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ESSENTIALS OF DIAGNOSIS
Serum concentration of magnesium may be normal even in the presence of magnesium depletion. Check urinary magnesium excretion if renal magnesium wasting is suspected.
Causes neurologic symptoms and arrhythmias.
Associated with hypocalcemia.
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GENERAL CONSIDERATIONS
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Causes of hypomagnesemia are listed in Table 21–10. Hypomagnesemia and hypokalemia share many etiologies, including diarrhea, alcohol use disorder, and diuretic use. Renal potassium wasting also occurs from hypomagnesemia and is refractory to potassium replacement until magnesium is repleted. Hypomagnesemia also suppresses PTH release and causes end-organ resistance to PTH and low 1,25-dihydroxyvitamin D levels. The resultant hypocalcemia is refractory to calcium replacement until the magnesium is normalized. Normomagnesemia does not exclude magnesium depletion because only 1% of total body magnesium is in the extracellular fluid; magnesium repletion should be considered in patients with risk factors for hypomagnesemia and refractory hypokalemia or hypocalcemia. Molecular mechanisms of magnesium wasting have been revealed in some hereditary disorders. There is an FDA warning about hypomagnesemia for patients taking proton pump inhibitors. The presumed mechanism is decreased intestinal magnesium absorption, but it is not clear why this complication develops in only a small fraction of patients taking these medications. The potassium binder patiromer can cause hypomagnesemia by binding magnesium in the colon.
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A. Symptoms and Signs
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Because hypomagnesemia causes hypokalemia and hypocalcemia, it is difficult to determine whether symptoms are from hypomagnesemia or from potassium and calcium depletion. Marked neuromuscular and central nervous system hyperirritability may produce tremors, cramps, Trousseau and Chvostek signs, confusion, disorientation, and coma. Weakness is common. Cardiovascular manifestations include hypertension, tachycardia, and ventricular arrhythmias, including torsades de pointes.
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B. Laboratory Findings
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Urinary excretion of magnesium exceeding 10–30 mg/day or a fractional excretion greater than 3% indicates renal magnesium wasting. Hypocalcemia and hypokalemia are often present. The ECG may show widening of the QRS complex, peaked T waves with ultimate diminution, and a prolonged PR interval. PTH secretion is often suppressed (see Hypocalcemia).
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Magnesium oxide, 250–500 mg orally once or twice daily, is useful for treating chronic hypomagnesemia. Symptomatic hypomagnesemia requires intravenous magnesium sulfate 1–2 g over 5–60 minutes mixed in either dextrose 5% or 0.9% normal saline. Torsades de pointes in the setting of hypomagnesemia can be treated ...