Elusive cause of hypomagnesaemia

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 (figure⇓arrows 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