A 35-year-old male patient is scheduled for emergency laparotomy for acute abdomen. Induction is with propofol and fentanyl. During intubation, succinylcholine 1.5 mg/kg IV is administered. Within 30 seconds of drug administration, the patient develops severe muscle rigidity, trismus, and masseter muscle spasm. Core temperature begins to rise. What is the most appropriate immediate management?

Explanation

## Clinical Recognition of Malignant Hyperthermia Trigger **Key Point:** Masseter muscle spasm (trismus) following succinylcholine administration is a **red flag sign** for malignant hyperthermia (MH) susceptibility, even in the absence of overt temperature elevation at presentation. ### Pathophysiology Succinylcholine is a **depolarising agent** that causes sustained depolarisation of muscle membrane, leading to uncontrolled calcium release from the sarcoplasmic reticulum in MH-susceptible individuals. This results in: 1. Sustained muscle contraction and rigidity 2. Hypermetabolism and heat production 3. Rhabdomyolysis with myoglobinuria 4. Electrolyte derangements (hyperkalaemia, hypocalcaemia) 5. Disseminated intravascular coagulation (DIC) ### Early Signs vs Late Signs | Feature | Early (Intraoperative) | Late (Post-operative) | |---------|------------------------|----------------------| | **Masseter spasm** | ✓ (within 30 sec) | — | | **Muscle rigidity** | ✓ | ✓ | | **Tachycardia** | ✓ | ✓ | | **Tachypnoea** | ✓ | ✓ | | **Core temperature ↑** | Late sign | ✓ | | **Cyanosis** | ✓ | ✓ | | **Rhabdomyolysis** | Begins | Evident | **High-Yield:** Masseter spasm alone is sufficient to **abort surgery and treat for MH** — do not wait for fever. ### Immediate Management Protocol ```mermaid flowchart TD A[Masseter spasm + succinylcholine]:::urgent --> B[STOP all triggering agents]:::action B --> C[Abort surgery if possible]:::action C --> D[Hyperventilate 100% O₂]:::action D --> E[Administer dantrolene 2.5 mg/kg IV]:::action E --> F[Repeat every 5 min until signs resolve]:::action F --> G[Monitor core temperature]:::action G --> H[Aggressive cooling measures]:::action H --> I[Monitor for complications]:::action I --> J[Rhabdomyolysis, hyperkalaemia, DIC, acute kidney injury]:::outcome ``` ### Dantrolene Mechanism **Key Point:** Dantrolene sodium is a **ryanodine receptor antagonist** that blocks calcium release from the sarcoplasmic reticulum, directly addressing the pathophysiology of MH. - **Dose:** 2.5 mg/kg IV bolus; repeat every 5 minutes up to 10 mg/kg total - **Onset:** 5–10 minutes - **Reconstitution:** Each 20 mg vial requires 60 mL sterile water (no bacteriostatic agents) - **Availability:** Must be immediately available in all ORs where triggering agents are used ### Supportive Care 1. **Cooling:** Ice packs, cold IV fluids, gastric/bladder lavage, extracorporeal cooling if core temp > 39°C 2. **Acidosis correction:** Sodium bicarbonate for metabolic acidosis 3. **Hyperkalaemia management:** Calcium gluconate, insulin + dextrose, hyperventilation 4. **Rhabdomyolysis:** Aggressive fluid resuscitation (target urine output > 200 mL/h), mannitol, furosemide 5. **Coagulopathy:** Fresh frozen plasma, platelets, cryoprecipitate as indicated 6. **Post-operative:** ICU monitoring for 24–48 hours; risk of recrudescence **Clinical Pearl:** Masseter spasm in isolation (without other signs) occurs in ~0.1% of general anaesthetics and is not always MH, but the **risk of progression is ~50%** — treat prophylactically. **Mnemonic — MH Early Signs: CHOP** — **C**yanosis, **H**ypertension/Hyperthermia, **O**xygen desaturation, **P**ressure (muscle rigidity) [cite:Stoelting's Pharmacology in Anesthesia Ch 12]