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Hyponatraemia

This empty salt shaker is suffering from hyponatraemia.

Definition

Serum sodium<135mmol/L (<120mmol/L is severe).

Epidemiology

Common.jpg
It is the most common electrolyte abnormality. 4.4% of postoperative patient have it. Almost 100% will have had an operation.

Pathophysiology

In a nutshell:
Lowsodium.jpg

Hypovolaemic

Not hypovolaemic

Clinical Features

Investigations

Dehydration

  • U+Es are useful for sodium (obviously) but also assessing dehydration (urea and creatinine).

Further diagnosis

Firstly determine their hydration status. Then the most important things are:

  • Urinary sodium
  • Urine osmolality

The reason being the help to determine which of the causes is most likely. In hypovolaemia:

  • Urinary sodium >20mmol/L - renal cause
  • Urinary sodium <20mmol/L - non-renal cause

In normovolaemia:

  • Urine osmolality >100mOsmol/kg - SIADH

Otherwise, it must be something else. See pathophysiology.

Management

  • Hypovolaemia - isotonic saline replacement
  • Normovolaemia - fluid restrict to 500ml/24h.
  • Hypervolaemia - furosemide, ACE inhibitors, fluid restrict.

Prognosis