Antacid-induced hypermagnesemia in a patient with normal renal function and bowel obstruction

To report a case of severe hypermagnesemia caused by magnesium hydroxide in a woman with normal renal function.

A 42-year-old Hispanic woman with schizophrenia and bipolar affective disorder was transported from jail to the emergency department with confusion, abdominal pain, vomiting, and constipation. She had been treated in jail with magnesium hydroxide, ordered as milk of magnesia 30 mL po each night and Maalox 30 mL po three times daily. Additional medications included lithium carbonate 300 mg po three times daily, chlorpromazine 150 mg po three times daily, benztropine mesylate 1 mg po twice daily, and docusate sodium 100 mg po each morning. Her temperature was 35.1 degrees C, blood pressure 108/58 mm Hg, heart rate 112 beats/min, and respiratory rate 24 breaths/min. She would respond only briefly to voice or painful stimuli. Her abdomen was distended and diffusely tender. Laboratory tests included serum magnesium concentration 9.1 mEq/L (normal 1.3-2), blood urea nitrogen 16 mg/dL (8-22), creatinine 0.9 mg/dL (0.5-1.1), calcium 3.9 mEq/L (4.2-5.2), and lithium 1.0 mEq/L. A laparotomy was performed, and an adhesive band from a previous oophorectomy was found to be compressing the sigmoid colon. Hypermagnesemia, hypothermia, and hypotension continued in the intensive care unit. Despite successful treatment of the hypermagnesemia with calcium, intravenous fluids, and furosemide, the patient's cardiac rhythm degenerated into fatal, pulseless electrical activity on postoperative day 2.

This case of severe hypermagnesemia from magnesium hydroxide ingestion illustrates many of the risk factors for hypermagnesemia in patients with normal renal function. People using magnesium-containing medications for relief of gastrointestinal distress may be at increased risk for hypermagnesemia. A brief review of magnesium physiology, clinical effects, and treatment is provided. Frequent use of the laboratory to identify hypermagnesemia is encouraged because it is often a clinically unexpected finding and responds well to early treatment.