19PClinico-patho-serological correlation for classification of idiopathic inflammatory myopathies and therapeutic outcomes - a real-world retrospective cohort

The classification of idiopathic inflammatory myopathies (IIMs) has undergone a paradigm shift with the application of muscle specific antibodies (MSAs) and biology-specific immunohistochemical stains. Many of these techniques are being gradually adopted in developing nations. We aimed to study the spectrum of subtypes in a retrospective cohort of IIMs with specific focus on the roles of clinical, pathological and serological evaluation and the therapeutic choices. We screened the electronic medical records between April 2017 to March 2024 for patients diagnosed as IIM with a follow up of at least one year. Patients where an alternate diagnosis could not be ruled out were excluded. We applied the European Neuromuscular Centre criteria for classification into dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM), antisynthetase syndrome (ASyS), and sporadic inclusion body myositis (sIBM). Those with a well-defined systemic autoimmune disease were classified as overlap myositis and the rest were labelled unclassified. Descriptive analysis of the clinical, investigational and therapeutic aspects of the subcategories were done. We identified 124 case records of IIM of whom 67 (53.7% females) were included for analysis. The mean age of onset was 42.1 (± 16.9) years with pediatric onset in 7 (10.4%). Symptom duration at presentation was 18.9 (35.3) months. Extramuscular features were seen in 19 (28.4%); 16 (23.9) with cutaneous manifestations, 3 (4.5%) each with arthritis and pulmonary disease and 1 (1.5%) with cardiomyopathy. Four (6%) had association with malignancies. The patient subclassification was DM in 14 (20.9%), IMNM in 11 (16.4%), ASyS in 3 (4.5%), sIBM in 15 (22.4%), and OM in 6 (8.9%) while 18 (26.9) were unclassified. Serology profile was incomplete in 7(38.9%) unclassified. Muscle biopsy was available for 61. Among DM, 6 were Mi2+, 3 NXP2+, and 2 MDA5+. Classical DM muscle pathology was recorded in 6. IMNM has 2 SRP+ and 3 HMGCR+ while ASyS had 2 Jo1+ and 1 PL-7+. Glucocorticoids were given in 97%, oral immunotherapies in 87.9%, and cyclophosphamide or rituximab were given in 25.3%. Excluding sIBM, 15.4% had a favourable immunotherapy response. The immunotherapy response did not differ between unclassified and other groups. Nearly three-fourths of the cohort could be classified into a specific IIM subcategory with judicious application of the criteria. Use of serology and biopsy were suboptimal in seronegative and unclassified groups. Higher therapeutic options were needed in a quarter.