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Hypomagnesemia

Mike Tozier

Background

  • Definition: Mg2+ \< 1.8 mg/dL, most pts asymptomatic until \<1.2 mg/dL. Severe [Mg+2] \< 1 mg/dL

  • Causes:

    • GI losses: Diarrhea, malabsorption, acute pancreatitis, EtOH use, TPN, vomiting, NG suction, GI fistulas, anorexia, short gut syndrome, small bowel bypass

    • Drugs: PPIs, loop diuretics, thiazides, digoxin, amphotericin, aminoglycosides, foscarnet, cisplatin, calcineurin inhibitors, laxatives, pentamidine

    • Kidney losses: post-ATN diuresis, Bartter syndrome and Gitelman syndrome

    • Cellular shifts: DKA treatment/recovery, refeeding, hungry bone syndrome, correction of metabolic acidosis, pancreatitis, EtOH withdrawal

    • Other: DM, hyper Ca, hyperthyroid, hyperaldosteronism, burns, lactation, Vit D deficiency, heat, prolonged exercise, mitral valve prolapse, pseudohypomagnesemia 2/2 EDTA tube, lactation

Presentation

  • Refractory hypocalcemia or hypokalemia, arrhythmias, muscle weakness

  • Severe symptoms: seizures, drowsiness, confusion, coma, arrhythmias

  • Vertical nystagmus, tetany (Chvostek sign, Trousseau), tremors, fasciculations

Evaluation

  • EKG: Initially wide QRS, peaked Ts. Progresses to wide PR, diminished T, arrhythmias

  • Labs: Ca+2, K+, can use FEMg (order urine Mg+2 and Cr, serum Cr and Mg) or 24-hour urine for Mg to distinguish renal vs GI etiology (FEMg>2% renal, \<2% GI)

Management

  • Correct underlying cause, replete based on severity (Dosing below for normal GFR)

  • Oral: asymptomatic pts, can cause GI symptoms, not well absorbed

    • Sustained release (Mg Chloride or Mg L-lactate) better tolerated and absorbed, though standard preparations (Mg oxide) are faster acting

    • Mg chloride: 3-4 tabs BID (total 30 to 56 meq [15 to 28 mmol]) for severe hypo Mg

    • 2-4 tabs daily (total 10 to 28 meq [5 to 14 mmol]) for mild hypo Mg

    • Mg oxide: 400-800 mg BID (20 to 40 mmol [40 to 80 meq]) for mod-severe hypo Mg

  • Intravenous: for symptomatic patients or if GI intolerance to oral

    • Mg \<1 mg/dL: 4 to 8g of MgSO4 (32 to 64 meq [16 to 32 mmol]) over 12 to 24 hrs

    • Mg 1 to 1.5 mg/dL: 4 g MgSO4 (16 to 32 meq [8 to 16 mmol]) over 4 to 12 hrs

    • Mg 1.6 to 1.9 give 1 to 2 grams MgSO4 (8 to 16 meq [4 to 8 mmol]) 1-2 hrs

      • VUMC only has 4g bags of IV mag so would need to ask nurses to only infuse ½ bag

    • Infusion rate should not exceed 2 g/hr to minimize urinary excretion

Additional Information

  • Renal impairment: replete with caution, reduce dose by 50-75% and monitor closely

  • If persistent hypo Mg in pts requiring diuresis, try K-sparing diuretic (e.g. Amiloride)

  • Treat concomitant hypokalemia, hypocalcemia or hypophosphatemia

  • In pts with concomitant hypophos and hypocalcemia, IV Mg alone -> worse hypophos