A 32-year-old male patient is scheduled for emergency laparotomy for acute appendicitis. During rapid sequence intubation, succinylcholine 1.5 mg/kg IV is administered. Within 30 seconds of administration, the patient develops severe muscle rigidity, hyperthermia (core temperature 38.5°C), tachycardia (HR 120/min), and dark-colored urine. Serum potassium is 6.8 mEq/L (normal 3.5–5.0). What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** The constellation of muscle rigidity, rapid-onset hyperthermia, tachycardia, hyperkalemia, and myoglobinuria (dark urine) occurring within seconds to minutes of succinylcholine administration is pathognomonic for malignant hyperthermia (MH). ## Mechanism of Malignant Hyperthermia Malignant hyperthermia is a pharmacogenetic disorder of skeletal muscle calcium regulation, most commonly triggered by depolarizing muscle relaxants (succinylcholine) and volatile anesthetics. 1. Succinylcholine causes sustained depolarization of muscle membrane 2. In MH-susceptible individuals, this triggers uncontrolled calcium release from the sarcoplasmic reticulum via defective ryanodine receptors (RYR1) or calcium release channel mutations 3. Sustained muscle contraction → rapid ATP depletion → heat generation → hyperthermia 4. Muscle breakdown (rhabdomyolysis) → myoglobin release → dark urine 5. Cellular potassium leak → severe hyperkalemia ## Diagnostic Features in This Case | Feature | Present | Significance | |---------|---------|-------------| | Trigger agent | Succinylcholine | Classic MH trigger | | Onset timing | Within 30 sec | MH is rapid-onset | | Muscle rigidity | Yes | Sustained contraction | | Hyperthermia | 38.5°C | Late sign; core temp rises 1–2°C/5 min | | Hyperkalemia | 6.8 mEq/L | Severe; risk of cardiac arrhythmias | | Myoglobinuria | Dark urine | Rhabdomyolysis marker | | Tachycardia | Yes | Sympathetic response to crisis | **High-Yield:** Early signs of MH are muscle rigidity and tachycardia; hyperthermia is a LATE sign. Hyperkalemia and myoglobinuria are secondary to rhabdomyolysis. ## Management Algorithm ```mermaid flowchart TD A[Suspected MH during anesthesia]:::urgent --> B[STOP volatile anesthetics & succinylcholine]:::action B --> C[Hyperventilate with 100% O2]:::action C --> D[Administer dantrolene 2.5 mg/kg IV]:::action D --> E[Repeat dantrolene q5min up to 10 mg/kg]:::action E --> F[Aggressive cooling measures]:::action F --> G[Monitor K+, CK, myoglobin, urine color]:::action G --> H[ICU admission for continued monitoring]:::outcome H --> I[Genetic testing & family counseling]:::action ``` **Clinical Pearl:** Dantrolene acts by inhibiting calcium release from the sarcoplasmic reticulum, directly addressing the pathophysiology of MH. Each vial must be reconstituted with sterile water (no bacteriostatic agents), making preparation time-critical. **Mnemonic: CHOP (Core temperature, Hyperkalemia, Oxygen consumption, Potassium leak)** - Core temperature rises rapidly - Hyperkalemia from muscle breakdown - Oxygen consumption increases dramatically - Potassium leaks from damaged myocytes ## Why This Is NOT Succinylcholine-Induced Hyperkalemia Alone While succinylcholine does cause transient hyperkalemia (0.5–1.0 mEq/L rise in normal patients), the degree of hyperkalemia here (6.8 mEq/L) combined with muscle rigidity, hyperthermia, and myoglobinuria indicates rhabdomyolysis from MH, not simple succinylcholine hyperkalemia. [cite:Miller's Anesthesia 8e Ch 12]