Hypomagnesemia

• Etiology/Pathophysiology
  • Decreased Intake
    • Alcohol use disorder (multifactorial: poor intake, renal wasting, diarrhea).
    • Malnutrition.
    • Total parenteral nutrition (TPN) with insufficient Mg²⁺.
  • GI Losses
    • Diarrhea: Chronic diarrhea from any cause (e.g., IBD, celiac disease).
    • Malabsorption syndromes (e.g., celiac disease, Crohn’s disease, short bowel syndrome).
    • Medications: Proton pump inhibitors (PPIs) (chronic use), laxatives.
  • Renal Losses
    • Diuretics: Loop and thiazide diuretics are a major cause.
    • Alcohol: Directly causes renal magnesium wasting.
    • Uncontrolled diabetes (osmotic diuresis).
    • Nephrotoxic drugs: Amphotericin B, cisplatin, aminoglycosides, cyclosporine.
    • Inherited tubular disorders: Gitelman syndrome, Bartter syndrome.
  • Redistribution
    • Refeeding syndrome.
    • Treatment of diabetic ketoacidosis (DKA).
• Clinical Features
  • Neuromuscular:
    • Hyperexcitability: Tremors, muscle cramps, fasciculations.
    • Tetany: Positive Chvostek sign (facial muscle twitch) and Trousseau sign (carpopedal spasm).
    • Seizures, lethargy, weakness.
  • Cardiovascular:
    • Arrhythmias: Torsades de pointes (polymorphic ventricular tachycardia), premature ventricular contractions (PVCs), atrial fibrillation.
    • EKG findings: Prolonged QT interval, prolonged PR interval, widened QRS.
  • Associated Electrolyte Abnormalities:
    • Refractory Hypokalemia: Mg²⁺ is a cofactor for ROMK channels in the kidney, which excrete K⁺. Low Mg²⁺ leads to increased K⁺ wasting. Hypokalemia will not correct without Mg²⁺ repletion.
    • Hypocalcemia: Low Mg²⁺ impairs PTH secretion and causes end-organ resistance to PTH.
• Diagnostics
  • Serum Mg²⁺ level < 1.8 mg/dL (< 0.7 mmol/L).
  • Check other electrolytes, especially K⁺ and Ca²⁺, which are often concurrently low.
  • EKG to evaluate for arrhythmias and QT prolongation.
  • To differentiate renal vs. GI losses:
    • Fractional excretion of magnesium (FEMg):
      • <2% suggests GI/inadequate intake (kidneys are conserving Mg²⁺).

      • 2% suggests renal wasting.

• Treatment
  • Asymptomatic/Mild:
    • Oral magnesium oxide.
  • Symptomatic/Severe (e.g., arrhythmias, seizures, Torsades):
    • IV magnesium sulfate (MgSO₄).
  • Always correct Mg²⁺ before or concurrently with K⁺ in patients with refractory hypokalemia.

Clinical features

• Neuromuscular excitability
  • Tetany, tremor
• Cardiac arrhythmias
• Hypocalcemia
• Hypokalemia