Purpose: To report a case of idiopathic orbital inflammation presenting with isolated myositis of the superior oblique muscle. Case summary: A 57-year-old male presented with pain in his left eye and diplopia for three months. His past history was unremarkable except surgery for a thyroid nodule six months prior. At initial presentation, his corrected vision and intraocular pressure were normal. Biomicroscopy and fundus examination were unremarkable. Nasal side swelling of the left upper eyelid

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... was observed, and 1.5-mm proptosis of the left eye was revealed on exophthalmometry. Two and four prism diopter right hypertropia were noted at up and right-up gaze, respectively. Axial and coronal orbital computed tomography (CT) showed enhancement of isolated superior oblique muscle hypertrophy in the left eye. Systemic work-up was negative for any inflammatory disease. A presumptive diagnosis of isolated myositis of superior oblique muscle was made, and the patient was started on 30 mg of oral prednisolone daily. Pain and diplopia disappeared and proptosis of the left eye decreased to 0.5 mm after one week of treatment. Orbital CT images obtained one month after treatment showed a significant decrease in size of the left superior oblique muscle. The steroid was tapered for two months, and the patient has shown no signs of recurrence for four months after cessation of treatment. Conclusions: Idiopathic orbital inflammation presenting with isolated myositis of the superior oblique muscle is very rare and has not been previously reported in Korea. A patient presenting with pain and diplopia was diagnosed as having isolated myositis of the superior oblique muscle based on CT scan, and good results were achieved with oral steroid therapy.