Malignant Hyperthermia

Intro & Pathophysiology - Fiery Muscle Mayhem

• Rare, life-threatening pharmacogenetic disorder of skeletal muscle hypermetabolism.
• Triggered by: All volatile halogenated anesthetics (e.g., halothane, sevoflurane, isoflurane), succinylcholine.
• Pathophysiology: "Fiery Muscle Mayhem" due to uncontrolled sarcoplasmic $Ca^{2+}$ release.
  • Primary defect: Abnormal ryanodine receptor (RYR1) on sarcoplasmic reticulum (SR).
  • Less common: Dihydropyridine receptor (DHPR, CACNA1S gene) defects.
  • Mechanism: Triggers → massive, sustained $Ca^{2+}$ efflux from SR → sustained muscle contraction.
  • Consequences:
    • Hypermetabolism: ↑$O_2$ consumption, ↑$CO_2$ production (early sign).
    • ATP depletion, heat production (hyperthermia).
    • Muscle damage: Rhabdomyolysis (↑CK, ↑myoglobin), hyperkalemia.
    • Lactic acidosis.

⭐ RYR1 gene mutations (chromosome 19q13.1) account for >70% of MH susceptibility; inheritance is typically autosomal dominant.

Triggers & Clinical Features - Alarming Signs Ignite

• Primary Triggers:
  • Volatile halogenated anesthetics (e.g., halothane, isoflurane, sevoflurane, desflurane)
  • Succinylcholine (depolarizing muscle relaxant)
• Predisposition:
  • Genetic susceptibility (e.g., RYR1, CACNA1S gene mutations)
• Rare:
  • Extreme physiological stress, heatstroke in susceptible individuals

Start["⚠️ Malignant Triggers
• Succinylcholine• Volatile gases"]

Calcium["📋 Calcium Release
• ⬆️ Sarcoplasmic Ca2+• RYR1 gene defect"]

Contraction["🩺 Muscle Contraction
• Sustained state• Uncontrolled ATP"]

Hypermetabolism["📋 Hypermetabolism
• ⬆️ O2 consumption• Massive energy use"]

Rigidity["🩺 Muscle Rigidity
• Masseter spasm• Jaw stiffness"]

Rhabdo["🩺 Rhabdomyolysis
• Muscle breakdown• Cell death"]

CO2Prod["🔬 CO2 Production
• ETCO2 > 55 mmHg• Early sign MH"]

HeatProd["📋 Heat Production
• ⬆️ Metabolic heat• Rapid thermogenesis"]

Vitals["⚠️ Vital Signs
• Tachycardia• Tachypnea"]

Hyperthermia["⚠️ Hyperthermia
• Temp > 40 deg C• Late presentation"]

Labs["🔬 Lab Findings
• Hyperkalemia• Myoglobinuria"]

Failure["⚠️ Organ Failure
• Severe acidosis• Multiple failure"]

Start --> Calcium Calcium --> Contraction Contraction --> Hypermetabolism Contraction --> Rigidity Contraction --> Rhabdo

Hypermetabolism --> CO2Prod Hypermetabolism --> HeatProd Hypermetabolism --> Failure

CO2Prod --> Vitals HeatProd --> Hyperthermia HeatProd --> Failure Rigidity --> Failure Rhabdo --> Labs Rhabdo --> Failure Labs --> Failure

style Start fill:#FDF4F3, stroke:#FCE6E4, stroke-width:1.5px, rx:12, ry:12, color:#B91C1C style Calcium fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style Contraction fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style Hypermetabolism fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style Rigidity fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style Rhabdo fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style CO2Prod fill:#FFF7ED, stroke:#FFEED5, stroke-width:1.5px, rx:12, ry:12, color:#C2410C style HeatProd fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style Vitals fill:#FDF4F3, stroke:#FCE6E4, stroke-width:1.5px, rx:12, ry:12, color:#B91C1C style Hyperthermia fill:#FDF4F3, stroke:#FCE6E4, stroke-width:1.5px, rx:12, ry:12, color:#B91C1C style Labs fill:#FFF7ED, stroke:#FFEED5, stroke-width:1.5px, rx:12, ry:12, color:#C2410C style Failure fill:#FDF4F3, stroke:#FCE6E4, stroke-width:1.5px, rx:12, ry:12, color:#B91C1C

> ⭐ An unexplained and persistent rise in End-Tidal CO₂ (**ETCO₂ >55 mmHg**) is often the earliest, most sensitive, and most specific sign of an acute Malignant Hyperthermia episode.


