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Key Features

Essentials of Diagnosis

  • Serum magnesium < 1.8 mg/dL (< 0.75 mmol/L)

  • Causes neurologic symptoms and arrhythmias

  • Serum concentration may be normal even in the presence of magnesium depletion

  • Check urinary magnesium excretion if renal magnesium wasting is suspected

  • Impairs release of parathyroid hormone (PTH)

General Considerations

  • Normomagnesemia does not exclude magnesium depletion because only 1% of total body magnesium is in the extracellular fluid

  • Renal potassium wasting 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 D3 levels and consequent hypocalcemia is refractory to calcium replacement until the magnesium is normalized

  • Molecular mechanisms of magnesium wasting have been revealed in some hereditary disorders

Etiology

  • Diminished absorption or intake (Table 21–11)

    • Malabsorption

    • Chronic diarrhea

    • Laxative abuse

    • Prolonged gastrointestinal suction

    • Small bowel bypass

    • Malnutrition

    • Alcoholism

    • Total parenteral alimentation with inadequate Mg2+ content

  • Increased renal loss

    • Diuretic therapy

    • Hyperaldosteronism

    • Gitelman syndrome (a variant of Bartter syndrome)

    • Hyperparathyroidism

    • Hyperthyroidism

    • Hypercalcemia

    • Volume expansion

    • Tubulointerstitial diseases

    • Transplant kidney

    • Drugs (aminoglycoside, cisplatin, amphotericin B, pentamidine)

  • Others

    • Diabetes mellitus

    • Postparathyroidectomy (hungry bone syndrome)

    • Respiratory alkalosis

    • Pregnancy

Table 21–11.Causes of hypomagnesemia.

Clinical Findings

Symptoms and Signs

  • Weakness, muscle cramps

  • Marked neuromuscular and central nervous system hyperirritability

    • Tremors

    • Athetoid movements

    • Jerking

    • Nystagmus

    • Babinski response

    • Confusion and disorientation

  • Hypertension, tachycardia, and ventricular arrhythmias

Diagnosis

Laboratory Tests

  • Serum magnesium < 1.8 mg/dL (< 0.75 mmol/L)

  • Urinary excretion of magnesium exceeding 10–30 mg/day or a fractional excretion more than 2% indicates renal magnesium wasting

  • Hypokalemia and hypocalcemia are often present

  • PTH secretion is often suppressed

Diagnostic Procedures

  • ECG may show a prolonged QT interval because of lengthening of the ST segment

Treatment

Medications

  • Symptomatic hypomagnesemia

    • 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 with 1–2 g of magnesium sulfate in 10 mL of dextrose 5% solution pushed intravenously over 15 minutes

    • Severe, non–life-threatening deficiency can be treated at a rate to 1–2 g/h over 3–6 hours

  • Chronic hypomagnesemia

    • Magnesium oxide, 250–500 mg ...

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