Malignant Hyperthermia and Anesthetic Pharmacogenetics

Malignant hyperthermia and anesthetic pharmacogenetics encompasses two of the founding disorders of human pharmacogenetics that manifest as acute, life-threatening emergencies in the perioperative setting: Malignant Hyperthermia Susceptibility (MHS) and butyrylcholinesterase (BCHE, pseudocholinesterase) deficiency. Both conditions are clinically silent in daily life until a genetically susceptible patient is exposed to specific general anesthetic or neuromuscular blocking agents in the operating room or intensive care unit.

In Malignant Hyperthermia Susceptibility, autosomal dominant gain-of-function variants in the skeletal muscle excitation–contraction coupling machinery—principally RYR1 (encoding the sarcoplasmic reticulum ryanodine receptor type 1 Ca²⁺ release channel) and CACNA1S (encoding the transverse-tubule dihydropyridine receptor Ca_v1.1)—cause uncontrolled myoplasmic Ca²⁺ release when triggered by potent volatile halogenated inhalational anesthetics or the depolarizing muscle relaxant succinylcholine. In butyrylcholinesterase deficiency, autosomal recessive loss-of-function variants in BCHE impair plasma enzymatic hydrolysis of succinylcholine and mivacurium, converting a standard 5- to 10-minute neuromuscular blockade into 2 to 8 hours of prolonged paralysis and apnea.

Excitation–Contraction Coupling and Molecular Genetics of MHS

In normal vertebrate skeletal muscle, physiological excitation–contraction (EC) coupling occurs at the triad junction, where invaginations of the sarcolemma termed transverse tubules (T-tubules) directly abut the terminal cisternae of the sarcoplasmic reticulum (SR). When an action potential depolarizes the T-tubule membrane, the voltage-sensing dihydropyridine receptor (DHPR, Ca_v1.1)—whose pore-forming α_1S subunit is encoded by CACNA1S on chromosome 1q32.1—undergoes a conformational shift. Assisted by the adaptor protein STAC3, the cytosolic II–III loop of Ca_v1.1 mechanically and allosterically gates open the homotetrameric (2.2-MDa, 5,038-amino-acid per monomer) ryanodine receptor type 1 (RYR1) channel embedded in the SR membrane (encoded by RYR1 across 106 exons on chromosome 19q13.2). Opening of RYR1 releases stored Ca²⁺ from the SR lumen into the myoplasm (raising cytosolic [Ca²⁺] from ~10⁻⁷ M to ~10⁻⁵ M), where Ca²⁺ binds troponin C to initiate actin–myosin cross-bridge cycling. Relaxation occurs when sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA1) pumps Ca²⁺ back into the SR at the expense of ATP hydrolysis.

Malignant Hyperthermia Susceptibility (MHS) is inherited predominantly as an autosomal dominant trait (genetic prevalence ~1 in 1,500 to 1 in 3,000 individuals; clinical crisis incidence ~1 in 10,000 to 1 in 100,000 anesthetics) caused by variants in three genes:

1. RYR1 (19q13.2, ~70% to 80% of MHS families): Over 60 pathogenic/likely pathogenic missense variants curated by CPIC and the European Malignant Hyperthermia Group (EMHG) cluster in three cytosolic "hotspots" (N-terminal residues 35–614, central bridging solenoid residues 2,163–2,458, and C-terminal pore residues 4,550–4,940). These gain-of-function mutations destabilize inter-subunit interactions that keep the RYR1 channel closed at resting Mg²⁺ concentrations, hypersensitizing the channel to opening.
2. CACNA1S (1q32.1, ~1% to 2% of MHS families): Pathogenic missense variants (such as c.3257G>A p.Arg1086His and c.520C>T p.Arg174Trp) alter voltage-sensor suppression of RYR1 gating.
3. STAC3 (12q13.3, Native American Myopathy): Homozygosity for the autosomal recessive variant c.851G>C (p.Trp284Ser) in the SH3 domain of the triad adaptor protein STAC3 causes congenital myopathy and MHS.

Triggering Anesthetics, Hypermetabolic Crisis, and Dantrolene Antidote

MHS crises are triggered specifically by:

• All potent volatile halogenated inhalational anesthetics: sevoflurane, desflurane, isoflurane, enflurane, halothane, and methoxyflurane.
• The depolarizing neuromuscular blocking agent succinylcholine (suxamethonium).

In contrast, nitrous oxide (N₂O), xenon, intravenous anesthetics (propofol, etomidate, ketamine, barbiturates), opioids, benzodiazepines, local anesthetics (lidocaine, bupivacaine), and non-depolarizing neuromuscular blockers (rocuronium, vecuronium, cisatracurium) are safe non-triggering agents.

