Malignant Hyperthermia

Joseph BlommesteynLeanne KongRyan Marko

Dario Moscoso PHM142 Fall 2014 Instructor: Dr. Jeffrey Henderson

Overview

• Genetic Basis• Biochemical Mechanisms• Diagnosis and Treatment• Susceptibility Testing and Prevention

Malignant Hyperthermia

• Inherited disorder that is usually triggered by exposure to certain general anesthetics, specifically volatile anesthetic agents and succinylcholine

• Susceptible individuals may experience excessive production of heat and lactic acid, which can lead to acidosis and death if not treated immediately

Muscle Contractions – EC Coupling

1. Contractile signal received at NMJ2. ACh release and sarcolemma

depolarization3. DHPRs undergo conformational change4. RYR1s undergo conformational change5. Ca2+ released from SR6. Myofilament contraction7. Ca2+ re-sequestering by ATP-dependent

pumps on the SR8. Process is ready to start again

Anaesthesia and Intensive Care Medicine, 12(6), 263-265

Genetic Basis of MH• Pharmacogenetic disorder• Autosomal dominant inheritance

Epidemiology:• Incidence:

– ~ 1/15,000 children – ~ 1/50,000 adults

• Genetic susceptibility:– Between 1/3,000 and 1/10,000

• ~ 2X more common in males than in females

Orphanet Journal of Rare Diseases, 2, 1-14.Pflugers Arch. 460(2), 467-480.

Genetic Basis of MH• Heterogeneous genetic disorder: 6 different loci containing

MH-associated mutations

Locus Chromosomal Location Gene

MHS1 19q13.1 RYR1

MHS2 17q11.3-q24 Unidentified

MHS3 7q21-q22 Unidentified

MHS4 3q13.1 Unidentified

MHS5 1q32 CACNA1S (DHPR)

MHS6 5p Unidentified

Loci and genes involved in MH:

Isr. Med. Assoc. J., 9(1), 39-41.

RYR1 (~ 50-70%)DHPR (~ 1%)

Anaesthesia and Intensive Care Medicine, 12(6), 263-265

Ryanodine Receptor 1

• RYR1: Skeletal muscle isoform• ~ 2 MDa homotetramer• 300 known genetic variants• 151 MH-associated point

mutations • 34 causative mutations (so far)

(Top-down view)

1 2

3 4

Nature, 468(7323), 585-588

Mutation site:

RyR1 AA sequence:

RYR1 Mutation Hot-Spots

T-tubule

SR

DHPR

RYR1 Mutations cluster in “hot-spots”:

Domain Amino Acids

N-terminal C35 – R614

Central A2129 – R2458

C-terminal I3916 – G4942

These clusters correspond to regions at the interface between the individual subunits

Cold Spring Harbor Perspectives in Biology, 2(11), a003996. Biochem. Biophys. Res. Comm., 322(4), 1280-1285.

RYR1s and MH

Mutations make the RYR1 more hypersensitive to channel-opening stimuli

Triggering Agents of MHDepolarizing Muscle Relaxants• Succinylcholine is more potent than

ACh• Longer duration of effect, does not

allow the muscle cells to repolarize• Ca2+ normally removed independent of

repolarization• With leaky RYR1s Ca2+ conc. remains

high in cytoplasm

Inhalant Anesthetics• Direct effect on DHPR-RYR1 complex

that occurs irrespective depolarization• Mg2+ binds to RYR1s and inhibit their

opening• Mutant RYR1s have less affinity for

Mg2+ • Inhalant anesthetics can overcome the

inhibiting effects of the weakly associated Mg2+

People susceptible to MH should avoid strenuous activity in hotter conditions as well since Ca2+ reuptake cannot keep up with the leaky RYR1’s

Biochemical Consequences of MH

• ATP depletion from an increase in compensatory Ca2+ reuptake processes and sustained muscle contractions

• ADP stimulates metabolic processes, and thus increases oxygen consumption, CO2 production

• Heat generated from sustained muscle contractions

• Increase in permeability of cell membrane due to hyperthermia and muscle activity

• Leakage of cellular constituents

Anaesthesia and Intensive Care Medicine, 12(6), 263-265

Diagnosis and Treatment

of Malignant Hyperthermia

Differential Diagnosis

• Certain other disorders can imitate• Common symptoms: masseter spasm

hypercapnia (ETCO2>55mmHg), hyperthermia, tachycardia, arrhythmia, ECG changes

• Patient history: thyroid storm, infection/sepsis, pheochromocytoma

MH Crisis Confirmed

• Primary treatment centered on 3 pillars:• Increasing inspired O2

• Discontinuing triggering agents• Administering dantrolene: dose dependent on

anesthetic dose used

Reasoning Behind Treatment

• High CO2 needs to be brought to equilibrium with O2

• Triggering agents will continue to cause pathophysiology

• Dantrolene inhibits calcium release into the SR by binding ryanodine receptors

Treatment Options

Charcoal filter prevents residual anesthetic from reaching patient

Dantrolene sodium to be reconstituted: 2 formulations, new is hyperconcentrated

Secondary Treatment

• Treatment of hyperkalemia: CaCl2, insulin + dextrose, furosemide

• Treatment of metabolic acidosis: HCO3

• Cooling of internal/external body surfaces/cavities: IV saline, ice (body temperature>39 C)

