular fluid (ECF) compartment (1)

  • water moves freely between the ECF and the intracellular fluid (ICF) compartments
    • therefore an excess of total body water relative to total body solute is present as well


isotonic or hypertonic hyponatraemia:

  • hyponatraemia with normal or even increased osmolality can occur:
    • when effective solutes other than sodium are present in the plasma
      • hyperosmolality produced by the additional solute causes an osmotic shift of water from the ICF to the ECF compartment
        • causes a dilutional decrease in the serum sodium
    • most commonly is seen with hyperglycemia
      • depending on the severity of hyperglycemia and the duration and magnitude of the accompanying glucose-induced osmotic diuresis, such patients actually may be hypertonic despite hyponatraemia