## Recognition and Immediate Management of Malignant Hyperthermia ### Clinical Presentation in This Case **Key Point:** The constellation of jaw rigidity (masseter muscle spasm), rapid rise in end-tidal CO₂, unexplained hyperthermia, muscle stiffness, and hyperkalemia within minutes of succinylcholine exposure is pathognomonic for malignant hyperthermia (MH). ### Pathophysiology Malignant hyperthermia is a pharmacogenetic disorder of skeletal muscle calcium regulation triggered by depolarizing neuromuscular blockers (succinylcholine) and/or volatile anesthetics. The defect in the ryanodine receptor (RYR1) or CACNA1S gene leads to uncontrolled calcium release from the sarcoplasmic reticulum, causing: 1. Sustained muscle contraction and rigidity 2. Hypermetabolism → rapid CO₂ production 3. Rhabdomyolysis → myoglobinuria and hyperkalemia 4. Uncontrolled heat generation → hyperthermia (late sign) **High-Yield:** Early signs are jaw rigidity and masseter spasm (within seconds to minutes), NOT fever. Hyperthermia is a late sign and should never be waited for before diagnosis. ### Immediate Management Algorithm ```mermaid flowchart TD A[Suspected MH: Jaw rigidity + ↑ETCO₂ + Muscle stiffness]:::urgent --> B[STOP all triggering agents]:::action B --> C[Abort surgery if possible]:::action C --> D[Hyperventilate with 100% O₂]:::action D --> E[Administer dantrolene 2.5 mg/kg IV]:::action E --> F[Repeat dantrolene q5min up to 10 mg/kg]:::action F --> G[Active cooling: cold IV fluids, ice packs, cold peritoneal lavage]:::action G --> H[Monitor: ECG, ABG, K⁺, CK, myoglobin, urine output]:::action H --> I[Treat hyperkalemia if K⁺ > 6 mEq/L]:::action I --> J[Aggressive hydration + furosemide for rhabdomyolysis]:::action J --> K[Admit to ICU for 24–48 hr monitoring]:::outcome ``` ### Dantrolene Sodium: Mechanism and Dosing **Key Point:** Dantrolene is a selective ryanodine receptor antagonist that prevents calcium release from the sarcoplasmic reticulum. It is the ONLY specific treatment for MH. | Parameter | Details | | --- | --- | | **Mechanism** | Blocks SR calcium release; reduces muscle contractility | | **Initial dose** | 2.5 mg/kg IV rapid bolus | | **Repeat dosing** | Every 5 minutes if MH signs persist, up to max 10 mg/kg | | **Onset** | 5–10 minutes | | **Preparation** | Each 20 mg vial requires 60 mL sterile water (difficult to reconstitute; have multiple vials ready) | | **Side effects** | Weakness, phlebitis, hepatotoxicity (rare) | ### Management of Complications **Hyperkalemia (K⁺ 6.2 mEq/L in this case):** - Calcium gluconate 10% (10 mL IV) for cardiac membrane stabilization - Insulin 10 units + 25 g dextrose IV - Sodium bicarbonate 1–2 mEq/kg IV - Hyperventilation (already done) **Rhabdomyolysis & Myoglobinuria:** - Aggressive IV hydration (target urine output 200–300 mL/hr) - Furosemide to maintain urine flow - Alkalinize urine with sodium bicarbonate (target urine pH > 6.5) to prevent myoglobin precipitation in renal tubules - Monitor CK, myoglobin, creatinine **Acidosis:** - Correct with hyperventilation and sodium bicarbonate - Acidosis worsens hyperkalemia and rhabdomyolysis ### Post-Crisis Management **Clinical Pearl:** After acute MH crisis, continue dantrolene 1 mg/kg IV every 4–6 hours for 24–48 hours to prevent recrudescence (recurrence of MH signs after initial control). **High-Yield:** All first-degree relatives must be counseled about MH risk and referred for malignant hyperthermia susceptibility testing (MHAUS caffeine halothane contracture test or genetic testing). [cite:Miller's Anesthesia 8e Ch 34]
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