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Cancer patients frequently suffer from hyponatraemia, the physiopathology of which is not always clearly understood. Generally, it is fortuitously discovered on a systematic ionogram. However, hyponatraemia increases patient fatigue and can possibly be complicated by somnolence or seizures.

Hyponatraemia is often observed in preterminal stages of severe disease. Patients excrete the sodium excesses administered in order to correct hyponatraemia and liquid restriction leads to more severe dehydration than that already observed. Surprisingly, if patients are able to resume normal eating, a spontaneous correction of hyponatraemia is observed.

Thus, this preterminal hyponatraemia should be distinguished from an inappropriate secretion of ADH which is a genuine paraneoplastic syndrome (occurring in lung cancer for instance). Hyponatraemia leads to severe neurological disorders and to an exaggerated water excretion by the kidney (excessive urinary osmolality). In such cases, fluid restriction is useful, but more particularly the treatment of cancer, where possible, will improve the clinical situation.

Another frequent cause of hyponatraemia in cancer patients is the presence of oedema leading to a dilution syndrome. Such a clinical situation can be observed when ascitis or major pleural effusion occur. Whereas symptomatic treatment (ascitic puncture, diuretics) may be useful, only aetiological treatment will actually correct hyponatraemia for a reasonably long time.