When an MHS individual receives a volatile anesthetic or succinylcholine, mutant RYR1 channels lock in an open conformation, flooding the myoplasm with unregulated Ca²⁺. Sustained micromolar myoplasmic [Ca²⁺] drives persistent myofilament contraction (masseter muscle spasm and generalized board-like muscular rigidity) and forces SERCA1, the Na⁺/K⁺-ATPase, and mitochondrial Ca²⁺ uniporters into a furious, futile cycle of ATP hydrolysis and oxidative phosphorylation. This hypermetabolic storm produces a characteristic sequence of clinical signs:

1. Early Hypercapnia and Tachycardia: A rapid, unexplained rise in end-tidal carbon dioxide (ETCO₂ > 55–60 mmHg) despite increased minute ventilation is the earliest and most sensitive indicator of an acute MH crisis, accompanied by unexplained sinus tachycardia and mixed respiratory and lactic acidosis (arterial pH < 7.15).
2. Fulminant Hyperthermia: Heat released from uncoupled glycolytic and mitochondrial ATP turnover causes core body temperature to climb rapidly (at 1 °C to 2 °C every 5 minutes, often exceeding 40 °C to 43 °C).
3. Rhabdomyolysis and Hyperkalemic Arrest: ATP depletion compromises sarcolemmal integrity, leaking potassium ions (K⁺ > 6.5–8.0 mEq/L), myoglobin, and creatine kinase (CK > 20,000–100,000 IU/L) into the bloodstream, precipitating peaked-T-wave ventricular fibrillation, myoglobinuric acute renal failure, and disseminated intravascular coagulation (DIC).

Immediate treatment requires: (i) discontinuing all volatile anesthetics and succinylcholine (flushing the anesthesia circuit with 100% O₂ at ≥10 L/min or inserting activated charcoal filters), (ii) active cooling and correction of hyperkalemia/acidosis, and (iii) immediate intravenous administration of the specific antidote dantrolene sodium (2.5 mg/kg IV bolus, repeated every 5–10 minutes up to 10 mg/kg until ETCO₂ and rigidity resolve). Dantrolene is a hydantoin derivative that binds to a specific cleft in the N-terminal domain of RYR1 (residues 590–609) in the presence of physiological Mg²⁺ and calmodulin, allosterically stabilizing the closed state of the RYR1 channel and halting SR Ca²⁺ release. Introduction of dantrolene reduced acute MH mortality from >70%–80% in the 1960s to <5% today.

Because DNA sequencing of RYR1, CACNA1S, and STAC3 has ~70–80% diagnostic sensitivity (meaning a negative genetic test in an uncharacterized family cannot exclude MHS), the gold-standard phenotypic test to rule out susceptibility remains the in vitro Caffeine–Halothane Contracture Test (CHCT / IVCT), which measures isometric tension development in freshly biopsied vastus lateralis muscle strips exposed to graded concentrations of caffeine and halothane.

Plasma Hydrolysis of Succinylcholine and Mivacurium

Succinylcholine (diacetylcholine) is an ultra-short-acting depolarizing neuromuscular blocker composed of two acetylcholine molecules linked via their acetate methyl groups. Used for rapid-sequence endotracheal intubation because of its rapid onset (45–60 seconds), succinylcholine binds nicotinic acetylcholine receptors (nAChRs) at the skeletal muscle motor endplate, inducing initial fasciculations followed by persistent Phase I depolarization paralysis.

Remarkably, the brief 5- to 10-minute clinical duration of succinylcholine (and of the benzylisoquinolinium non-depolarizing blocker mivacurium) does not result from metabolism at the neuromuscular junction (as synaptic acetylcholinesterase cannot hydrolyze succinylcholine). Instead, 90% to 95% of an intravenous succinylcholine bolus is rapidly hydrolyzed in the bloodstream by hepatic-synthesized plasma butyrylcholinesterase (BChE, pseudocholinesterase), encoded by the BCHE gene on chromosome 3q26.1, into inactive succinylmonocholine and choline before the drug ever reaches the neuromuscular junction.

