Makindo Medical Notes

Related Subjects: Acute Kidney Injury |Acute Rhabdomyolysis |Hyperkalaemia |Neuroleptic Malignant Syndrome |Malignant Hyperpyrexia (Malignant Hyperthermia) |Serotonin syndrome |Cholinergic crisis-syndrome |Anticholinergic syndrome

📉 Advances in understanding and treatment have reduced MH mortality from >80% thirty years ago to <5% today.

📖 About

• Triggered by certain anaesthetic drugs in genetically susceptible patients.
• Previous uneventful anaesthesia with triggers does not exclude MH risk.
• Characterised by: hypermetabolism, acidosis, rhabdomyolysis, and hyperkalaemia.
• 🚨 Invariably fatal if untreated.

🧬 Aetiology

• Autosomal dominant predisposition (~1 in 20,000).
• Commonly due to Ryanodine receptor (RYR1) mutation on chromosome 19q13.
• Excess intracellular calcium → sustained muscle contraction & breakdown.

⚙️ Pathogenesis

Uncontrolled calcium release from sarcoplasmic reticulum → hypermetabolic crisis with ↑ O₂ demand, acidosis, and muscle breakdown.

💉 Causes (Triggers)

• Inhalational anaesthetics: Halothane, Sevoflurane, Desflurane.
• Depolarising muscle relaxant: Succinylcholine.
• Rare: Vigorous exercise or extreme heat.

🩺 Clinical Features

• ⬆️ End-tidal CO₂ (ETCO₂) – one of the earliest signs.
• Generalised muscle rigidity (esp. trunk).
• Masseter spasm (early warning sign).
• Hyperthermia >40°C 🌡️, tachycardia, dark urine (myoglobinuria).
• Rapid progression → arrhythmias, cardiac arrest if untreated.

🔍 Differential Diagnosis

• Light anaesthesia.
• Hypoventilation.
• Thyroid storm.
• Overheating.
• Phaeochromocytoma.

🧪 Investigations

• Bloods: U&E (renal function), ↑ CK, ↑ K⁺.
• ABG: Mixed metabolic + respiratory acidosis, ↑ lactate.
• ECG: Monitor for arrhythmias and hyperkalaemia changes.
• Urine: Myoglobinuria (dark red/brown urine).

⚠️ Complications

• Acute Kidney Injury (AKI) 🚰.
• Cardiac arrhythmias → arrest ❤️.
• Severe hyperkalaemia ⚡.
• Disseminated Intravascular Coagulation (DIC).

🚑 Management (Do NOT use Calcium Channel Blockers)

• ABC resuscitation: MH is fatal if not rapidly treated. Call for help immediately.
• Stop triggers: Discontinue volatile agents & succinylcholine. Switch to IV anaesthetics (e.g. propofol).
• 100% Oxygen: Hyperventilate with O₂ ≥10 L/min using clean circuit or Ambu bag.
• Dantrolene: 2–3 mg/kg IV bolus, then 1 mg/kg as needed (max 10 mg/kg). ⚠️ Risk of extravasation injury.
• Cooling: Cool IV fluids, ice packs, cold lavage if needed. Antipyretics ineffective.
• Correct acidosis/hyperkalaemia: IV bicarbonate (if pH <7.2), insulin/dextrose, calcium gluconate if ECG changes.
• ICU admission: Central line, urinary catheter, monitor for AKI, rhabdomyolysis, arrhythmias.

📚 References

• FRCA Notes – Malignant Hyperthermia
• NCBI – Malignant Hyperthermia