Hypomagnesemia

Essentials of Diagnosis

• Serum concentration 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

• Impairs release of parathyroid hormone (PTH)

General Considerations

• Causes of hypomagnesemia are listed in Table 21–10

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

• Hypomagnesemia and hypokalemia share many etiologies, including

  • Diuretics

  • Diarrhea

  • Alcoholism

  • Aminoglycosides

  • Amphotericin

• 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

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

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

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

• Magnesium sulfate may also be given intramuscularly

  • Dosage: 200–800 mg/day (8–33 mmol/day) in four divided doses

  • Serum levels must be monitored daily and dosage adjusted to keep the concentration from rising above 3 mg/dL (1.23 mmol/L)

• Chronic hypomagnesemia

  • Magnesium oxide, 250–500 mg once or twice daily orally, is useful for repleting stores

Follow-Up

• Check tendon reflexes because hypermagnesemia causes hyporeflexia

• In patients with chronic kidney disease,

  • Reduce doses (50–75% dose reduction)

  • Monitor serum magnesium levels frequently (at least twice daily)

  • Replace magnesium cautiously to avoid hypermagnesemia

Complications

• There is an FDA warning about hypomagnesemia for patients taking proton pump inhibitors

  • Presumed mechanism is decreased intestinal magnesium absorption

  • However, it is not clear why this complication develops in only a small fraction of patients taking these medications

When to Refer

• Refer to an endocrinologist or nephrologist for consultation about repletion in the setting of marked neuromuscular symptoms or cardiac arrhythmia

When to Admit

• Confusion, disorientation

• Marked neuromuscular and central nervous system hyperirritability

• Need for intravenous replacement