shutterstock_280146626Clinical features of hyponatraemia are relatively non-specific especially if gradual in onset.  However hyponatraemia can lead to:

  • Anorexia, nausea and vomiting.
  • Lethargy or poor concentration.
  • Agitation.
  • Headache.

Often the diagnosis of hyponatraemia is made based on routine biochemistry blood tests in the early post-operative period. If the onset is rapid, or sodium depletion is severe, the following clinical features can develop:

  • Delirium.shutterstock_192698828
  • Muscle weakness, tremor, hypotonia and/or incoordination.
  • Generalized seizures or coma.

While Syndrome of Inappropriate ADH secretion (SIADH) is the most common cause of hyponatraemia and is typically the assumed cause it is important to exclude other causes (though rare).

Causes of hypotnatraemia can be classified as follows:

1.Hypervolaemic causes of hyponatraemia:

  • It is typically associated with oedema and usually caused by nephrotic syndrome, cirrhosis, renal failure or cardiac failure.
  • These can usually be discriminated from SIADH because patients typically have oedema and reduced urine output. Clinical or metabolic features of the underlying conditions are typically quite overt or obvious. They tend to be associated with a relevant past medical history.

2. Euvolaemic causes of hyponatraemia (SIADH is also a euvolaemic cause of hyponatraemia):

  • Typically patients are neither oedematous nor dehydrated. It is usually caused by SIADH, Hypothyroidism or Addisons Disease. However excessive fluid intake can also cause a similar pattern.
  • Hypothyroidism or Addisons Disease can usually be discriminated from SIADH by blood tests:
    • Thyroid function tests for Hypothyroidism.
    • Cortisol levels, potassium and hydrogen ion/pH for Addisons Disease.

3. Hypovolaemic causes of hyponatraemia:

  • It is typically associated with dehydration and usually caused either by:
    • Renal salt loss (e.g. Cerebral Salt-Wasting; Diuretics; Conn’s Syndrome; Renal Tubular Acidosis).
    • Extra-renal salt loss (e.g. profuse diarrhoea; burns; excess sweating).
  • These can usually be discriminated from SIADH because:
    • Patients typically are clinically dehydrated.
    • Urine is very concentrated (Urinary osmolarity >450 mOsm/l).
    • Clinical features in the case of extra renal causes are typically quite overt or obvious.

The Algorithm can help to discriminate causes of Hyponatraemia.