RYR1 Channelopathy | Hypermetabolic Crisis | Dantrolene Rescue | Orthopaedic Relevance
CRISIS CLASSIFICATION
Critical Must-Knows
- RYR1 gene mutation causes uncontrolled sarcoplasmic Ca2+ release from skeletal muscle SR
- Triggers: ALL volatile agents (sevoflurane, desflurane, isoflurane, halothane) and suxamethonium
- Earliest signs: rising EtCO2 (unexplained), tachycardia, masseter rigidity
- Dantrolene: 2.5 mg/kg IV bolus, repeat every 5 min up to 10 mg/kg, then 1 mg/kg every 6 hours
- Total IV anaesthesia (TIVA) with propofol is safe in MH-susceptible patients
- Malignant hyperthermia is a clinical diagnosis: treat FIRST, investigate LATER
Clinical Pearls
- "Rising EtCO2 with no change in ventilation is the cardinal early sign
- "Masseter muscle rigidity after suxamethonium may herald MH - stop triggers immediately
- "Dantrolene works by inhibiting RYR1-mediated Ca2+ release, not by central action
- "Hyperkalaemia, not hyperthermia, is the principal cause of early cardiac arrest in MH
Critical MH Recognition Points for the Orthopaedic Surgeon
Earliest Signs
Rising EtCO2 is the most sensitive and specific early sign. Unexplained tachycardia follows. Masseter rigidity after suxamethonium is a warning sentinel. Temperature rise is a LATE sign - do not wait for it.
Triggers
ALL volatile anaesthetic agents (sevoflurane, desflurane, isoflurane, halothane, enflurane). Suxamethonium (succinylcholine). Safe agents: propofol, nitrous oxide, local anaesthetics, opioids, benzodiazepines, non-depolarising muscle relaxants.
Dantrolene
Reconstitute and give IMMEDIATELY. Dose: 2.5 mg/kg IV bolus. Repeat every 5 min (max cumulative 10 mg/kg). Then 1 mg/kg every 6-12 h for 24-48 h. Each vial contains 20 mg; a 70 kg patient needs 9 vials for the first dose alone.
Orthopaedic Relevance
Long cases, tourniquet use, and repetitive suxamethonium dosing raise MH risk. Orthopaedic patients with undiagnosed myopathies (e.g. Duchenne, Becker) are at elevated risk. Surgeons must recognise the intraoperative crisis and support anaesthetic management.
Quick Decision Guide: Intraoperative Crisis
| Presentation | Diagnosis | Immediate Action | Key Pearl |
|---|---|---|---|
| Rapid EtCO2 rise + tachycardia under volatile anaesthesia | Probable evolving MH crisis | STOP volatile agent, hyperventilate with 100% O2, give dantrolene 2.5 mg/kg IV | Do NOT wait for temperature rise |
| Masseter rigidity after suxamethonium | Sentinel MH event (may be isolated or prodrome) | Abandon intubation attempt, switch to non-triggering technique, monitor closely | Up to 50% of masseter rigidity cases progress to fulminant MH |
| Generalised rigidity + hyperthermia + dark urine | Fulminant MH crisis | Dantrolene IV maximum dose, active cooling, treat hyperkalaemia, ICU | Mortality rises dramatically with delayed dantrolene |
CHOPPEDRecognising Malignant Hyperthermia
| C | CO2 rising End-tidal CO2 rises rapidly despite unchanged ventilation |
| H | Heart rate up Unexplained tachycardia, then arrhythmias |
| O | Oxygen desaturation Late sign: falling SpO2 from increased O2 consumption |
| P | PEAK pressures Increased peak airway pressures from rigid chest wall |
| P | Potassium rises Hyperkalaemia from muscle cell lysis - causes cardiac arrest |
| E | Enzymes elevated CK rises dramatically (often greater than 20,000 U/L) |
| D | Dark urine Myoglobinuria from rhabdomyolysis, cola-coloured urine |
| C | CO2 rising End-tidal CO2 rises rapidly despite unchanged ventilation | P | PEAK pressures Increased peak airway pressures from rigid chest wall | D | Dark urine Myoglobinuria from rhabdomyolysis, cola-coloured urine |
| H | Heart rate up Unexplained tachycardia, then arrhythmias | P | Potassium rises Hyperkalaemia from muscle cell lysis - causes cardiac arrest | ||
| O | Oxygen desaturation Late sign: falling SpO2 from increased O2 consumption | E | Enzymes elevated CK rises dramatically (often greater than 20,000 U/L) |
Hook:CHOPPED by volatile agents - the anaesthetic chops down the calcium gate!
