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Hypomagnesemia

by James L. Lewis, III, MD

Hypomagnesemia is serum Mg concentration < 1.4 mEq/L (< 0.70 mmol/L). Causes include inadequate Mg intake and absorption or increased excretion due to hypercalcemia or drugs such as furosemide. Clinical features are often due to accompanying hypokalemia and hypocalcemia and include lethargy, tremor, tetany, seizures, and arrhythmias. Treatment is with Mg replacement.

Serum Mg concentration, even when free Mg ion is measured, may be normal even with decreased intracellular or bone Mg stores. Mg depletion usually results from inadequate intake plus impairment of renal conservation or GI absorption. There are numerous causes of clinically significant Mg deficiency (see Causes of Hypomagnesemia).

Causes of Hypomagnesemia

Cause

Comment

Alcoholism

Due to inadequate intake and excessive renal excretion

GI losses

Chronic diarrhea

Steatorrhea

Small-bowel bypass

Pregnancy-related

Preeclampsia/eclampsia (see Preeclampsia and Eclampsia)

Lactation (increased Mg requirements)

Primary renal losses

Rare disorders that cause inappropriately high Mg excretion (eg, Gitelman syndrome)

Secondary renal losses

Loop and thiazide diuretics

Hypercalcemia

After removal of parathyroid tumor

Diabetic ketoacidosis

Hypersecretion of aldosterone, thyroid hormones, or ADH

Nephrotoxins (eg, amphotericin B, cisplastin, cyclosporine, aminoglycosides)

Symptoms and Signs

Clinical manifestations are anorexia, nausea, vomiting, lethargy, weakness, personality change, tetany (eg, positive Trousseau or Chvostek sign or spontaneous carpopedal spasm, hyperreflexia), and tremor and muscle fasciculations. The neurologic signs, particularly tetany, correlate with development of concomitant hypocalcemia, hypokalemia, or both. Myopathic potentials are found on electromyography but are also compatible with hypocalcemia or hypokalemia. Severe hypomagnesemia may cause generalized tonic-clonic seizures, especially in children.

Diagnosis

  • Considered in patients with risk factors and with unexplained hypocalcemia or hypokalemia

  • Serum Mg concentration < 1.4 mEq/L (< 0.70 mmol/L)

Hypomagnesemia is diagnosed by a serum Mg concentration. Severe hypomagnesemia usually results in concentrations of < 1.0 mEq/L (< 0.50 mmol/L). Associated hypocalcemia and hypocalciuria are common. Hypokalemia with increased urinary K excretion and metabolic alkalosis may be present. Mg deficiency should be suspected even when serum Mg concentration is normal in patients with unexplained hypocalcemia or refractory hypokalemia. Mg deficiency should also be suspected in patients with unexplained neurologic symptoms and alcoholism, with chronic diarrhea, or after cyclosporine use, cisplatinum-based chemotherapy, or prolonged therapy with amphotericin B or aminoglycosides.

Treatment

  • Oral Mg salts

  • IV or IM Mg sulfate for severe hypomagnesemia or inability to tolerate or adhere to oral therapy

Treatment with Mg salts is indicated when Mg deficiency is symptomatic or persistently < 1 mEq/L (< 0.50 mmol/L). Patients with alcoholism are treated empirically. In such cases, deficits approaching 12 to 24 mg/kg are possible. About twice the amount of the estimated deficit should be given in patients with intact renal function, because about 50% of the administered Mg is excreted in urine. Oral Mg salts (eg, Mg gluconate 500 to 1000 mg po tid) are given for 3 to 4 days. Oral treatment is limited by the onset of diarrhea. Parenteral administration is reserved for patients with severe, symptomatic hypomagnesemia who cannot tolerate oral drugs. Sometimes a single injection is given in patients with alcoholism who are unlikely to adhere to ongoing oral therapy. When Mg must be replaced parenterally, a 10% Mg sulfate (MgSO 4 ) solution (1 g/10 mL) is available for IV use and a 50% solution (1 g/2 mL) is available for IM use. The serum Mg concentration should be monitored frequently during Mg therapy, particularly when Mg is given to patients with renal insufficiency or in repeated parenteral doses. In these patients, treatment is continued until a normal serum Mg concentration is achieved.

In severe, symptomatic hypomagnesemia (eg, Mg < 1 mEq/L [< 0.5 mmol/L] with seizures or other severe symptoms), 2 to 4 g of MgSO 4 IV is given over 5 to 10 min. When seizures persist, the dose may be repeated up to a total of 10 g over the next 6 h. In patients in whom seizures stop, 10 g in 1 L of 5% D/W can be infused over 24 h, followed by up to 2.5 g q 12 h to replace the deficit in total Mg stores and prevent further drops in serum Mg. When serum Mg is 1 mEq/L (< 0.5 mmol/L) but symptoms are less severe, MgSO 4 may be given IV in 5% D/W at a rate of 1 g/h as slow infusion for up to 10 h. In less severe cases of hypomagnesemia, gradual repletion may be achieved by administration of smaller parenteral doses over 3 to 5 days until the serum Mg concentration is normal.

Key Points

  • Hypomagnesemia may occur in alcoholics, in patients with uncontrolled diabetes, and with hypercalcemia or use of loop diuretics.

  • Symptoms include anorexia, nausea, vomiting, lethargy, weakness, personality change, tetany (eg, positive Trousseau or Chvostek sign, spontaneous carpopedal spasm, hyperreflexia), tremor, and muscle fasciculations.

  • Treat with Mg salts when Mg deficiency is symptomatic or persistently < 1 mEq/L (< 0.50 mmol/L).

  • Give oral Mg salts unless patients have seizures or other severe symptoms, in which case, give 2 to 4 g of MgSO 4 IV over 5 to 10 min.

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  • LASIX
  • NEORAL, SANDIMMUNE

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