↑↓Na (Hypernaturaemia and Hyponaturemia

Overview Hyponatremia is commonly defined as a serum sodium concentration below 130 mmol/L. 

  • Defined as a serum Na+ <130mmo/L
  • It affects 1% of hospital in patients (on call book), 15% (BMJ)
  • Most cases require no treatment.


According to serum ADH levels:

  • True volume depletion - gastrointestinal losses (i.e vomiting or diarrhea) or renal losses (i.e thiazides)
  • Decreased tissue perfusion (also called effective arterial volume depletion) due to reduced cardiac output in heart failure or to systemic vasodilation in cirrhosis
  • A primary (i.e not hypovolemic) increase in ADH release in the syndrome of inappropriate ADH secretion (SIADH)

According to volume status:

  • Hypovolemia
  • Normovolemia
  • Hypervolemia
Remember Syndrome of Inappropriate ADH (SIADH) is inappropriate secretion of ADH resulting in increased ADH levels in serum and increase ADH activity.


  • Pseudohyponatraemia (arefactual)
    • Laboratory analysis technique
    • Hyponatraemia with normal serum osmolality
      • Hyperlipidaemia, hyperproteinaemia
    • Hyponatraemia with increased serum osmolality
    • Hyperglycaemia, mannitol, excess urea
    • Toxic alcohols (ethanol, methanol, isopropyl alcohol, ethylene glycol)
  • Hyponatreamia with high urinary Na+ Indicates inappropriate renal wasting of sodium (rather than retention, which should occur in hyponaturaemia) – Low urine osmolality.
    • Hypovolaemia
      • Diuretic excess (increase in water and sodium excretion, ADH kicks in > increase water retention).
      • Vomiting, NG suction (loss of acid -> increase pH -> kidneys compensate to excrete HCO- with Na+ -> more Na excretion by kidneys)
      • Hypoaldosteronism, addisons’ disease, spironolactone (Decrease sodium retention – increase sodium excretion with water).
    • Euvolaemia – Due to SIADH
      • Malignancy
      • CNS disorders – tumours, meningitis, Gullian BS.
      • Pulmonary disorders
      • GIT
      • Drugs – neuroleptics, antidepressants
    • Hypervolaemia
      • Chronic renal failure (normal urine sodium)
      • Hypothyroidism
    • Hyponaturaemia with low urinary Na+ indicates appropriate renal conservation of sodium (High Urine osmolality)
      • Hypovolaemia
        • Diarrhoea (more bicarb excrete in faeces with Na+ kidneys retain Na).
        • Sweating, burns, pancreatitis (water and sodium loss – renal system retains more water and sodium – decrease Na excretion from Kidneys. ADH increase more water retention).
      • Euvolaemia
        • Hypotonic post-op fluids (water moves into RBC)
        • Elderly patients with poor diet (tea and toast diet)
        • Large volume binge beer drinking
      • Hypervolaemia (excess water + sodium in ECF space – reduce Na in blood and urine)
        • CCF
        • Cirrhosis of liver
        • Nephrotic syndrome
        • Hypoalbuminaemia
Remember Increasing sodium to quick can lead to central pontine myelonosis

Osmotic demyelination syndrome (central pontine myelonosis)

  • Rapidly correcting hyponatraemia may produce permanent central nervous system injury, due to osmotic demyelination.
  • Patients with chronic hyponatraemia (ie known duration more than 48 hours) are particularly at risk.
  • Clinical manifestations typically delayed for 2-6 days.

Symptoms include dysarthria, dysphagia, paraparesis or quadriparesis, behavioural disturbances, movement disorders, seizures, lethargy, confusion, disorientation, obtundation, and coma. Severely affected patients may become "locked in"; they are awake but are unable to move or verbally communicate

Clinical Presentation

  •  Mild-moderate
    • Lethargy, weakness and ataxia
    • Nausea and vomiting
    • Headache
  • Severe (<120mmols)
    • Confusion
    • Seizures and coma