A 28-year-old male from rural Maharashtra is brought to the operating room for emergency appendectomy. Fifteen minutes after induction with propofol and intubation with succinylcholine, the patient develops masseter muscle rigidity, followed by generalized muscle rigidity, tachycardia (HR 145/min), and a core temperature of 39.2°C. Which is the most common genetic mutation associated with this malignant hyperthermia crisis?

Explanation

## Genetic Basis of Malignant Hyperthermia ### Most Common MH Susceptibility Gene **Key Point:** RYR1 gene mutations account for approximately 50–80% of all malignant hyperthermia cases, making it the most common genetic cause of MH susceptibility worldwide. **High-Yield:** The RYR1 gene encodes the ryanodine receptor 1, a calcium release channel in the sarcoplasmic reticulum of skeletal muscle. Mutations cause abnormal calcium handling and uncontrolled muscle contraction in response to volatile anesthetics and succinylcholine. ### Genetic Mutations in Malignant Hyperthermia | Gene | Protein | Frequency | Inheritance | Clinical Features | |------|---------|-----------|-------------|-------------------| | **RYR1** | **Ryanodine receptor 1** | **50–80%** | **Autosomal dominant** | **Most common; classic MH presentation** | | CACNA1S | Calcium channel α-1S subunit | 10–20% | Autosomal dominant | Second most common; similar phenotype | | STAC3 | STAC family member 3 | <1% | Autosomal recessive | Rare; native American populations | | TRPV4 | TRP cation channel | Rare | Autosomal dominant | Not a primary MH gene; associated with other myopathies | **Clinical Pearl:** Both RYR1 and CACNA1S mutations affect calcium homeostasis in the excitation–contraction coupling pathway, leading to similar clinical presentations of MH. However, RYR1 is far more prevalent in clinical practice. ### Pathophysiology of RYR1-Mediated MH ```mermaid flowchart TD A[Volatile anesthetic or succinylcholine exposure]:::action --> B[RYR1 mutation in sarcoplasmic reticulum]:::outcome B --> C[Abnormal calcium release from SR]:::action C --> D[Sustained muscle contraction]:::urgent D --> E[Hypermetabolism]:::urgent E --> F[Heat production]:::urgent F --> G[Hyperthermia, rhabdomyolysis, hyperkalemia]:::urgent G --> H[Cardiac arrhythmias, acute kidney injury]:::urgent ``` **Mnemonic — MH Genetics: RYR-STAC** - **RYR1** — most common (Ryanodine Receptor 1) - **STAC3** — rare, recessive - **CACNA1S** — second most common (Calcium chANnel) ### Clinical Presentation in This Case **Key Point:** Masseter muscle rigidity (MMR) following succinylcholine is a classic early sign of MH susceptibility, particularly in RYR1-positive individuals. This patient's rapid progression to generalized rigidity, hyperthermia, and tachycardia is pathognomonic for MH crisis. **Warning:** Masseter muscle rigidity alone (without other signs) is sometimes called "awake rigidity" and may not always predict MH, but in the context of volatile anesthetic use and other MH signs (hyperthermia, tachycardia, muscle rigidity), it is highly suggestive of MH susceptibility.