DANTROLENEMH Treatment Steps
| D | Dantrolene 2.5 mg/kg IV First and definitive treatment, reconstitute fast |
| A | Airway: 100% O2 hyperventilation Flush volatile agent, high-flow fresh gas |
| N | Nix the trigger Stop ALL volatile agents immediately |
| T | Temperature: active cooling Cold IV fluids, surface cooling, cold lavage |
| R | Replace fluids, treat hyperkalaemia Insulin/glucose, calcium chloride, bicarbonate |
| O | Organ support: ICU Monitor for DIC, renal failure, compartment syndrome |
| L | Laboratory: ABG, CK, K+, myoglobin Track metabolic acidosis, rhabdomyolysis |
| E | Exclusions ruled out Not sepsis, not thyroid storm, not phaeochromocytoma |
| N | Notify: MHAUS hotline Call for guidance, plan genetic testing post-crisis |
| E | End: dantrolene 1 mg/kg q6-12h for 24-48h Prevent recrudescence, observe in ICU minimum 24h |
| D | Dantrolene 2.5 mg/kg IV First and definitive treatment, reconstitute fast | T | Temperature: active cooling Cold IV fluids, surface cooling, cold lavage | L | Laboratory: ABG, CK, K+, myoglobin Track metabolic acidosis, rhabdomyolysis | E | End: dantrolene 1 mg/kg q6-12h for 24-48h Prevent recrudescence, observe in ICU minimum 24h |
| A | Airway: 100% O2 hyperventilation Flush volatile agent, high-flow fresh gas | R | Replace fluids, treat hyperkalaemia Insulin/glucose, calcium chloride, bicarbonate | E | Exclusions ruled out Not sepsis, not thyroid storm, not phaeochromocytoma | ||
| N | Nix the trigger Stop ALL volatile agents immediately | O | Organ support: ICU Monitor for DIC, renal failure, compartment syndrome | N | Notify: MHAUS hotline Call for guidance, plan genetic testing post-crisis |
Hook:DANTROLENE spells the treatment - from first dose to post-crisis care!
SAFE-TRIGGERSSafe vs Unsafe Agents in MH-Susceptible Patients
| S | Safe: Propofol TIVA Total IV anaesthesia with propofol is the standard safe technique |
| A | Safe: All opioids Fentanyl, morphine, remifentanil - all safe |
| F | Foreign: Avoid ALL volatiles Sevoflurane, desflurane, isoflurane, halothane - ALL trigger MH |
| E | Endure: No suxamethonium Succinylcholine is absolutely contraindicated in MH-susceptible patients |
| T | Trust: Non-depolarising relaxants Rocuronium, atracurium, vecuronium - all safe |
| R | Relax: Nitrous oxide is safe N2O does not trigger MH |
| I | Injectables: Local anaesthetics safe Lidocaine, bupivacaine, ropivacaine - all safe |
| G | Guarantee: Pre-warn anaesthetist Known MH susceptibility requires a triggered-free anaesthetic machine |
| E | Equipment: Flush anaesthetic machine Flush vaporiser-free circuit with high-flow O2 for minimum 20 min |
| R | Ready: Dantrolene immediately available Keep dantrolene drawn up or immediately accessible |
| S | Surgery: Regional is safest Spinal or epidural anaesthesia avoids all triggering agents |
| S | Safe: Propofol TIVA Total IV anaesthesia with propofol is the standard safe technique | E | Endure: No suxamethonium Succinylcholine is absolutely contraindicated in MH-susceptible patients | I | Injectables: Local anaesthetics safe Lidocaine, bupivacaine, ropivacaine - all safe | R | Ready: Dantrolene immediately available Keep dantrolene drawn up or immediately accessible |
| A | Safe: All opioids Fentanyl, morphine, remifentanil - all safe | T | Trust: Non-depolarising relaxants Rocuronium, atracurium, vecuronium - all safe | G | Guarantee: Pre-warn anaesthetist Known MH susceptibility requires a triggered-free anaesthetic machine | S | Surgery: Regional is safest Spinal or epidural anaesthesia avoids all triggering agents |
| F | Foreign: Avoid ALL volatiles Sevoflurane, desflurane, isoflurane, halothane - ALL trigger MH | R | Relax: Nitrous oxide is safe N2O does not trigger MH | E | Equipment: Flush anaesthetic machine Flush vaporiser-free circuit with high-flow O2 for minimum 20 min |
Hook:SAFE-TRIGGERS tells you what is safe and what triggers MH!
