We report a case of idiopathic orbital myositis with sudden onset of unilateral visual loss and hypertrophy of the lateral rectus muscle at the apex of the orbit that was diagnosed using orbital biopsy and treated with intravenous corticosteroids.

We examined a 20-year-old woman who complained of decreased visual acuity and binocular diplopia for 2 weeks. She showed 40-prism diopter left esotropia at the primary position. Her visual acuity with eyeglasses was 0.7 and 0.2 in the right and left eyes, respectively. Brain magnetic resonance imaging (MRI) revealed a round mass on the left lateral rectus muscle at the apex of the orbit with optic nerve compression. We planned orbital biopsy to exclude orbital lymphoma and other biopsy-requiring diseases. After 3 weeks, the visual acuity of her left eye decreased to 0.02. Repeated MRI revealed enlargement of all extraocular muscles on the left orbit. With biopsy showing several lymphocytes infiltrating muscle fibers, we made a diagnosis of idiopathic orbital myositis. Intravenous injection of corticosteroids was administered during hospitalization. After 2 weeks of corticosteroid therapy, the visual acuity of her left eye was improved to 0.63, the esotropia disappeared, and the enlargement of the left lateral rectus muscle was improved on orbital MRI.

For non-specific idiopathic orbital myositis located at the apex of the orbit and enlargement of the extraocular muscle in a short period with decreased visual acuity, administration of intravenous injection of corticosteroids before orbital biopsy could help restore vision.