The EMHG has modified the ACMG scoring matrix in order to better fit for malignant hyperthermia susceptibility. There are three categories of pathogenetic variant classification: strong, moderate and supporting. Then there are three categories of benign variant classification: stand alone, strong and supporting.

• A pathogenic variant is defined by at least 1 PS criterion and 2 PP criteria

• A likely pathogenic variant is defined as at least 1 PM criterion and 2 PP criteria

Pathogenic and likely pathogenic variants can be used for the diagnosis of MH susceptibility. As outlined in the guidelines absence of RYR1 and CACNA1S variants does not exclude MH. The only possibility to establish an MH negative diagnosis is in-vitro contracture testing.

Pathogenic strong (PS)

PSa. Same amino acid change as a previously established pathogenic variant regardless of nucleotide change.

PSb: Well established in vitro functional studies supportive of a gain of function effect on intracellular calcium release.

Pathogenic moderate (PM)

PMa: Well established ex vivo functional studies in two independent families supportive of a gain of function effect on intracellular calcium release

PMb: Prevalence of the variant in an affected cohort is significantly increased (P< 1 x 10-7) compared with the prevalence of a relevant low risk population.

Pathogenic supporting (PP)

PPa: Segregation with the IVCT phenotype in multiple affected (> 4) family members in a gene definitively known to cause the disease. (Note. Absence of a variant in an affected individual of a family that otherwise shows co-segregation does not preclude pathogenicity).

PPb: Computational evidence (conservation, evolutionary, impact of amino acid change) concordantly support a deleterious effect on the gene.

PPc: Association with a clinical reaction consistent with malignant hyperthermia under anaesthesia and confirmed by a positive IVCT

• Benign variant: at least one BA criterion or 2 x BS criteria

• Likely Benign variant: one BS criterion and one BP criterion

Benign Stand Alone (BA)

BAa: Allele frequency is greater than expected for disorder (>0.1%)

BAb: The variant is present in two unrelated individuals tested MHN (negative) by IVCT where there is no history suggestive of a clinical malignant hyperthermia reaction under anaesthesia in the family

BAc: A RYR1 variant that results in a loss of function of the protein. Nonsense, frameshift or splice site variants that are likely leading to absence of the protein by nonsense mediated decay

Benign Strong (BS)

BSa: The variant is present in one individual tested MHN (negative) by IVCT where there is no history suggestive of a clinical malignant hyperthermia reaction under anaesthesia in the family

BSb: Well established in vitro functional studies using skeletal muscle cells show no gain of function effect on intracellular calcium release.

Benign Supporting (BP)

BPa: Computational evidence (conservation, evolutionary, impact of amino acid change) concordantly suggest no impact on the gene or gene product.