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

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Malignant Hyperpyrexia (Malignant Hyperthermia)

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