Autoantibody-Based Clinicoradiopathologic Phenotyping of Idiopathic Inflammatory Myopathies: An Indian Cohort.

Objectives: We aim to characterize the clinical, pathological, laboratory and imaging features of various antibody defined IIM subgroups in Indian population.

Methodology: 103 patients who satisfied 2017 ACR/ EULAR Classification criteria for IIM, and tested seropositive for myositis antibodies using Immunoblot technique were retrospectively identified. Patients were classified into following subgroups - Mi2B group, SRP group, Anti RNA Synthetase antibody group (Jo 1, PL 7, PL 12, OJ), multiple MSA, only MAA group (U1RNP, Ro 52, SS-A, SS-B, PM Scl 75, PM Scl 100). Clinical, laboratory, histopathology and imaging parameters were compared among different groups.

Results: Ro52 (n = 31; 22.46%) was the most common autoantibody seen in our cohort followed by Mi2B (n = 26; 18.84%) and SRP (n = 20; 14.49%). Skin manifestations (p = 0.053) and joint involvement (p = 0.023) were seen more frequently among Mi2B and MAA sub groups. Three out of 4 patients with persisting antibody positivity on serial measurements developed clinical relapse between 2 and 3 years after the initial episode. Remaining 2 patients showed declining titres of antibodies and developed no clinical relapses during follow up period of 5 and 6 years. Among histopathology features, perifascicular atrophy was found to be more prevalent among Mi2B (55.6%) and MAA subgroups (31.3%) (p value = 0.037). In thigh muscle MRI, Mi2B group showed a pattern of diffuse involvement of affected muscles without regional preference, with sparing of hamstring group. SRP group showed characteristic pattern of edema involving peripheral regions of Quadriceps femoris and central regions of Adductor group along with severe atrophy involving hamstring and adductor compartment muscles. ILD was seen in 27.5% of patients who underwent CT Chest. 3 patients were found to have underlying malignancy at the time of diagnosis of myositis (Mi2B = 2 and PM Scl = 1).

Conclusion: Classification of IIM patients based on myositis antibodies yields subgroups with certain differences in clinical, laboratory, histopathological and imaging features.