Hyponatraemia

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Hyponatraemia, a sodium below 135, is relatively commonly in palliative care patients and is often found incidentally when blood tests are taken. In many cases (e.g. in a patient declining gradually from progressive cancer who is in the last few weeks of life) it is of little significance, however at other times a finding of severe hyponatraemia is a major concern. Very low levels of sodium can cause drowsiness, confusion and precipitate seizures. Causes of hyponatraemia seen in the palliative care context can include vomiting, medications and SIADH.

Causes

Hyponatremia is often categorized by volume state:

  • Hypovolemic hyponatraemia - causes include:
    • Gastrointestinal losses - vomiting or diarrhoea
    • Renal losses - diuretics
  • Normovolaemic hyponatraemia - causes include:
    • SIADH
    • Primary polydipsia
  • Hypervolaemic hyponatraemia - causes include:
    • Heart failure
    • Cirrhosis
  • Pseudohyponatraemia, e.g. hyperglycaemia

Determining the cause

History

Look for causes of fluid loss including vomiting or diuretics.

Is the patient on any medications that might cause hyponatraemia such as diuretics?

Does the patient have a history of a condition that would could SIADH such as lung or brain pathology, e.g. small cell lung cancer?

Does the patient have a history of a condition that is associated with volume overload such as cardiac failure or cirrhosis?

Examination

Are there signs of extra-vascular volume depletion such as a dry mouth and reduced skin turgor or intra-vascular volume depletions such as hypotension and tachycardia?

Are there signs of fluid overload with peripheral oedema, ascites or a raised JVP?

Investigations

The key tests are:

  • Serum osmolality
  • Urinary osmolality
  • Urinary sodium

The typical findings in SIADH are: a low serum osmolality; a high urine osmolality, usually > 300; high urinary sodium > 40.

Serum osmolality

A normal sermum osmolality is between 275 and 290. A low serum osmolality is almost always present in hyponatraemia. In cases of pseudohyponatraemia osmolality will be normal and the hyponatraemia does not need treatment.

Urinary osmolality

Hyponatraemia normally causes suppression of anti-diuretic hormone secretion which leads to dilate urine. Thus in hyponatraemia the urine osmolality should reduce, commonly to a value of less than 100. In SIADH there is an inability to dilute the urine and so the osmolality will usually be greater than 300.

Urinary sodium

The urinary sodium concentration is helpful in distinguishing between SIADH and hypovolaemic hyponatraemia. In hypovolaemia that is not due to diuretics, the urinary sodium is usually less than 25. In SIADH the urinary sodium is usually greater than 40.

Authors

Graham Llewellyn Grove