## Diagnosis & Differentials - Spotting the Inferno

*   **Clinical Suspicion:** Intraoperative ↑ETCO₂, tachycardia, rigidity, hyperthermia.
*   **Lab Markers:**
    -   ↑ ETCO₂ (earliest sign).
    -   ABG: Mixed acidosis.
    -   ↑ K⁺, ↑ CK (often > **20,000 IU/L**).
    -   Myoglobinuria.
*   **Confirmatory Tests:**
    -   Caffeine Halothane Contracture Test (CHCT) - Gold standard (muscle biopsy).
    -   Genetic testing (RYR1 mutations).

> ⭐ Unexplained, persistent ↑ETCO₂ refractory to increased ventilation is a hallmark early sign of MH.

| Differential          | Differentiator                                      |
| :-------------------- | :-------------------------------------------------- |
| Thyroid Storm         | Thyroid history, slower onset, goiter               |
| Sepsis                | Infection signs, hypotension, ↑WBC                  |
| NMS                   | Antipsychotics, slow onset, lead-pipe rigidity      |
| Pheochromocytoma      | Episodic HTN, headache                              |
| Heat Stroke           | Heat exposure, dry skin (classic)                   |
| Serotonin Syndrome    | Serotonergic drugs, myoclonus, GI sx                |