Overview and Epidemiology
Why This Matters
Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle presenting as a hypermetabolic crisis when a genetically susceptible individual is exposed to volatile anaesthetic agents or suxamethonium. Although primarily managed by anaesthetists, orthopaedic surgeons must recognise the crisis, support resuscitation, and understand which of their patients carry elevated risk - particularly those with undiagnosed myopathies (Duchenne, Becker) who may present for orthopaedic procedures. MH remains a core basic science viva topic across FRCS, FRACS, and ABOS examinations.
Incidence and Demographics
- Incidence: approximately 1 in 100,000 adult anaesthetics involving volatile agents; higher in children (1 in 30,000)
- Mortality: less than 5% with prompt dantrolene treatment; historically 70-80% before dantrolene
- Genetics: autosomal dominant with variable penetrance; RYR1 mutations account for approximately 70% of cases; CACNA1S mutations approximately 1%
- Sex: males more frequently affected (higher muscle mass amplifies the hypermetabolic response)
Orthopaedic Clinical Impact
- Long surgical cases: prolonged volatile exposure increases risk
- Myopathy patients: Duchenne and Becker muscular dystrophy carry elevated MH-like risk (distinct mechanism but overlapping crisis)
- Tourniquet use: may mask or delay recognition of limb rigidity
- Scoliosis surgery: neuromuscular scoliosis patients (e.g. Duchenne) are a high-risk group
- Post-operative pain: opioid-safe analgesia is important; regional techniques preferred
Pathophysiology
RYR1 Calcium Channel Mechanism
Malignant hyperthermia results from a defect in the ryanodine receptor type 1 (RYR1), the major calcium release channel of the sarcoplasmic reticulum (SR) in skeletal muscle. In normal physiology, depolarisation of the motor endplate activates dihydropyridine receptors (DHPR) on the T-tubule membrane, which mechanically couple to RYR1, triggering brief Ca2+ release from the SR. SERCA pumps then actively sequester Ca2+ back into the SR, restoring baseline. In MH-susceptible individuals, RYR1 mutations render the channel hyper-responsive to triggering agents: volatile anaesthetics and suxamethonium cause prolonged, uncontrolled Ca2+ release. The result is sustained myofibrillar contraction (rigidity), massive ATP consumption, and a hypermetabolic cascade producing heat, CO2, lactate, and potassium.
Pathophysiology Cascade: Trigger to Crisis
| Step | Event | Clinical Correlate | Time Course |
|---|---|---|---|
| 1. Trigger exposure | Volatile agent or suxamethonium binds RYR1 complex | Onset: within minutes of exposure | Minutes to hours |
| 2. Uncontrolled Ca2+ release | RYR1 channel locked open, massive SR calcium efflux | Masseter rigidity, then generalized muscle rigidity | 5-30 minutes |
| 3. Hypermetabolism | ATP consumed by SERCA and myosin ATPase at extreme rates | Rising EtCO2, tachycardia, metabolic acidosis | 10-40 minutes |
| 4. Thermogenesis | Muscle contraction generates heat; impaired thermoregulation | Core temperature rising up to 1-2 degrees Celsius every 5 min | 15-60 minutes |
| 5. Cell lysis | ATP depletion causes membrane failure; Ca2+ overload activates proteases | Hyperkalaemia, CK rise, myoglobinuria, DIC | 30-90 minutes |
| 6. Organ failure | Arrhythmias from hyperkalaemia; renal failure from myoglobin; cerebral oedema | Cardiac arrest, acute kidney injury, compartment syndrome | Hours |
Genetics of MH Susceptibility
RYR1 gene (chromosome 19q13.2): encodes the ryanodine receptor type 1
- Over 400 variants identified; approximately 30 are causally linked to MH
- Autosomal dominant inheritance with variable penetrance
- Most common pathogenic variants: R614C, G2434R, R2163C, T2206M
CACNA1S gene (chromosome 1q32.1): encodes the DHPR alpha-1 subunit
- Less common (approximately 1% of cases)
- Links the voltage sensor to RYR1 opening
Inheritance: 50% chance of transmission to offspring; however, penetrance is variable and many mutation carriers never develop a crisis
Muscle Pathology and Histology
- No diagnostic histological features on routine H+E staining in MH-susceptible muscle
- In vitro contracture test (IVCT): European protocol exposes fresh muscle biopsy strips to halothane and caffeine; a positive contracture confirms susceptibility
- North American protocol (CHCT): similar principle with caffeine-halothane contracture test
- Central core disease: a myopathy caused by RYR1 mutations that is strongly associated with MH susceptibility - key exam link
- Multi-minicore disease: another RYR1-related myopathy with MH risk
Classification and Types
