MALIGNANT HYPERTHERMIA SUSCEPTIBILITY

Malignant Hyperthermia Susceptibility — a silent genetic trait that is completely benign until a single exposure to the wrong anesthetic triggers a potentially fatal hypermetabolic crisis in the operating room.

RYR1 and CACNA1S are on the ACMG SF v3.2 mandatory secondary findings list. Whole genome sequencing identifies malignant hyperthermia susceptibility — preventing an anesthetic crisis before surgery even begins.

RYR1
CACNA1S

About this condition

Malignant Hyperthermia Susceptibility

Malignant hyperthermia susceptibility (MHS) is a pharmacogenomic disorder in which pathogenic variants in RYR1 (ryanodine receptor 1) or CACNA1S (L-type voltage-sensitive calcium channel alpha-1 subunit) cause abnormal calcium regulation in skeletal muscle sarcoplasmic reticulum. MHS is clinically silent under normal conditions but becomes life-threatening when susceptible individuals are exposed to triggering agents — volatile inhalational anesthetics (halothane, isoflurane, sevoflurane, desflurane) or succinylcholine (a depolarizing neuromuscular blocking agent). Incidence of MH crisis is estimated at 1 in 10,000 to 1 in 50,000 anesthetic exposures; susceptibility (MHS) is more common, estimated at 1 in 2,000-3,000.

During a malignant hyperthermia crisis, uncontrolled skeletal muscle calcium release causes a hypermetabolic state with rapidly rising temperature (>40°C), severe metabolic acidosis, hypercapnia, muscle rigidity, rhabdomyolysis, and cardiovascular instability. Without immediate treatment with dantrolene sodium (the specific MH antidote), mortality exceeds 70%; with prompt dantrolene administration and supportive care, mortality has fallen to approximately 1-2%. Despite this improvement, MH remains a leading cause of preventable anesthetic death — approximately 150 deaths occur annually in the United States among unrecognized susceptible patients.

RYR1 pathogenic variants account for approximately 70-80% of MH-susceptible families; CACNA1S variants account for approximately 1%. A substantial fraction of clinically MH-susceptible families (confirmed by caffeine-halothane contracture test) have no identifiable pathogenic variant, because hundreds of RYR1 variants have been described and many have not achieved sufficient evidence for ACMG pathogenic classification. Both RYR1 and CACNA1S are included on the ACMG SF v3.2 secondary findings list — variants in these genes discovered opportunistically during genome sequencing are required to be reported, reflecting the life-saving nature of preoperative identification. MH susceptibility is also associated with certain myopathies (King-Denborough syndrome, central core disease) caused by RYR1 variants.

Central core disease and multi-minicore disease — caused by the same RYR1 variants — are associated with MH susceptibility. Patients with these myopathies should be assumed susceptible to MH regardless of genetic testing results.

Gene locus: RYR1 (19q13.2), CACNA1S (1q32.1)

RYR1 is a massive 106-exon gene with hundreds of variants — many not validated by contracture testing. Whole genome sequencing detects all RYR1 and CACNA1S variants, and the ACMG SF v3.2 mandate means these results are reported in genome analysis.

Finding an MH susceptibility variant before the first surgery prevents a crisis that has no pre-anesthetic warning signs

Malignant hyperthermia susceptible individuals look and feel completely normal. They have no symptoms, no medical history, and no abnormal laboratory findings until the moment they receive a triggering anesthetic agent. The first clinical presentation of MH is often the crisis itself — in the operating room or recovery room, after anesthetic induction. Once the crisis begins, the diagnosis must be made clinically and dantrolene administered within minutes to prevent death. Pre-identification of susceptibility — through whole genome sequencing of RYR1 and CACNA1S — allows the anesthesiologist to use trigger-free anesthesia protocols before any exposure occurs, completely preventing the crisis.

RYR1 with 106 exons requires complete gene analysis — limited panels detect only the common variants

RYR1 is one of the largest genes in the human genome with 106 coding exons, and over 400 RYR1 variants associated with MH have been described. Targeted MH panels typically test for the subset of variants that have been validated by caffeine-halothane contracture test correlation — approximately 48-50 variants with strong evidence. The remaining hundreds of RYR1 variants exist in a classification gray zone. Whole genome sequencing sequences all 106 RYR1 exons and the complete CACNA1S gene, providing maximum sensitivity across the full variant spectrum. When a known pathogenic variant is identified, the result directly enables MRC precautionary anesthesia planning.

Malignant Hyperthermia Susceptibility — a silent genetic trait that is completely benign until a single exposure to the wrong anesthetic triggers a potentially fatal hypermetabolic crisis in the operating room.

Malignant Hyperthermia Susceptibility

RYR1 is a massive 106-exon gene with hundreds of variants — many not validated by contracture testing. Whole genome sequencing detects all RYR1 and CACNA1S variants, and the ACMG SF v3.2 mandate means these results are reported in genome analysis.

Finding an MH susceptibility variant before the first surgery prevents a crisis that has no pre-anesthetic warning signs

RYR1 with 106 exons requires complete gene analysis — limited panels detect only the common variants

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