Histopathologic Findings in Idiopathic Orbital Myositis.
To report the histologic and clinical features of idiopathic orbital myositis (IOM) cases who underwent extraocular muscle (EOM) biopsy over 20 years, to provide the first methodical reference to the expected histopathologic findings and to discuss the histopathologic differences from common differential diagnoses.Cohort study.All patients with a diagnosis of IOM who underwent EOM biopsy in the years 2000-2019 were included. Patients who had a different final diagnosis were excluded.Tissue samples of EOM and medical records of all participants were reviewed.Histopathologic features of muscle biopsy including tissue morphology and cellular composition.13 patients met both inclusion and exclusion criteria and their tissue samples were revisited. 9 patients had histopathology findings that suggested a conclusive diagnosis of IOM and the study focuses on them. The average age at presentation was 49 years and 66.7% of patients were women. The most commonly biopsied EOM was the medial rectus (44.4%). The most common indications for biopsy were non resolving orbital disease with inadequate response to corticosteroids (44.4%), or a high suspicion for malignancy due to known pre-existing systemic malignancy or the presence of an atypical orbital mass in addition to enlarged muscles (44.4%). The histopathology findings that suggested a diagnosis of IOM were splaying of muscle fibers by inflammatory infiltrates (n=9) and mild fibrosis (n=8) in the endomysium or replacing muscle fibers, with no granulomas or vasculitis. The inflammatory infiltrates identified were of chronic inflammatory cells, consisting of lymphocytes (n=9), plasma cells and histiocytes (n=6 each). Other less commonly identified cells were eosinophils (n=4), polymorphonuclears and giant cells (n=1 each). Muscle fibre degeneration or regeneration were evident in five cases. Four patients from the initial cohort had inconclusive histologic findings upon revision and were re-assigned as suspected IOM.The histopathologic features of involved muscles in IOM resemble those seen in idiopathic orbital inflammation (IOI) and differ from those seen in common differential diagnoses. Extraocular muscle biopsy should be strongly considered whenever the presentation of orbital myositis is not typical or when significant underlying conditions are a possibility.