Patterns of MH Presentation
| Pattern | Onset | Key Features | Outcome |
|---|---|---|---|
| Fulminant MH | Rapid (minutes) | Generalised rigidity, hyperthermia greater than 39 degrees, rapid EtCO2 rise, arrhythmias | High mortality if dantrolene delayed |
| Insidious onset | Gradual (20-60 min) | Slow EtCO2 rise, unexplained tachycardia, mild acidosis | Good outcome if recognised early |
| Post-operative presentation | PACU recovery | Rigidity, fever, dark urine, cardiac instability in recovery | Often initially misdiagnosed as sepsis |
| Masseter rigidity alone | After suxamethonium | Jaw clonus/rigidity preventing intubation, with or without progression | 50% progress to MH; observe minimum 12 hours |
| Abortive form | Variable | Mild symptoms that self-resolve when trigger withdrawn | Must still investigate for MH susceptibility |
Not every MH episode follows a textbook cascade. The clinical picture ranges from mild abortive episodes to fulminant crisis.
Clinical Assessment
Early Recognition (Minutes)
- Rising EtCO2: unexplained increase despite unchanged ventilation settings (most sensitive sign)
- Tachycardia: unexplained, often the first vital sign change
- Masseter rigidity: jaw stiffness after suxamethonium administration
- Mixed respiratory and metabolic acidosis: on blood gas analysis
- Tachypnoea: if spontaneously breathing
Late Signs (10-60 minutes)
- Generalised skeletal muscle rigidity: trunk and limbs
- Hyperthermia: core temperature rising 1-2 degrees Celsius every 5 min; may exceed 40 degrees
- Dark urine: myoglobinuria (cola-coloured)
- Cardiac arrhythmias: ventricular tachycardia/fibrillation from hyperkalaemia
- Skin mottling, sweating, cyanosis
Temperature Rise Is a LATE Sign
Hyperthermia is NOT an early sign of MH. By the time core temperature is significantly elevated, the hypermetabolic crisis is well established. Waiting for fever to make the diagnosis delays life-saving treatment. The earliest signs are rising EtCO2 and unexplained tachycardia in a patient receiving a volatile agent.
Laboratory Findings in MH Crisis
| Parameter | Expected Change | Mechanism | Clinical Significance |
|---|---|---|---|
| EtCO2 | Markedly elevated (doubling or more) | Massive CO2 production from hypermetabolism | Most sensitive real-time indicator |
| Arterial blood gas | Mixed respiratory and metabolic acidosis | CO2 accumulation + lactic acidosis | Base deficit greater than 8 suggests evolving crisis |
| Serum potassium | Elevated (may be markedly high) | Muscle cell lysis releases intracellular K+ | Principal cause of cardiac arrhythmias and arrest |
| Creatine kinase (CK) | Massively elevated (greater than 20,000 U/L) | Skeletal muscle destruction | Peak at 12-24 hours; degree reflects rhabdomyolysis severity |
| Myoglobin (urine/serum) | Elevated; cola-coloured urine | Muscle breakdown product released into circulation | Risk of acute tubular necrosis and renal failure |
| International normalised ratio (INR) / platelets | Disseminated intravascular coagulation | Tissue factor release from damaged muscle | DIC is a late, life-threatening complication |
Investigations
Investigation Pathway
End-tidal CO2 monitoring: continuous; rapid rise is diagnostic
Core temperature monitoring: oesophageal, nasopharyngeal, or bladder
Arterial blood gas: metabolic and respiratory acidosis, hyperkalaemia, rising lactate
ECG: arrhythmia detection (peaked T-waves from hyperkalaemia)
Serum CK: baseline and serial; levels over 20,000 U/L confirm significant rhabdomyolysis
Serum and urine myoglobin: myoglobinuria indicates ongoing muscle destruction
Coagulation profile: fibrinogen, D-dimer, INR for DIC surveillance
Renal function: creatinine and urea for acute kidney injury
In vitro contracture test (IVCT): European MH Group protocol; fresh muscle biopsy exposed to halothane and caffeine in the laboratory; sensitivity approximately 99%, specificity approximately 94%
Caffeine-halothane contracture test (CHCT): North American MH Group equivalent
Genetic testing: targeted RYR1 and CACNA1S sequencing; identifies causative variant in approximately 70% of cases; if a pathogenic variant is found, family screening is possible without biopsy
Investigation Pearl
The IVCT remains the gold standard for confirming MH susceptibility. It requires a fresh (not frozen) muscle biopsy specimen, typically from the vastus lateralis, performed in a specialised centre. Genetic testing is increasingly used first-line in families where a known pathogenic variant exists. A positive genetic test in a first-degree relative of a confirmed MH survivor is sufficient to label the patient MH-susceptible without biopsy.