Susceptibility Testing

• In vitro caffeine-halothane contracture test (IVCT)– Involves a muscle biopsy in

which the biopsy is bathed in solutions of halothane receptor agonists, caffeine and halothane, and tested for contraction

– European Malignant Hyperthermia Group

– North American Malignant Hyperthermia Group

• DNA Testing– RYR1 receptor mutation

http://bestpractice.bmj.com/best-practice/monograph/1053/diagnosis/step-by-step.html

Prevention

• Patients should be screened before any procedure with anesthesia is being performed

• If patient is MH susceptible, trigger (potent volatile) anesthetics should be avoided

• Avoid use of depolarizing muscle relaxants• Anesthesia machines should also be flushed

to remove remnants of trigger anesthetics • Ensure that dantrolene is available• Performing regular MH drills to be prepared

Summary• Malignant hyperthermia is a heterogeneous pharmacogenetic disorder that's typically inherited in an autosomal dominant

fashion.• 6 MH susceptibility genes have been mapped, but only two have been unambiguously identified: ryanodine receptor 1

(RYR1) and dihydropyridine receptor (DHPR).• RYR1 mutations are associated with 50-70% of MH cases.• RYR1 is a large homotetrameric calcium channel in the sarcoplasmic reticulum. MH-associated mutations tend to cluster in

"hot-spots" at the interfaces between the four subunits.• Mutant RYR1s are hypersensitive to Ca2+ releasing stimuli.• High cytoplasmic Ca2+ concentration leads to increased activity of compensatory reuptake mechanisms (ATP-dependent Ca 2+

pumps).• Depletion of ATP from these compensatory mechanisms as well as muscle contractions leads to increased metabolic

processes.• Sustained muscle contractions produce excess heat, increased oxygen consumption, increased CO 2 production, and lactate

buildup.• There is also leakage of cellular constituents (hyperkalemia and rhabdomyolysis).• First one must establish patient is actually experiencing MH crisis; consider susceptibility, history, and administered agent.• MH patients exhibit signs such as elevated body temperature, tachycardia, arrhythmia, muscle spasm and ECG changes.• Primary treatment include ventilation with O2, IV dantrolene, and removing triggers.• Secondary treatment involves insulin/glucose, bicarbonate, diuretics and ice/cool saline.• The gold standard for susceptibility testing of MH is the in vitro caffeine-halothane contracture test (IVCT), which involves

taking a skeletal muscle biopsy and testing the contracture with soaking the biopsy in ryanodine receptor agonists (caffeine and halothane). High contracture denotes susceptibility to MH.

• Preventive measures: Avoid triggering anesthetics, avoid depolarizing muscle relaxants, and be prepared for the possibility of an episode occurring upon anesthesia by stocking dantrolene and doing drills

References

• Benkusky, N. A., Farrell, E. F., & Valdivia, H. H. (2004). Ryanodine receptor channelopathies. Biochemical and Biophysical Research Communications, 322(4), 1280-1285.

• Betzenhauser, M. J., & Marks, A. R. (2010). Ryanodine receptor channelopathies. Pflugers Archiv: European Journal of Physiology, 460(2), 467-480.

• Brandom, B.W., & Lehman, E.B. (2010). Clinical presentation, treatment, and complications of malignant hyperthermia in North America from 1987 to 2006. Anesthesia & Analgesia, 110, 498-507.

• Denborough, M. (1998). Malignant hyperthermia. The Lancet, 352, 1131-1136.• Greenbaum, I., Weigl, Y., & Pras, E. (2007). The genetic basis of malignant hyperthermia. The Israel Medical Association

Journal, 9(1), 39-41.• Hopkins, P. (2011) Malignant Hyperthermia. Anaesthesia and Intensive Care Medicine, 12(6), 263-265• Kobayashi, S., Bannister, M. L., Gangopadhyay, J. P., Hamada, T., Parness, J., & Ikemoto, N. (2005). Dantrolene stabilizes

domain interactions within the ryanodine receptor. The Journal of Biological Chemistry, 280(8), 6580-6587. • Lanner, J. T., Georgiou, D. K., Joshi, A. D., & Hamilton, S. L. (2010). Ryanodine receptors: structure, expression, molecular

details, and function in calcium release. Cold Spring Harbor Perspectives in Biology, 2(11), a003996. • Larach, M.G., Gronert, G.A., Allen, G.C., MacLennan, D.H., & Phillips, M.S. (1992) Malignant hyperthermia. Science, 256, 789-

194.• Litman, R., & Rosenberg, H. (2005) Malignant Hyperthermia: Update on Susceptibility Testing. Journal of the American

Medical Association, 293, 2918-2924.• Rosenberg, H., Davis, M., James, D., Pollock, N., & Stowell, K. (2007) Review: Malignant Hyperthermia. Orphanet Journal of

Rare Diseases, 2, 1-14.• Stowell, K. M. (2008). Malignant hyperthermia: a pharmacogenetic disorder. Pharmacogenomics, 9(11), 1657-1672. • Tung, C. C., Lobo, P. A., Kimlicka, L., & Van Petegem, F. (2010). The amino-terminal disease hotspot of ryanodine receptors

forms a cytoplasmic vestibule. Nature, 468(7323), 585-588.