Atypical (*\*A*), Kalow (*\*K*), Fluoride-Resistant (*\*F*), and Silent (*\*S*) Alleles

Discovered by Werner Kalow in 1956, autosomal recessive variants in BCHE impair or abolish plasma esterase activity:

1. Atypical Allele (*BCHE\*A* or E₁^a, c.293A>G, p.Asp70Gly, rs1799807): Replaces negatively charged aspartate-70 at the peripheral anionic binding site of the catalytic gorge with neutral glycine, reducing the enzyme's affinity for positively charged quaternary ammonium substrates like succinylcholine by >100-fold and rendering the enzyme resistant to inhibition by the local anesthetic dibucaine. Homozygotes (*\*A/\*A*, ~1 in 3,000 Europeans) have a dibucaine number of <20–30 (vs. 70–80 in wild-type *\*U/\*U* and 50–65 in *\*U/\*A* heterozygotes) and experience 2 to 4 hours of complete neuromuscular paralysis and apnea after a standard 1–1.5 mg/kg dose of succinylcholine.
2. Kalow Variant (*BCHE\*K*, c.1699G>A, p.Ala567Thr, rs1803274): Very common allele (~12% frequency) that reduces quantitative plasma BChE concentration by ~30%; clinically mild alone, but causes prolonged apnea (1–2 hours) when inherited in trans with an atypical allele (*\*A/\*K*).
3. Fluoride-Resistant (*BCHE\*F*) and Silent (*BCHE\*S*) Alleles: Null frameshift or nonsense *\*S/\*S* homozygotes (~1 in 100,000 globally, with high founder frequency of ~1 in 1,000 in the Arya Vysya community of India and Alaska Native populations) have 0% enzymatic activity and experience 4 to 8 hours of paralysis following succinylcholine.

Because neostigmine and other anticholinesterases inhibit whatever residual BChE activity remains (paradoxically prolonging Phase I block), management of BChE-deficient succinylcholine apnea consists of continued mechanical ventilation and maintenance of anesthesia/sedation until succinylcholine diffuses away from the motor endplate, or rapid-sequence intubation using rocuronium (reversed on demand by the cyclodextrin encapsulates sugammadex).

Why is a rise in end-tidal carbon dioxide (ETCO2) the earliest clinical sign of malignant hyperthermia rather than high fever?

During an acute malignant hyperthermia crisis, uncontrolled sarcoplasmic reticulum Ca²⁺ release immediately stimulates SERCA1 ATP hydrolysis, glycogenolysis, and mitochondrial respiration inside skeletal muscle fibers, generating large volumes of carbon dioxide (CO₂) within minutes. Because heat has a high specific heat capacity in body water and takes time to equilibrate across deep tissue compartments, hypercapnia (rising ETCO₂), tachycardia, and muscular rigidity appear first, whereas core hyperthermia (>40 °C) is often a later sign.

How does dantrolene stop a malignant hyperthermia crisis without causing complete flaccid paralysis of breathing?

Dantrolene binds to the N-terminal region of RYR1 (residues 590–609) and requires physiological cytosolic Mg²⁺ (~1 mM) to stabilize the closed conformation of the channel. At therapeutic doses (2.5 mg/kg IV), dantrolene selectively suppresses the pathological, unregulated Ca²⁺ leak through mutant RYR1 channels while leaving sufficient voltage-gated T-tubule Ca_v1.1 signaling intact to allow partial physiological muscle contraction, and it does not block cardiac RYR2 channels in the heart.

Why should neostigmine be avoided when a patient with butyrylcholinesterase deficiency fails to wake up after succinylcholine?

Neostigmine and pyridostigmine are reversible cholinesterase inhibitors that inhibit both synaptic acetylcholinesterase and plasma butyrylcholinesterase (BChE). If administered during a prolonged Phase I depolarizing block caused by unhydrolyzed succinylcholine in a BChE-deficient patient, neostigmine inhibits any remaining plasma BChE activity and increases synaptic acetylcholine concentrations, further intensifying and prolonging the depolarizing neuromuscular blockade.

1. Gonsalves, S. G., Dirksen, R. T., Sangkuhl, K., Pulk, R., Alvarellos, M., Vo, T., et al. (2019). "Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline for the use of potent volatile anesthetic agents and succinylcholine in the context of RYR1 or CACNA1S genotypes." Clinical Pharmacology & Therapeutics, 105(6), 1398–1407. https://doi.org/10.1002/cpt.1319
2. Rosenberg, H., Pollock, N., Schiemann, A., Bulger, T., & Stowell, K. (2015). "Malignant hyperthermia: a review." Orphanet Journal of Rare Diseases, 10, 93. https://doi.org/10.1186/s13023-015-0310-1
3. Lockridge, O. (2015). "Review of human butyrylcholinesterase structure, function, genetic variants, history of use in the clinic, and potential therapeutic uses." Pharmacology & Therapeutics, 148, 34–46. https://doi.org/10.1016/j.pharmthera.2014.11.011
4. Horstick, E. J., Linsley, J. W., Dowling, J. J., Hauser, M. A., McDonald, K. K., Ashley-Koch, A., et al. (2013). "Stac3 is a component of the excitation–contraction coupling machinery and mutated in Native American myopathy." Nature Communications, 4, 1952. https://doi.org/10.1038/ncomms2952