MH Susceptibility Testing Comparison
| Test | Sensitivity | Specificity | Invasive | Availability |
|---|---|---|---|---|
| IVCT (European protocol) | Approximately 99% | Approximately 94% | Yes - open muscle biopsy under local | Specialised MH centres only |
| CHCT (North American protocol) | Approximately 97% | Approximately 78% | Yes - open muscle biopsy under local | Limited centres in North America |
| RYR1/CACNA1S genetic testing | Approximately 70% (identifies known variants only) | High for known pathogenic variants | No - blood sample | Widely available; cost decreasing |
Management Algorithm
Immediate Management of MH Crisis
Goal: Stop the hypermetabolic cascade with dantrolene while providing supportive care
Treatment Protocol
STOP all volatile anaesthetic agents immediately
DO NOT give suxamethonium
Hyperventilate with 100% oxygen at high flow (greater than 10 L/min) to flush volatile agent
Call for help and request dantrolene immediately
Inform the surgeon - may need to abort or rapidly complete the procedure
Prepare: each 20 mg vial requires mixing with 60 mL sterile water (warmed if possible for faster dissolution)
Administer: 2.5 mg/kg IV bolus
Repeat: every 5 minutes until symptoms abate, up to cumulative 10 mg/kg
Note: a 70 kg adult needs approximately 9 vials for the initial dose alone; organise a team for rapid reconstitution
Active cooling: cold IV saline (4 degrees Celsius), surface cooling (ice packs to groins, axillae, neck), cold body cavity lavage if needed
Treat hyperkalaemia: calcium chloride 10 mL of 10% IV, insulin 10 units with 50 mL 50% dextrose, sodium bicarbonate 1-2 mmol/kg
Treat arrhythmias: standard ACLS algorithms; avoid calcium channel blockers (may worsen hyperkalaemia or interact with dantrolene)
Continue dantrolene: 1 mg/kg IV every 6-12 hours for 24-48 hours
ICU monitoring: minimum 24 hours observation for recrudescence
Serial labs: CK, K+, ABG, urine output, coagulation every 2-4 hours initially
Maintain urine output: target greater than 1-2 mL/kg/hour with IV fluids and mannitol to prevent myoglobin-induced renal failure
Dantrolene Pearl
Dantrolene sodium works by directly inhibiting RYR1-mediated calcium release from the sarcoplasmic reticulum. It does NOT act centrally. It causes mild muscle weakness but does not produce full paralysis. Newer formulations (e.g. Ryanodex) allow reconstitution in much smaller volumes, dramatically speeding administration. The key exam fact: dantrolene is the ONLY specific treatment for MH and must be given as rapidly as possible.
Complications
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Cardiac arrest (hyperkalaemic) | Principal early cause of death | Severe rhabdomyolysis, delayed dantrolene | ACLS with aggressive K+ lowering; calcium chloride, insulin/dextrose, bicarbonate |
| Acute kidney injury | Common in severe cases | Myoglobinuria, hypovolaemia | Aggressive IV fluids, mannitol, maintain urine output |
| Disseminated intravascular coagulation | Late complication | Massive tissue destruction, shock | Blood product support, treat underlying cause |
| Compartment syndrome | In limbs with severe rhabdomyolysis | Muscle oedema within fascial compartments | Fasciotomies if compartment pressures elevated |
| Cerebral oedema / neurological injury | Severe cases with prolonged hypoxia | Hyperthermia, hypotension, cardiac arrest | Neurocritical care, targeted temperature management |
| Recrudescence (MH recurrence) | Approximately 20% of cases | Inadequate dantrolene dosing or early cessation | Resume full dantrolene protocol; ICU monitoring minimum 24 h |
Hyperkalaemia Is the Killer, Not Hyperthermia
Although the condition is called "malignant hyperthermia," it is hyperkalaemia from rhabdomyolysis that causes the lethal cardiac arrhythmias in the early phase. Aggressive potassium management (calcium chloride for membrane stabilisation, insulin-dextrose for intracellular K+ shift, bicarbonate) is as critical as dantrolene itself. The hyperthermia reflects the hypermetabolic state but is not the primary cause of death.
