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.

Classification

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

• 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

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