Learn Anesthesia

Malignant Hyperthermia

🔍 What is Malignant Hyperthermia (MH)?

Malignant Hyperthermia (MH) is a rare, life-threatening condition triggered by certain anesthetic agents. It results from a genetic mutation in the ryanodine receptor (RYR1), leading to uncontrolled calcium release in skeletal muscle. This causes continuous muscle contraction, hypermetabolism, and severe metabolic derangements.

🛑 Pathophysiology: Why Does MH Occur?

In MH-susceptible individuals, the ryanodine receptor (RYR1) is defective, meaning that:

  1. When triggered by certain anesthetics, calcium channels in the sarcoplasmic reticulum stay open.
  2. This results in sustained muscle contraction, which drastically increases oxygen consumption, CO₂ production, and heat generation.
  3. The body enters a hypermetabolic state, leading to acidosis, rhabdomyolysis, hyperkalemia, and multiorgan failure if untreated.

📌 Why Does MH Cause Hyperkalemia?

  • Constant muscle contraction leads to ATP depletion.
  • Breakdown of muscle cells (rhabdomyolysis) releases potassium into the bloodstream.
  • This can lead to life-threatening cardiac arrhythmias.

🚨 Triggers of Malignant Hyperthermia

MH is triggered by two main classes of anesthetic agents:

  • Volatile Anesthetics (Inhalational Agents)
    • Sevoflurane
    • Desflurane
    • Isoflurane
    • Halothane
  • Depolarizing Neuromuscular Blocker
    • Succinylcholine

📌 Note: MH does NOT occur with non-depolarizing muscle relaxants (e.g., rocuronium, vecuronium) or total intravenous anesthesia (TIVA).

🩺 Clinical Presentation: Early & Late Signs of MH

📌 Early Signs (First 30 Minutes)

  • 📈 Rising EtCO₂ (End-Tidal CO₂) → Despite increased ventilation
  • 🦵 Generalized Muscle Rigidity → Masseter muscle spasm (especially after succinylcholine)
  • ❤️ Tachycardia & Hypertension → Sympathetic overactivity
  • 🧪 Metabolic Acidosis → Increased lactic acid due to hypermetabolism

📌 Late Signs (If Untreated)

  • 🔥 Hyperthermia → Temperature rises rapidly (>39°C or >102.2°F)
  • 🩸 Hyperkalemia → From muscle breakdown, leading to arrhythmias
  • 💔 Cardiac Arrest → Due to severe hyperkalemia & acidosis
  • 🦠 Rhabdomyolysis → Myoglobinuria (cola-colored urine) & acute kidney injury (AKI)

💉 Treatment: MH is Reversible if Treated Promptly!

🚑 Immediate Steps (STOP the Trigger)

✅ Discontinue all volatile anesthetics & succinylcholine immediately
✅ Switch to 100% Oxygen & Hyperventilate (to wash out CO₂)
✅ Start High-Flow IV Fluids to maintain perfusion & prevent renal failure

💊 Dantrolene: The Only Specific Antidote

  • Dose: 2.5 mg/kg IV, repeat every 5-10 min as needed (max 10 mg/kg)
  • Mechanism: Direct RYR1 receptor antagonist → Stops calcium release → Relaxes muscles
  • Post-Treatment: Continue 1 mg/kg/hr IV infusion for 24-48 hours to prevent recurrence

📌 Additional Supportive Care

  • Active Cooling Measures → Ice packs, cold IV fluids
  • Sodium Bicarbonate (50 mEq IV) → Corrects metabolic acidosis
  • Calcium Gluconate or Insulin + Dextrose → Treats hyperkalemia
  • Diuretics (Mannitol/Furosemide) → Prevents myoglobin-induced kidney injury
  • Monitor in ICU for 24-48 Hours → Risk of recurrence within 24 hours

🧬 Who is at Risk? (Genetics & Diagnosis)

  • MH is an autosomal dominant disorder, meaning 50% of first-degree relatives of an MH patient are at risk.
  • Preoperative Screening:
    • Ask about family history of MH, unexplained fevers under anesthesia, or prior anesthetic complications.
  • Definitive Diagnosis:
    • Caffeine-Halothane Contracture Test (CHCT) – Gold standard (performed on muscle biopsy).
    • RYR1 or CACNA1S Gene Testing – Identifies MH susceptibility.

🔬 Prevention: What If a Patient Has MH Risk?

  • Use Total Intravenous Anesthesia (TIVA) instead of volatile anesthetics.
  • Avoid succinylcholine – Use rocuronium or vecuronium for neuromuscular blockade.
  • Ensure MH treatment supplies (Dantrolene) are available in the OR.