Outcomes and Prognosis
Outcomes by Treatment Timing
| Timing of Dantrolene | Expected Outcome | Complication Rate | Mortality |
|---|---|---|---|
| Within 10 minutes of onset | Rapid resolution of hypermetabolism | Low (less than 10%) | Less than 2% |
| 10-30 minutes delay | Moderate rhabdomyolysis, possible AKI | Moderate (20-40%) | Less than 5% |
| Greater than 30 minutes delay | Severe rhabdomyolysis, DIC, multi-organ failure risk | High (greater than 50%) | 10-30% |
| No dantrolene available | Fulminant crisis, cardiac arrest likely | Near universal | Greater than 70% |
Prognostic Factors
Best prognosis: Early recognition (rising EtCO2), immediate dantrolene within 10 minutes, young healthy patient, abortive form
Poor prognosis: Delayed recognition, fulminant presentation, extreme hyperthermia (greater than 41 degrees), pre-existing myopathy, limited dantrolene availability
Key threshold: Dantrolene administration within 10 minutes of symptom onset is associated with near-zero mortality. Every 10-minute delay significantly increases mortality.
Evidence Base and Key Trials
Malignant hyperthermia: a review
- Comprehensive review establishing RYR1 as the primary gene responsible for MH susceptibility
- Over 400 RYR1 variants identified, with approximately 30 confirmed causative
- Dantrolene reduces mortality from approximately 70% to less than 5%
- IVCT remains the diagnostic gold standard with approximately 99% sensitivity
Cardiac arrests and deaths associated with malignant hyperthermia in north america from 1987 to 2006
- Analysis of the North American MH Registry identifying risk factors for cardiac arrest and death
- Increased risk with greater than 6 hour delay between symptom onset and dantrolene administration
- Muscle rigidity at onset and extreme temperature elevation were independent predictors of mortality
- Body mass index and myopathy history were additional risk factors
Availability of dantrolene for the management of malignant hyperthermia crises: European Malignant Hyperthermia Group guidelines
- EMHG consensus guidelines on dantrolene availability and dosing for MH crisis management
- Confirmed dantrolene 2.5 mg/kg initial bolus with repeat dosing as the gold standard
- Recommended minimum 36 vials of conventional dantrolene be immediately available wherever general anaesthesia is administered
- Highlighted that reconstitution time is a critical barrier and nanocrystalline formulations address this
European Malignant Hyperthermia Group guidelines for investigation of malignant hyperthermia susceptibility
- Recommended genetic testing as first-line investigation in families with a known proband variant
- IVCT remains the gold standard when no familial variant is identified or genetic testing is inconclusive
- RYR1 screening identifies causative variants in approximately 70% of MH-susceptible individuals
- Defined criteria for interpreting IVCT results and classifying MH susceptibility
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Scenario 1: Intraoperative Crisis Recognition
"You are assisting at a total knee arthroplasty in a 55-year-old male under general anaesthesia with sevoflurane. Forty minutes into the case, the anaesthetist reports a rapidly rising end-tidal CO2 from 38 to 65 mmHg despite unchanged ventilation settings, unexplained tachycardia to 120 bpm, and the patient feels warm. What is your diagnosis and what do you do?"
Scenario 2: Orthopaedic Patient with Known Myopathy
"A 14-year-old boy with Duchenne muscular dystrophy (DMD) is scheduled for scoliosis correction. His anaesthetist asks your opinion on the risk of MH during the case. What are the specific risks, how should anaesthesia be conducted, and what preparations are mandatory?"
