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Learning points
History, especially drug history, and focused examination of heart rate, blood pressure, venous filling, peripheral oedema, and cognitive function should identify possible causes, severity, and the fluid status in patients with hyponatraemia
Hyponatraemia usually has multiple causes with SIADH and drugs (especially thiazides) the commonest
The criteria to diagnose SIADH are hyponatraemia, urine osmolality >100 mOsm/kg and urinary sodium >=30 mmol/L in the absence of hypovolaemia and hypervolaemia
The patient
A 70 year old woman with chronic obstructive pulmonary disease who smoked 30 cigarettes daily was admitted to the emergency department after a fall and a fracture of the left neck of femur. Her daughter stated that her mother had lately seemed confused and unsteady, and that she had lost weight. She was using a [beta] agonist inhaler and taking no other drugs. On examination, she looked emaciated, had nicotine stained fingers, and was disorientated in time. Blood pressure was 140/85 mm Hg with no postural drop. Chest examination found scattered wheeze; no cardiac, abdominal, or other neurological abnormalities were found. Results of biochemistry were sodium 122 mmol/L (normal range 136-146 mmol/L), potassium 4.8 mmol/L (3.5-5.3 mmol/L), urea 1.7 mmol/L (2.1-7.6 mmol/L), and creatinine 55 mmol/L (51-107 mmol/L). Liver function tests, plasma glucose concentration, and lipids were normal. Computed tomography of the brain was normal.
What is the next investigation?
Hyponatraemia (a serum sodium concentration <136 mmol/L) is found in up to 42% of inpatients. 1 Hyponatraemia may be asymptomatic if it is mild to moderate (>125 mmol/L) and chronic (>48 hours). Symptoms and signs due to brain oedema occur in severe (<=125 mmol/L) or acutely developing (<48 hours) hyponatraemia. 2 As hyponatraemia advances, clinical features are headache, malaise, nausea, vomiting, cramps, lethargy, disorientation, unsteadiness and seizure, respiratory arrest, and death. 3 Additionally, patients may present with the symptoms and signs of the underlying cause. In a study on the aetiological factors of severe hyponatraemia (<=125 mmol/L), 79 out of 105 inpatients had multiple causes, including heart failure in 27, dehydration in 20, thiazide drugs in 29, liver disease in 21, pneumonia in 15, central nervous system lesions in 13, and other drugs in 46. Causes of syndrome of inappropriate antidiuretic hormone (SIADH), also known as...