A new system that incorporates clinical and serologic data may help classify idiopathic inflammatory myopathies, according to an analysis published online ahead of print September 10 in JAMA Neurology.
By analyzing the patterns of relationships between 47 variables in this observational, retrospective cohort study, investigators identified four clusters of patients that corresponded to known subtypes of idiopathic inflammatory myopathy. Myositis-specific autoantibodies played a key role in predicting whether a patient belonged in a given cluster, according to the investigators. Myositis-specific antibodies known to be associated with certain subgroups were observed in the corresponding clusters that the researchers identified.
“This [finding] emphasizes that muscle biopsy may no longer be necessary for diagnosis of idiopathic inflammatory myopathies in patients with myositis-specific antibodies and corresponding phenotypes,” said Kubéraka Mariampillai, PhD, of the Université Pierre et Marie Curie, Institut National de la Santé et de la Recherche Médicale (INSERM) in Paris, and colleagues.
The study was based on data for 260 patients in the database of the French Myositis Network. Patients’ mean age was 60, and 63% were women.
Investigators conducted a multiple correspondence analysis based on 47 selected variables, including age, ethnicity, historical and recent diagnoses, dermatologic changes, creatine kinase levels, myositis-specific antibodies, and pathologic characteristics. They identified four subgroups of patients corresponding to dermatomyositis, inclusion body myositis, immune-mediated necrotizing myopathy, and antisynthetase syndrome.
Using decisional algorithm trees, investigators found that the pathologic data were “dispensable,” said the authors. The best tree omitted variables related to muscle biopsy and had a 78% correct estimation based on antisynthetase syndrome antibodies, dermatomyositis rash, and finger flexor scores of 3 or less, said the investigators. “The classification quality of the tree was appreciated on the basis of all classification criteria, with an overall sensitivity of 77.0% and a specificity of 92.0%.”
Patients with polymyositis were included in the study, but were grouped mainly in the clusters corresponding to immune-mediated necrotizing myopathy and antisynthetase syndrome. “This finding indicates that patients with polymyositis do not represent a subgroup of patients, and use of this term should probably be discontinued,” Dr. Mariampillai and colleagues concluded.
—Andrew D. Bowser
Mariampillai K, Granger B, Amelin D, et al. Development of a new classification system for idiopathic inflammatory myopathies based on clinical manifestations and myositis-specific autoantibodies. JAMA Neurol. 2018 Sep 10 [Epub ahead of print].