MCQ Practice Points
Pathophysiology Question
Q: What is the molecular basis of malignant hyperthermia? A: MH is caused by mutations in the RYR1 gene (approximately 70% of cases) encoding the ryanodine receptor type 1 on the sarcoplasmic reticulum of skeletal muscle. These mutations render the calcium release channel hyper-responsive to volatile anaesthetic agents and suxamethonium, causing sustained uncontrolled Ca2+ efflux from the SR. The resulting hypermetabolism produces heat, CO2, lactate, and muscle cell lysis.
Diagnosis Question
Q: What is the earliest and most sensitive sign of MH during anaesthesia? A: Rising end-tidal CO2 (EtCO2) despite unchanged ventilation settings. This reflects the massive CO2 production from skeletal muscle hypermetabolism. Unexplained tachycardia typically follows. Hyperthermia is a LATE sign and should not be awaited before initiating treatment.
Treatment Question
Q: What is the dose and mechanism of dantrolene in MH? A: Dose: 2.5 mg/kg IV bolus, repeated every 5 minutes up to 10 mg/kg cumulative, then 1 mg/kg every 6-12 hours for 24-48 hours. Mechanism: dantrolene directly inhibits RYR1-mediated calcium release from the sarcoplasmic reticulum, breaking the hypermetabolic cascade. It does not act centrally. It causes mild muscle weakness but not full paralysis.
Safety Question
Q: Which anaesthetic agents are safe in MH-susceptible patients? A: Safe: propofol, all opioids, nitrous oxide, local anaesthetics, non-depolarising muscle relaxants (rocuronium, atracurium, vecuronium), benzodiazepines, ketamine. Unsafe (triggering): ALL volatile agents (sevoflurane, desflurane, isoflurane, halothane) and suxamethonium. The safest overall technique is total IV anaesthesia with propofol or regional anaesthesia.
Genetics Question
Q: What is the inheritance pattern and genetic testing strategy for MH? A: Autosomal dominant with variable penetrance. RYR1 mutations account for approximately 70% of cases, CACNA1S approximately 1%. Genetic testing of RYR1/CACNA1S is offered first-line in families with a known proband variant. If no variant is identified, the in vitro contracture test (IVCT) on fresh muscle biopsy (vastus lateralis) is the gold standard diagnostic test with approximately 99% sensitivity.
Guidelines, Registries & Global Practice
Global Epidemiology
- Incidence: approximately 1 per 100,000 adult anaesthetics; 1 per 30,000 paediatric anaesthetics where volatile agents are used
- Geographic variation: incidence reporting varies by country and depends on anaesthetic practices (volatile vs TIVA use rates)
- Registries: North American MH Registry (MHAUS), European MH Group registry, Japanese MH registry, and Australian/New Zealand case reports contribute to global data
- Mortality: less than 5% with prompt dantrolene; historically 70-80% before dantrolene era
Practice Variation by Resource Setting
- High-resource settings: TIVA widely available, dantrolene stocked in all ORs, IVCT and genetic testing accessible, specialist MH centres for follow-up
- Limited-resource settings: volatile anaesthesia predominates (cheaper, simpler), dantrolene may not be immediately available, IVCT centres are scarce
- Critical gap: dantrolene availability varies enormously worldwide; some regions rely on Dan-Alarm or MHAUS hotline networks for guidance
- Universal principle: regardless of resources, early recognition (rising EtCO2) and stopping volatile agents saves lives even before dantrolene arrives
Society and Reference Guidance (Side by Side)
| Source | Trigger avoidance | Dantrolene dosing | Testing strategy |
|---|---|---|---|
| European MH Group (EMHG) 2023 | All volatiles and suxamethonium contraindicated; propofol TIVA safe | 2.5 mg/kg bolus, repeat to 10 mg/kg; then 1 mg/kg q6h for 24-48h | Genetic testing first if familial variant known; IVCT if not |
| MHAUS / North American MH Registry | Identical trigger avoidance; Ryanodex (nanocrystalline dantrolene) approved and recommended for faster reconstitution | Same dosing; Ryanodex allows 250 mg/vial vs 20 mg/vial conventional | CHCT (caffeine-halothane contracture test) is the biopsy standard; genetic testing complementary |
| Association of Anaesthetists (UK) / AAGBI | Aligns with EMHG guidance; dantrolene must be available within minutes in every OR | EMHG-aligned dosing | Referral to one of the UK MH investigation units (Leeds, St George's) for IVCT |
| ANZCA (Australia/NZ) | Aligns with EMHG; mandatory dantrolene availability in all facilities providing general anaesthesia | EMHG-aligned dosing | Referral to MH investigation units; genetic testing increasingly used first-line |
Registry and Evidence Note
There is no single global MH implant registry (as MH is a pharmacogenetic event, not a device-related condition). However, the North American MH Registry (maintained by MHAUS) and the European MH Group registry are the two largest databases capturing MH events, outcomes, and genotype-phenotype correlations. These registries demonstrated that mortality fell from approximately 70% in the 1970s to less than 5% with modern dantrolene protocols.