## Management & Prevention - Dousing the Flames

**Acute MH Crisis:**
1.  **STOP** triggers! Call for help.
2.  **Hyperventilate:** **100% O2** (≥**10 L/min**).
3.  **Dantrolene:** **2.5 mg/kg** IV. Repeat prn (max **10 mg/kg**).
4.  **Cool Patient:** IV cold saline, ice packs. Stop at **38.5°C**.
5.  **Treat:**
    *   Hyperkalemia: Glucose-insulin, $NaHCO_3$.
    *   Acidosis (pH < **7.2**): $NaHCO_3$ **1-2 mEq/kg**.
    *   Arrhythmias: Amiodarone/Lidocaine (NO Ca-channel blockers).
6.  **Monitor:** ETCO2, core temp, ABG, K+, CK, UO.
7.  **ICU Transfer:** Min **24-36h** observation.

```mermaid
%%{init: {'flowchart': {'htmlLabels': true}}}%%
flowchart TD
    Start["<b>⚠️ MH Crisis</b><br><span style='display:block; text-align:left; color:#555'>• Suspected Malignant</span><span style='display:block; text-align:left; color:#555'>• Hyperthermia event</span>"]
    Init["<b>🛑 Initial Steps</b><br><span style='display:block; text-align:left; color:#555'>• Call for help</span><span style='display:block; text-align:left; color:#555'>• Stop triggers</span>"]
    Hypervent["<b>🫁 Ventilation</b><br><span style='display:block; text-align:left; color:#555'>• 100% Oxygen use</span><span style='display:block; text-align:left; color:#555'>• Hyperventilate</span>"]
    Dantrolene["<b>💊 Dantrolene</b><br><span style='display:block; text-align:left; color:#555'>• Give 2.5 mg/kg IV</span><span style='display:block; text-align:left; color:#555'>• Muscle relaxant</span>"]
    SymptomCheck{"<b>📋 Persist?</b><br><span style='display:block; text-align:left; color:#555'>• Check symptoms</span><span style='display:block; text-align:left; color:#555'>• Monitor status</span>"}
    RepeatDantro["<b>💊 Repeat Dose</b><br><span style='display:block; text-align:left; color:#555'>• q5-10min interval</span><span style='display:block; text-align:left; color:#555'>• Max 10 mg/kg</span>"]
    Cooling["<b>❄️ Active Cooling</b><br><span style='display:block; text-align:left; color:#555'>• Cold IV fluids</span><span style='display:block; text-align:left; color:#555'>• Surface cooling</span>"]
    Arrest["<b>🔬 Lab Monitor</b><br><span style='display:block; text-align:left; color:#555'>• Treat acidosis</span><span style='display:block; text-align:left; color:#555'>• Check K+ levels</span>"]
    Supportive["<b>🩺 Support Care</b><br><span style='display:block; text-align:left; color:#555'>• Maintain output</span><span style='display:block; text-align:left; color:#555'>• Monitor dysrhythmia</span>"]
    ICU["<b>👁️ Post-Op Care</b><br><span style='display:block; text-align:left; color:#555'>• ICU admission</span><span style='display:block; text-align:left; color:#555'>• 24hr monitoring</span>"]

    Start --> Init
    Init --> Hypervent
    Hypervent --> Dantrolene
    Dantrolene --> SymptomCheck
    SymptomCheck -->|Yes| RepeatDantro
    RepeatDantro --> SymptomCheck
    SymptomCheck -->|No| Cooling
    Cooling --> Arrest
    Arrest --> Supportive
    Supportive --> ICU

    style Start fill:#FDF4F3, stroke:#FCE6E4, stroke-width:1.5px, rx:12, ry:12, color:#B91C1C
    style Init fill:#FDF4F3, stroke:#FCE6E4, stroke-width:1.5px, rx:12, ry:12, color:#B91C1C
    style Hypervent fill:#F1FCF5, stroke:#BEF4D8, stroke-width:1.5px, rx:12, ry:12, color:#166534
    style Dantrolene fill:#F1FCF5, stroke:#BEF4D8, stroke-width:1.5px, rx:12, ry:12, color:#166534
    style SymptomCheck fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E
    style RepeatDantro fill:#F1FCF5, stroke:#BEF4D8, stroke-width:1.5px, rx:12, ry:12, color:#166534
    style Cooling fill:#F1FCF5, stroke:#BEF4D8, stroke-width:1.5px, rx:12, ry:12, color:#166534
    style Arrest fill:#FFF7ED, stroke:#FFEED5, stroke-width:1.5px, rx:12, ry:12, color:#C2410C
    style Supportive fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8
    style ICU fill:#EEFAFF, stroke:#DAF3FF, stroke-width:1.5px, rx:12, ry:12, color:#0369A1

Prevention:

• Identify at-risk: Family Hx, genetic tests (RYR1), muscle biopsy.
• Safe anesthesia: TIVA, regional, N2O.
• Machine prep: Flush O2 (20-100 min), new circuit/absorbent.
• MH cart accessible.

⭐ Dantrolene's mechanism: Blocks Ca²⁺ release from sarcoplasmic reticulum by antagonizing RYR1 receptors.

High‑Yield Points - ⚡ Biggest Takeaways

• MH: A pharmacogenetic disorder (often autosomal dominant, RYR1 gene) of skeletal muscle.
• Triggers: Volatile anesthetics (e.g., halothane, sevoflurane) and succinylcholine.
• Features: Hypermetabolism - early ↑ETCO2, tachycardia, muscle rigidity (masseter spasm); late hyperthermia.
• Pathophysiology: Defective RYR1 receptor causes uncontrolled Ca²⁺ release in muscle cells.
• Treatment: Stop triggers, 100% O2, IV Dantrolene (2.5 mg/kg), active cooling.
• Dantrolene specifically blocks Ca²⁺ release from the sarcoplasmic reticulum.
• Monitor for: Hyperkalemia, rhabdomyolysis, and metabolic acidosis post-episode.