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Episodic weakness in a young woman
  1. N Joss,
  2. K Simpson
  1. Renal Unit, Glasgow Royal Infirmary, Glasgow, Scotland
  1. Dr Nicola Joss, Renal Unit, Glasgow Royal Infirmary, 84 Castle Street, Glasgow G4 0SF, UK njoss{at}compuserve.com

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A 37 year old women presented with a 10 year history of episodes of weakness and palpitations. On one or two occasions each year she was admitted to her local hospital where her serum potassium concentration was always below the reference range with the lowest concentration reported at 2.2 mmol/l (3.5–5.0 mmol/l). The episodes were terminated with intravenous potassium. At other times she had mild weakness on exertion and occasional palpitations. These symptoms disappeared with oral potassium supplements. She was always normotensive. She had no diarrhoea or vomiting, her drug history consisted of oral amiloride 15 mg/day and a variable amount of effervescent potassium chloride supplements. She had a sister who had one minor episode of weakness but had not been investigated. She was referred to a renal unit for further investigations and to obtain a diagnosis.

On examination she looked well, with a blood pressure of 115/70 mm Hg, she was clinically euvolaemic, she had no signs of self induced vomiting, and she was not cushingoid.

Her initial investigations revealed the following serum electrolyte concentrations: sodium 137 mmol/l (135–145), potassium 3.0 mmol/l (3.5–5.0), chloride 100 mmol/l (97–107), bicarbonate 30 mmol/l (23–30), urea 4.9 mmol/l (2.5–8.0), creatinine 90 μmol/l (40–130), and calcium 2.6 mmol/l (2.2–2.6). Arterial blood gases: arterial oxygen tension 12 kPa (12–15), arterial carbon dioxide tension 4.8 kPa (4.4–5.6), and hydrogen ion 32 nmol/l (35–45). Her 24 hour urine sample performed when her serum potassium was 3.0 mmol/l revealed a urinary potassium of 63 mmol/24 hours. Her plasma cortisol and thyroid function tests were normal. A supine plasma renin activity of 32.8 ng/ml/hour (0.5–2.4) and aldosterone of 990 pmol/l (100–400) and ambulatory values of 40.4 ng/ml/hour (0.98–4.18) and 2500 pmol/l (400–800) were measured.

Questions

(1)
What is the metabolic abnormality?
(2)
What are the differential diagnoses?
(3)
What investigations will help distinguish between the possible diagnoses?
(4)
What is the likely diagnosis?

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