Histopathologic Findings in Idiopathic Orbital Myositis

Purpose: To report the histologic and clinical features of idiopathic orbital myositis (IOM) patients 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.

Design: Cohort study.

Participants: All patients with a diagnosis of IOM who underwent EOM biopsy from 2000 through 2019 were included. Patients who had a different final diagnosis were excluded.

Methods: Tissue samples of EOM and medical records of all participants were reviewed.

Main outcome measures: Histopathologic features of muscle biopsy, including tissue morphologic features and cellular composition.

Results: Thirteen patients met both inclusion and exclusion criteria, and their tissue samples were revisited. Nine patients showed histopathologic findings that suggested a conclusive diagnosis of IOM, and the study focused 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 nonresolving orbital disease with inadequate response to corticosteroids (44.4%) or a high suspicion of malignancy because of known pre-existing systemic malignancy or the presence of an atypical orbital mass in addition to enlarged muscles (44.4%). The histopathologic 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 (n = 6), and histiocytes (n = 6). Other less commonly identified cells were eosinophils (n = 4), polymorphonuclears (n = 1), and giant cells (n = 1). Muscle fiber degeneration or regeneration was evident in 5 patients. Four patients from the initial cohort showed inconclusive histologic findings on revision and were reassigned as suspected IOM.

Conclusions: The histopathologic features of involved muscles in IOM resemble those seen in idiopathic orbital inflammation 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.