Mariampillai K, et al. - Researchers sought to develop a new classification scheme for idiopathic inflammatory myopathies based on phenotypic, biological, and immunologic criteria. A classification of idiopathic inflammatory myopathies with four subgroups (dermatomyositis, inclusion body myositis, immune-mediated necrotizing myopathy, and antisynthetase syndrome) was suggested in these findings. Based on the findings it is justifiable to implement a targeted clinical-serologic approach for identifying idiopathic inflammatory myopathies.

Methods

• Researchers performed an observational, retrospective cohort study using a database of the French myositis network.
• They included patients identified from referral centers for neuromuscular diseases from January 1, 2003 to February 1, 2016.
• Out of 445 initial patients, they excluded 185 patients and enrolled 260 adult patients with myositis who had complete data and defined historical classifications for polymyositis, dermatomyositis, and inclusion body myositis.
• Experts tested all patients for anti–histidyl-ARN-t- synthetase (Jo1), anti–threonine-ARN-t-synthetase (PL7), anti–alanine-ARN-t-synthetase (PL12), anti–complex nucleosome remodeling histone deacetylase (Mi2), anti-Ku, anti–polymyositis/systemic scleroderma (PMScl), anti–topoisomerase 1 (Scl70), and anti–signal recognition particle (SRP) antibodies.
• They collected a total of 708 variables per patient (eg, cancer, lung involvement, and myositis-specific antibodies).
• Main outcomes and measures were unsupervised multiple correspondence analysis and hierarchical clustering analysis to aggregate patients in subgroups.

Results

• Findings suggested that among 260 participants (163 [62.7%] women; mean age, 59.7 years; median age [range], 61.5 years [48-71 years]), four clusters of patients emerged.
• Cluster 1 (n=77) included patients who were male, white, and older than 60 years, had finger flexor and quadriceps weakness, and findings of vacuolated fibers and mitochondrial abnormalities.
• Patients who had inclusion body myositis (72 of 77 patients [93.5%]; 95% CI, 85.5%-97.8%; P < .001) were regrouped by cluster 1.
• Patients who were women and had high creatine phosphokinase levels, necrosis without inflammation, and anti–SRP or anti–3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibodies corresponding to immune-mediated necrotizing myopathy were regrouped by cluster 2 (n=91) (53 of 91 [58.2%]; 95% CI, 47.4%-68.5%; P < .001).
• Cluster 3 (n=52) regrouped patients who had dermatomyositis rash and anti-Mi2, anti–melanoma differentiation-associated protein 5 (MDA5), or anti–transcription intermediary factor-1γ (TIF1γ) antibodies, mainly corresponding with patients who had dermatomyositis (43 of 52 [82.7%]; 95% CI, 69.7%-91.8%; P < .001).
• The presence of anti-Jo1 or anti-PL7 antibodies corresponding to antisynthetase syndrome were defined by cluster 4 (n=40) (36 of 40 [90.0%]; 95% CI, 76.3%-97.2%; P < .001).
• The 4 clusters were confirmed by the classification of an independent cohort (n=50) (Cohen κ light, 0.8; 95% CI, 0.6-0.9).