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Case History

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Image not available. A 62-year-old female smoker with chronic obstructive airways disease presents to the emergency department with a collapse. She gives a six-week history of cough, anorexia and weight loss with recent onset of dizziness, nausea and confusion. The admitting doctor noted that she was vague, with an abbreviated mental test score of 7 out of 10. A chest X-ray shows a large mass at the right hilum with widening of the mediastinum. Her serum sodium returns at 111 mmol/l.

What is the differential diagnosis?

How would you manage this patient?

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Background

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Image not available. Hyponatraemia occurs when there is an excess of water in the extracellular fluid compartment relative to its sodium content. Patients can be asymptomatic, or they may report headache, difficulty concentrating, weakness, muscle cramps and dysgeusia. A rapid drop in sodium can cause more dramatic neurological manifestations, including confusion, seizures, reduced level of consciousness and respiratory arrest.

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Hyponatraemia is commonly a consequence of excess sodium loss from either the gastrointestinal tract or the kidneys, or excess dilution due to cardiac, liver or renal impairment. In addition, patients with malignant disease, inappropriate diuresis is typically due to ectopic production of antidiuretic hormone by the tumour.1-3 The patient should always be assessed in terms of their fluid volume status: are they hypo-, hyper- or euvolaemic? A careful drug history should always be sought, and other causes of hyponatraemia should be looked for before a diagnosis of syndrome of inappropriate antidiuretic hormone (SIADH) is made. Treatment of hyponatraemia involves correcting the underlying cause (Table 40.1), and, therefore, recognizing the pathophysiological process leading to low sodium is crucial in determining the most appropriate management.

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Table Graphic Jump Location
Table 40.1*Causes of hyponatraemia in cancer patients
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What is the differential diagnosis?

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Initially, the key differential is whether the hyponatraemia is caused by SIADH, fluid overload or depletion. A full history and physical examination should be undertaken, bearing in mind the differential diagnoses in Table 40.1. The key to understanding SIADH is the inappropriate levels of serum versus urine osmolality. If the serum is concentrated then the urine should be concentrated, and it ...

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