Unmasking Orbital Myopathy: When Enlarged Extraocular Muscles Indicate More Than Thyroid Eye Disease: Report of Two Cases.

INTRODUCTION: Orbital myopathy is a condition involving the extraocular muscles, and establishment of a differential diagnosis is crucial to determine its etiology. However, the broad spectrum of the etiologies and the overlap in their clinical and radiological findings often make the diagnosis challenging. We report two cases of orbital myopathy that clinically mimicked thyroid eye disease (TED). CASE PRESENTATION: Both patients underwent unsuccessful medical ± surgical treatments, based on the presumed diagnosis of TED at other institutions. One patient presented with enlargement of multiple extraocular muscles, causing proptosis and extraocular motility restriction, whereas the other showed inferior rectus muscle enlargement and subsequent severe restrictive hypotropia. Incisional biopsy was performed at our institution in both cases: Lacrimal gland biopsy confirmed a histopathological diagnosis of immunoglobulin G4-related ophthalmic disease in one patient, whereas extraocular muscle biopsy revealed metastasis from breast carcinoma in the other. DISCUSSION: These cases underscore the importance of comprehensive diagnostic approach, including consideration of alternative etiologies and tissue biopsy, to ensure accurate diagnosis of orbital myopathy.