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Case history

image A 62-year-old female smoker with chronic obstructive airways disease presented to the emergency department with a collapse. She gave a 6 week history of cough, anorexia and weight loss with recent onset of dizziness, nausea and confusion. The admitting doctor noted that she was vague; her abbreviated mental test score was 7 out of 10. A chest X-ray showed a large mass at the right hilum with widening of the mediastinum. Her serum sodium level was 111 mmol/l.

What are the causes of hyponatraemia?

How should this patient be managed?

What are the causes of hyponatraemia?

Hyponatraemia is one of the most common biochemical abnormalities in cancer patients and occurs when there is an excess of water in the extracellular fluid compartment relative to its sodium content. Patients may be asymptomatic or report headache, difficulty concentrating, weakness, muscle cramps and dysgeusia. Patients with chronic hyponatraemia are often asymptomatic even with sodium values <125 mmol/l. In chronic hyponatraemia, as sodium levels become lower serious neurological symptoms may become apparent. A rapid drop in sodium can cause more dramatic neurological manifestations including confusion, seizures and reduced level of consciousness. Such patients are at high risk of fatal brainstem herniation.

The myriad causes of hyponatraemia are best grouped according to whether the patient’s extracellular fluid compartment is normal in volume, overloaded or contracted.13

Hypovolaemic hyponatraemia

Hypovolaemic hyponatraemia occurs when water and sodium are lost, but sodium is lost in greater proportion to fluid. Common causes of hypovolaemic hyponatraemia are excessive vomiting, sweating, diarrhoea or use of diuretic agents. It can be corrected with volume replacement using isotonic saline. Urine is typically (and appropriately) very concentrated (urine osmolality >500 mmol/kg) and urine sodium is often low (<20 mmol/l), except when diuretics are being used.

Euvolaemic hyponatraemia

Euvolaemic hyponatraemia is typically caused by the syndrome of inappropriate antidiuresis (SIAD). SIAD is now preferred to the term SIADH (syndrome of inappropriate antidiuretic hormone), as in many cases antidiuretic hormone may be appropriately released but with a more pronounced physiological effect. This may be caused by increasing sensitivity at the renal collecting ducts or by altered sensitivity of posterior pituitary secretory cells to changes in plasma osmolality (‘reset osmostat’ typically seen in neurological conditions). A common cause of SIAD in malignancy is ectopic production of antidiuretic hormone by the tumour. Many drugs used in cancer patients can also cause inappropriate release of antidiuretic hormone from the posterior pituitary or can potentiate the effect of the drug on the renal collecting ducts. The urine is normally (and inappropriately) concentrated relative to the plasma, and urinary sodium is usually high (>40 mmol/l). Administration of isotonic saline has little impact on sodium and may even worsen hyponatraemia.

Hypervolaemic hyponatraemia

Hypervolaemic hyponatraemia occurs when water is retained to a greater degree than sodium. Common causes are cardiac failure, nephrotic syndrome, cirrhosis, or excessive fluid consumption seen in psychosis or endurance sporting activity. The plasma and urine are usually dilute (urine osmolality <100 mmol/kg). Urinary sodium is usually low (<20 mmol/l), except when diuretics are being used. Administration of isotonic saline can worsen hyponatraemia as a result of further volume expansion.

How should this patient be managed?

Assessment

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