Hypomagnesemia

Hypomagnesemia (low serum magnesium) is very common in critically unwell patients but can occur in the community, particularly secondary to medication use. Magnesium has a vital role in membrane stabilization and while the symptoms of mild deficiency can be nonspecific, severe hypomagnesemia can cause serious complications, such as cardiac arrhythmias and seizures. Hypomagnesemia is commonly seen alongside other electrolyte abnormalities, and magnesium levels should always be checked in patients with hypokalemia or hypocalcemia. Management consists of magnesium replacement, treatment of associated electrolyte imbalances, and addressing the underlying causes.

• Serum magnesium concentration < 1.5 mEq/dL ^[1]
• Severe hypomagnesemia: magnesium concentration < 1.0 mEq/dL ^[1]

Low body magnesium stores can affect many organ systems. The presence and severity of symptoms do not always correlate with serum magnesium levels. ^[2]

• Gastrointestinal: anorexia, nausea, vomiting
• Neuromuscular
  • Muscle weakness, lethargy
  • Tremor
  • Tetany
  • Muscle cramps, spontaneous carpopedal spasm
  • Hyperreflexia
  • Paresthesia
  • Positive Chvostek sign and Trousseau sign
  • Ataxia, nystagmus
  • Seizures ^[4]
• Cardiac ^[5]
  • Prolonged PR interval
  • Widening of QRS complex
  • Prolonged QT interval
  • Peaking T waves (moderate hypermagnesemia) or T wave flattening (severe hypermagnesemia)
  • Possibly U waves
  • Supraventricular tachycardias
  • Ventricular arrhythmias
  • Torsades de pointes
  • Atrial fibrillation
  • Cardiac arrest ^[5]
• Metabolic disturbances
  • Abnormalities of calcium metabolism
    • Hypocalcemia (in severe hypomagnesemia)
    • Hypoparathyroidism
    • PTH resistance
    • ↓ Calcitriol synthesis
  • Hypokalemia

• Magnesium supplementation
  • Repletion should be considered in all patients with symptoms consistent with hypomagnesemia, including patients with normal serum magnesium levels. ^[2]
  • See “Repletion regimens for hypomagnesemia.”
• Identify and treat concurrent electrolyte abnormalities, particularly hypokalemia and hypocalcemia. ^[1][5]
• Identify and treat the underlying cause (see causes of hypomagnesemia).
• Consider continuous cardiac monitoring. ^[6]

Parenteral administration of magnesium can reduce serum calcium levels, which can worsen preexisting hypocalcemia.

• Cardiac arrest
• Seizures

We list the most important complications. The selection is not exhaustive.

Neonatal hypomagnesemia ^[7]

• Definition: low serum magnesium levels in a newborn
• Risk factors
  • Maternal diabetes
  • Prematurity
  • Fetal growth restriction
  • Hypocalcemia
  • Hypoparathyroidism
  • Renal tubular disorders (e.g., Gitelman syndrome)
• Pathophysiology: maternal hyperglycemia → abnormal maternal calcium-phosphorus metabolism → ↑ maternal urinary Mg excretion → maternal hypomagnesemia → fetal hypomagnesemia → impaired PTH synthesis in the fetus → fetal hypocalcemia and hypomagnesemia
• Clinical features
  • May be asymptomatic
  • Gastrointestinal symptoms (e.g., feeding intolerance)
  • Neuromuscular symptoms (e.g., jitteriness, seizures)
  • Respiratory symptoms (e.g., apnea)
• Diagnostics: serum magnesium level < 1.6 mg/dL
• Treatment: IV magnesium for symptomatic patients

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