Hypomagnesemia

Background

• Serum Magnesium (Mg⁺²) < 1.8 mg/dL, most pts asymptomatic until <1.2 mg/dL
  • • Severe [Mg⁺²] < 1 mg/dL
• Causes    
  • • GI: Diarrhea, malabsorption, acute pancreatitis, EtOH use, terminal cancer, TPN, vomiting, NG suction, GI fistulas, anorexia, short gut syndrome, small bowel bypass
    • Drugs: PPIs, loop diuretics, thiazides, digoxin, amphotericin, aminoglycosides, foscarnet, cisplatin, cetuximab, CNIs, laxatives, pentamidine
    • Renal: ATN recovery, transplant, others genetic or drugs as listed elsewhere
    • Extracellular->Intracellular shift: DKA treatment/recovery, refeeding, hungry bone syndrome, correction of metabolic acidosis, pancreatitis, EtOH withdrawal
    • Genetic: Gittleman, Bartter, multiple other rare conditions
    • Other: DM, hyperCa, hyperthyroid, hyperaldosteronism, burns, lactation, Vit D deficiency, heat, prolonged exercise, mitral valve prolapse, pseudohypomagnesemia 2/2 EDTA tube, lactation

Presentation

• Consider in ICU pts, refractory hypocalcemia or hypokalemia, heart failure, any arrhythmia, chronic diarrhea/GI losses, refeeding, pts on PPIs or diuretics
• Initial symptoms: nausea, vomiting, appetite loss, fatigue, weakness, paresthesia’s, contractions, cramps, depression, agitation, psychosis, dysphagia
• Severe symptoms: seizures, drowsiness, confusion, coma, arrhythmias
• Vertical nystagmus, tetany (Chvostek sign, Trousseau), tremors, fasciculations, seizures
• EKG: Initially wide QRS, peaked Ts. Progresses to wide PR, diminished T, arrhythmias
• Other labs: Ca⁺², K⁺, can use FE Mg⁺² (order urine Mg⁺² and Cr, serum Cr and Mg⁺²) or 24-hour urine for Mg⁺² to distinguish renal vs GI etiology (FE Mg⁺²>2% renal, <2% GI)
• Urine studies need to be done when pt at steady state, if checked shortly after IV magnesium given you will see elevated magnesium excretion no matter the cause

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⁺² Chloride or Mg⁺² L-lactate) better tolerated and absorbed, though standard preparations (Mg⁺² oxide) are faster acting
    • Mg⁺² chloride: 3-4 tabs BID (total 30 to 56 meq [15 to 28 mmol]) for severe hypo Mg⁺²
    • 2-4 tabs daily (total 10 to 28 meq [5 to 14 mmol]) for mild hypo Mg⁺²
    • Mg⁺² oxide: 400-800 mg BID (20 to 40 mmol [40 to 80 meq]) for mod-severe hypo Mg⁺²
• Intravenous: for symptomatic patients or if GI intolerance to oral
  • • Mg⁺² <1 mg/dL: 4 to 8g of MgSO4 (32 to 64 meq [16 to 32 mmol]) over 12 to 24 hrs
    • Mg⁺² 1 to 1.5 mg/dL: 4 g MgSO4 (16 to 32 meq [8 to 16 mmol]) over 4 to 12 hrs
    • Mg⁺² 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 1/2 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⁺² 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⁺² alone -> worse hypophos