Is polymyositis a rare or over-diagnosed entity? A descriptive follow-up study of patients initially admitted in a tertiary center with the diagnosis of polymyositis

Objective: This study aimed to describe a significant sample of patients with an initial diagnosis early-stage polymyositis (PM) in who were reclassified to different diagnoses after new evaluations during follow-ups. Subjects and Methods: In a single-center descriptive study conducted from 2000 to 2019, 108 patients with an initial presumptive diagnosis of PM were admitted to our tertiary center and were reevaluated during follow-up. Patients with promptly suspected or definite initial diagnoses of other autoimmune or hereditary myopathies were excluded. Results: The mean age of the 108 patients when admitted initially into our center was 45.3 ± 15.4 years; they were predominantly female (70.4%) and Caucasian (81.5%). During a median follow-up period of 4.0 years and constant diagnostic reinvestigation, only 30 (27.7%) out of the 108 participants maintained the PM diagnosis, whereas the other patients were re-diagnosed with immune-mediated necrotizing myopathy (20.4%), inclusion body myositis (18.5%), muscular dystrophy 13.0% as (13.0%), antisynthetase syndrome (8.3%), metabolic myopathies (5.6%), and other muscle diseases (7.3%). The initial clinical and laboratory parameters were not distinguishable between the PM and reclassified patients. Twenty-two of 30 patients with PM continued follow-up in our service, all with clinical remission or complete clinical response, full recovery of muscle strength, and normalization of the serum levels for muscle enzymes. Conclusions: Patients with PM should be reevaluated constantly, as PM can mimic other muscle diseases. Notably, only one-third of our large patient samples maintained diagnoses of PM until the end of the present study's data collection.