Hypomagnesemia

Hypomagnesemia – Mike Tozier

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