Hypomagnesemia

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

• Associated with hypocalcemia

General Considerations

• 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

• Hypomagnesemia causes renal potassium wasting that 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

• The potassium binder patiromer can cause hypomagnesemia by binding magnesium in the colon

Etiology

• Diminished absorption or intake (Table 21–10)

  • Malabsorption

  • Chronic diarrhea

  • Laxative abuse

  • Prolonged gastrointestinal suction

  • Small bowel bypass

  • Malnutrition

  • Alcohol use disorder

  • 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

Clinical Findings

Symptoms and Signs

• Because hypomagnesemia causes hypokalemia and hypocalcemia, it is difficult to determine if symptoms are from hypomagnesemia itself or from potassium and calcium depletion

• Marked neuromuscular and central nervous system hyperirritability

  • Tremors

  • Cramps

  • Trousseau and Chvostek signs

  • Confusion

  • Disorientation

  • Coma

• Weakness is common

• Cardiovascular manifestations include

  • Hypertension

  • Tachycardia

  • Ventricular arrhythmias, including torsades de pointes

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 3% indicates renal magnesium wasting

• Hypokalemia and hypocalcemia are often present

• PTH secretion is often suppressed

Diagnostic Procedures

• ECG may show widening of the QRS complex, peaked T waves with ultimate diminution and a prolonged PR ...