Global Dantrolene Availability
Dantrolene availability remains inconsistent worldwide. In many limited-resource settings, dantrolene is not stocked in every OR, and reconstitution time with conventional formulations (20 mg/vial requiring 60 mL sterile water each) is a significant barrier. Ryanodex (250 mg/vial, reconstitutes in seconds) addresses this but is not universally available. Orthopaedic surgeons operating in settings without guaranteed dantrolene access should advocate for its inclusion in OR emergency supplies.
Controversies & Areas of Uncertainty
MH vs anaesthesia-induced rhabdomyolysis in myopathies
Patients with Duchenne, Becker, and other myopathies develop rhabdomyolysis and hyperkalaemic cardiac arrest under volatile agents via membrane instability (absent or abnormal dystrophin) rather than RYR1 channelopathy. The clinical overlap with MH is significant, dantrolene is still used, but the underlying mechanism differs. This distinction is important for genetic counselling and family screening.
Genetic testing vs IVCT
Genetic testing identifies a causative variant in only approximately 70% of MH-susceptible individuals because not all pathogenic variants have been characterised. A negative genetic test does NOT exclude MH susceptibility if clinical suspicion is high - IVCT is still required. The trend toward genetics-first testing is growing but the IVCT remains the definitive investigation when genetics is inconclusive.
Minimum observation time post-crisis
The risk of recrudescence after an MH crisis is approximately 20%, typically occurring within 6-24 hours. Most guidelines recommend ICU observation for a minimum of 24 hours, but the optimal duration of continuing dantrolene (24 vs 48 hours) and the safe discharge threshold are not defined by randomised evidence.
MH-susceptible patients and statins
Statins have been implicated in rare cases of drug-induced myopathy that may unmask or complicate MH susceptibility. The interaction between statin myopathy and MH risk is poorly characterised. Current guidance does not recommend stopping statins pre-operatively in MH-susceptible patients, but clinicians should be aware of the potential for additive myotoxicity.
MALIGNANT HYPERTHERMIA
Clinical summary
Pathophysiology
- •RYR1 mutation (chromosome 19) causes uncontrolled Ca2+ release from skeletal muscle SR
- •Triggers: ALL volatile agents + suxamethonium cause sustained channel opening
- •Hypermetabolism: massive ATP consumption produces heat, CO2, lactate, K+
- •Rhabdomyolysis and hyperkalaemia are the lethal downstream effects
Clinical Recognition
- •Earliest: rising EtCO2 (most sensitive) + unexplained tachycardia
- •Masseter rigidity after suxamethonium is a sentinel warning
- •Late: generalised rigidity, hyperthermia, dark urine, arrhythmias
- •Temperature rise is LATE - never wait for it before treating
Dantrolene Protocol
- •2.5 mg/kg IV bolus immediately, repeat every 5 min up to 10 mg/kg
- •Mechanism: inhibits RYR1-mediated Ca2+ release (not central action)
- •Then 1 mg/kg IV every 6-12 hours for 24-48 hours post-crisis
- •Each 20 mg vial needs 60 mL sterile water; 70 kg patient needs 9 vials for first dose
Safe and Unsafe Agents
- •UNSAFE: ALL volatiles (sevoflurane, desflurane, isoflurane, halothane) + suxamethonium
- •SAFE: propofol, opioids, N2O, local anaesthetics, non-depolarising relaxants, benzodiazepines
- •Safest technique: propofol TIVA or regional anaesthesia
- •Machine preparation: remove vaporisers, flush circuit 20 min, replace CO2 absorber
Diagnosis and Testing
- •Clinical diagnosis: treat first, investigate later
- •IVCT (European) or CHCT (North American): gold standard on fresh muscle biopsy
- •Genetic testing: RYR1/CACNA1S identifies approximately 70% of cases
- •Autosomal dominant inheritance; 50% risk to first-degree relatives; offer family screening