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Elusive cause of hypomagnesaemia

BMJ 2011; 343 doi: http://dx.doi.org/10.1136/bmj.d5087 (Published 30 September 2011) Cite this as: BMJ 2011;343:d5087
  1. Krishnan Swaminathan, consultant endocrinologist 1,
  2. John Wilson, consultant gastroenterologist2
  1. 1Department of Endocrinology, Victoria Hospital, Kirkcaldy KY2 5AH, UK
  2. 2Department of Gastroenterology, Victoria Hospital, Kirkcaldy, UK
  1. Correspondence to: K Swaminathan k_swaminathan{at}hotmail.com

A 46 year old man presented with carpopedal spasm, muscle cramps, and a prolonged QTc interval on a background of a week’s history of diarrhoea and vomiting. He had a history of Barrett’s oesophagus and had been taking 40 mg omeprazole daily for many years. He had severe hypocalcaemia and hypomagnesaemia, with a corrected calcium and magnesium of 1.77 mmol/L (reference range 2.15-2.65) and 0.13 mmol/L (0.75-1), respectively. Serum parathyroid hormone was low at 3.1 ng/L (8-55). Intravenous replacement of fluids and electrolytes restored the biochemical abnormalities. He was discharged on oral alfacalcidol and magnesium glycerophosphate.

At six week follow-up, calcium had normalised (2.25 mmol/L) but symptomatic hypomagnesaemia persisted, with a serum magnesium of 0.27 mmol/L. Extensive evaluation, including upper and lower gastrointestinal endoscopy and distal duodenal biopsies, failed to identify a cause for his persistent hypomagnesaemia. He was admitted for recurrent intravenous magnesium infusions because his magnesium values were between 0.27 mmol/L and 0.29 mmol/L in spite of oral magnesium supplements. An intervention (figurearrows labelled A and B) resulted in dramatic normalisation of serum magnesium values with no need for further oral or intravenous magnesium replacement.

The patient’s serum magnesium concentrations

Questions

  • 1 What are the causes of hypomagnesaemia?

  • 2 What interventions on the graph (indicated by the arrows) are responsible for the fluctuations in magnesium …

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