What are the causes of hypomagnesemia?

Causes of Hypomagnesemia

Hypomagnesemia is primarily caused by decreased intestinal absorption, increased renal losses, or redistribution from extracellular to intracellular space, with medications being a major contributor in hospitalized patients. 1, 2

Common Causes of Hypomagnesemia

Gastrointestinal Causes

• Protein-calorie malnutrition
• Intravenous administration of magnesium-free fluids
• Total parenteral nutrition without adequate magnesium
• Chronic watery diarrhea and steatorrhea
• Short bowel syndrome
• Bowel fistula
• Continuous nasogastric suctioning
• Primary familial magnesium malabsorption (rare) 1

Renal Causes

• Medication-induced renal magnesium wasting:
  • Diuretics (loop and thiazide)
  • Aminoglycoside antibiotics
  • Cisplatin
  • Pentamidine
  • Foscarnet
  • Proton pump inhibitors 3, 1, 4
• Genetic disorders:
  • Bartter's syndrome (hypercalciuria)
  • Gitelman's syndrome (hypocalciuria)
  • Familial renal magnesium wasting 2
• Other renal conditions:
  • Post-obstructive diuresis
  • Post-acute tubular necrosis
  • Renal transplantation
  • Interstitial nephropathy 1

Other Common Causes

• Alcoholism (multifactorial)
• Diabetes mellitus
• Polypharmacy (≥5 medications) in elderly patients 3, 1
• Chemotherapeutic agents (cisplatin, cetuximab) 3

Clinical Significance and Diagnosis

Hypomagnesemia is defined as serum magnesium less than 1.8 mg/dL (< 0.74 mmol/L), with normal levels ranging from 1.5 to 2.5 mEq/L 5, 2. It is common in hospitalized patients, especially those in intensive care units 6.

Important diagnostic considerations:

• Serum magnesium can be normal despite intracellular depletion
• Low serum levels usually indicate significant deficiency 1
• Fractional excretion of magnesium <2% suggests gastrointestinal loss
• Fractional excretion >2% with normal kidney function indicates renal wasting 2

Clinical Manifestations

Symptoms typically appear when serum magnesium falls below 1.2 mg/dL and may include:

• Neuromuscular irritability, clonic twitching, tremors
• Cardiac arrhythmias (including life-threatening ventricular arrhythmias)
• Increased sensitivity to digoxin
• Associated electrolyte abnormalities:
  • Refractory hypokalemia
  • Hypocalcemia 7, 1

Management Considerations

For severe hypomagnesemia (< 1.2 mg/dL) with cardiac manifestations:

• IV magnesium 1-2 g of MgSO4 bolus is recommended for cardiotoxicity and cardiac arrest 7, 3
• Particularly important in polymorphic ventricular tachycardia (torsades de pointes) 7

For asymptomatic or mild hypomagnesemia:

• Oral magnesium supplementation (400-800 mg daily in divided doses) 3
• Verify adequate renal function before administering magnesium 2
• Address underlying causes, particularly medication-related factors

Important Clinical Pearls

• Always correct hypomagnesemia before addressing hypokalemia, as potassium replacement may be ineffective until magnesium is repleted 3
• Consider monitoring serum magnesium in patients on diuretics, proton pump inhibitors, or other high-risk medications 3
• Hypomagnesemia is frequently associated with alcoholism and diabetes through multiple mechanisms 1
• Patients with QTc prolongation (≥500 ms) may benefit from prophylactic magnesium to prevent torsades de pointes 3

Remember that hypomagnesemia may be asymptomatic but can lead to serious complications if not properly identified and treated, particularly in critically ill patients.