Malignant hyperthermia

🧬 Pathophysiology - Calcium Catastrophe

• Genetic Basis: Autosomal Dominant inheritance.

  • Primarily due to mutations in the RYR1 gene (ryanodine receptor 1 on sarcoplasmic reticulum).
  • Less commonly, the CACNA1S gene (dihydropyridine receptor).

• Mechanism:

  • Triggers: Volatile inhaled anesthetics (e.g., sevoflurane) or succinylcholine.
  • Defect: Triggers cause the defective RYR1 receptor to open abnormally, leading to a massive, uncontrolled release of $Ca^{2+}$ from the sarcoplasmic reticulum (SR) into the myoplasm.

⭐ The primary defect is in the ryanodine receptor (RYR1), leading to a massive, uncontrolled release of calcium from the sarcoplasmic reticulum in skeletal muscle.

🔥 Clinical Manifestations - The Heat is On

MH presents as a hypermetabolic crisis. Signs appear sequentially but can overlap, often developing minutes to hours after exposure to a triggering agent.

• Early Signs (First Clues):

  • Hypercarbia: Sudden, unexplained ↑ in end-tidal $CO_2$ ($ETCO_2$) is often the first sign.
  • Tachycardia & Tachypnea: Unexplained sinus tachycardia and rapid breathing.
  • Acidosis: Both respiratory (from ↑ $CO_2$) and metabolic acidosis develop.

• Intermediate Signs (Worsening State):

  • Muscle Rigidity: Generalized muscle rigidity, especially masseter spasm ("jaw of steel").
  • Cyanosis: Mottled, cyanotic skin due to ↑ oxygen consumption.

• Late & Ominous Signs (Crisis Peak):

  • Hyperthermia: Rapid ↑ in core body temperature (1-2°C every 5 minutes); a late but hallmark sign.
  • Myoglobinuria: Dark, cola-colored urine from rhabdomyolysis.

⭐ A sudden, unexpected increase in end-tidal $CO_2$ ($ETCO_2$) that is resistant to increasing minute ventilation is often the earliest and most sensitive sign of an acute MH episode.

🔥 Diagnosis - Spotting the Firestorm

Diagnosis is primarily clinical during an acute crisis, based on a high index of suspicion. Immediate lab tests confirm the metabolic chaos.

• Arterial Blood Gas (ABG):
  • Severe mixed respiratory (↑$PCO_2$) and metabolic acidosis (↑lactate).
• Key Lab Findings:
  • Hyperkalemia: Rapid ↑K+ from muscle breakdown.
  • Elevated Creatine Kinase (CK): Peaks >10,000 IU/L within 24 hours.
  • Myoglobinuria: Dark, tea-colored urine; risk of acute kidney injury.

⭐ While clinical suspicion is key for acute management, the gold standard for diagnosis is the caffeine-halothane contracture test (CHCT) on a skeletal muscle biopsy, performed electively after the acute event.

🧯 Management - Dousing the Flames

Immediate, aggressive intervention is critical. Follow a stepwise protocol.

⭐ Dantrolene is a specific ryanodine receptor (RYR1) antagonist. It inhibits Ca²⁺ release from the sarcoplasmic reticulum, directly counteracting the core pathophysiological defect in skeletal muscle.

• Treat Hyperkalemia: IV insulin, glucose, calcium gluconate.
• Correct Acidosis: Sodium bicarbonate for severe metabolic acidosis (pH <7.2).
• Monitor: Urine output (>1 mL/kg/hr), core temp, ETCO₂, ABG, CK levels.

⚡ Biggest Takeaways

• A life-threatening hypermetabolic crisis triggered by inhaled anesthetics (e.g., sevoflurane) and succinylcholine.
• Autosomal dominant mutation in the ryanodine receptor (RYR1) gene, causing massive sarcoplasmic Ca²⁺ release.
• Early signs: tachycardia, masseter muscle rigidity, and a sudden ↑ end-tidal CO₂.
• Late signs: fever (hyperthermia), rhabdomyolysis, hyperkalemia, and severe metabolic acidosis.
• Immediate treatment is dantrolene, which blocks the RYR1 channel, preventing further calcium efflux.