Isolated idiopathic myositis of the inferior oblique muscle biopsied through lateral orbitotomy

BACKGROUND: Isolated orbital myositis of the inferior oblique muscle (IObM) is rare, with few reported cases. CASE DESCRIPTION: A 65-year-old woman was aware of double vision and left dacryorrhea for 2 months. At presentation, the patient showed mild restriction on the downward gaze. In addition, a subcutaneous mass was palpated on the left eyelid. The blood examination showed normal findings. Cranial computed tomography revealed an isodense mass in the left orbit, located in the inferior, inferolateral, and posterior aspects of the bulb. On magnetic resonance imaging, the mass was well-demarcated, appeared isointense on both T1- and T2-weighted imaging, and was intensely enhanced after intravenous infusion of gadolinium. The patient underwent biopsy through lateral orbitotomy. Microscopically, the resected specimen showed sheet-like proliferation of small round nonneoplastic cells. These cells were positively immunostained for both B- and T-lymphocyte markers. Therefore, we diagnosed the patient with isolated idiopathic myositis of the IObM. The patient was managed with steroids, which resulted in a remarkable resolution of her orbital symptoms. CONCLUSION: Biopsy findings should be considered when the presentation of orbital myositis is atypical. Lateral orbitotomy may provide a useful surgical window when approaching the posterior belly of the IObM.