A 28-year-old female patient with a family history of anesthetic complications is scheduled for emergency appendectomy. During rapid sequence induction, succinylcholine 1.5 mg/kg is administered. Within 60 seconds, the patient develops severe masseter muscle rigidity, making intubation impossible. The anesthesiologist suspects malignant hyperthermia. What is the most appropriate immediate action?

Explanation

## Masseter Muscle Rigidity and Malignant Hyperthermia Susceptibility ### Clinical Context: Masseter Spasm as a Warning Sign Masseter muscle rigidity (MMR) occurring within 30–60 seconds of succinylcholine administration is a **red flag for malignant hyperthermia susceptibility**, even in the absence of other signs (fever, hypercarbia, tachycardia). **Key Point:** Masseter spasm is an **early and specific sign** of MH. It reflects abnormal calcium handling in skeletal muscle and indicates a high risk of fulminant MH crisis if volatile anesthetics are subsequently administered. **Mnemonic: EARLY SIGNS OF MH — "CHOP"** - **C**ore temperature rise (late sign) - **H**ypercarbia (elevated ETCO₂) — earliest sign - **O**pisthotonos / muscle rigidity (jaw clenching, masseter spasm) - **P**eak potassium (hyperkalemia with cardiac dysrhythmias) ### Management of Suspected MH During Induction ```mermaid flowchart TD A[Succinylcholine given]:::action A --> B[Masseter muscle rigidity develops]:::outcome B --> C{Suspected MH}:::decision C --> D[STOP succinylcholine immediately]:::urgent D --> E[Administer dantrolene 2.5 mg/kg IV]:::urgent E --> F[Hyperventilate with 100% O₂]:::action F --> G{Able to intubate?}:::decision G -->|Yes| H[Use rocuronium or vecuronium for paralysis]:::action G -->|No| I[Consider awake fiberoptic intubation or LMA]:::action H --> J[Proceed with TIVA only - NO volatile agents]:::action J --> K[Complete surgery if feasible, or abort]:::action K --> L[ICU admission, dantrolene q4-6h for 24-48h]:::action ``` ### Why Dantrolene Is Correct **High-Yield:** Even though the patient has not yet developed the full MH crisis (no fever, no hypercarbia), the presence of masseter rigidity is **sufficient to diagnose MH susceptibility** and mandate immediate dantrolene administration. **Rationale:** 1. **Masseter spasm = confirmed MH trigger response** — succinylcholine has activated the abnormal calcium release mechanism 2. **Prevent fulminant crisis** — if volatile anesthetics are given without dantrolene, the patient will rapidly deteriorate with hypercarbia, hyperthermia, hyperkalemia, and cardiac arrest 3. **Dantrolene is safe** — it has no contraindications and is the only effective preventive/therapeutic agent 4. **Proceed with surgery using safe anesthesia** — TIVA (propofol + opioid) is completely safe in MH-susceptible patients ### Safe Anesthetic Technique After Dantrolene | Aspect | Safe | Contraindicated | |--------|------|------------------| | **Induction** | Propofol, thiopental, etomidate | Succinylcholine | | **Maintenance** | Propofol infusion, opioids, nitrous oxide | Volatile anesthetics (sevoflurane, isoflurane, desflurane, halothane) | | **Paralysis** | Rocuronium, vecuronium, cisatracurium | Succinylcholine | | **Analgesia** | All opioids safe | — | | **Local anesthetics** | All safe | — | **Clinical Pearl:** Once dantrolene is administered, the patient can safely undergo surgery using TIVA. The operation should proceed because delaying emergency surgery (e.g., appendicitis) carries its own mortality risk. **Warning:** ~~Allowing the patient to wake up and rescheduling~~ is inappropriate for an emergency appendectomy; the patient needs surgery now, and it can be safely done with dantrolene + TIVA. [cite:Miller's Anesthesia 8e Ch 30]