A Case of Idiopathic Orbital Inflammation Presenting as Isolated Myositis of the Inferior Oblique Muscle

Purpose: To report a case of idiopathic orbital inflammation presenting with isolated myositis of the inferior oblique muscle. Case summary: A 54‐year‐old man presented with swelling on the left lower lid, pain on superonasal and inferonasal gaze, and binocular diplopia for 2 months. His head was tilted to the right by about 5° and mild conjunctival injection and 3 mm narrowing of palpebral fissure of the left eye compared to the other eye were observed. Eight prism diopter (PD) left hypertropia and 4 PD intermittent esotropia were noted on primary gaze, which worsened on leftward gaze, downward gaze, and left head tilt. Orbital magnetic resonance imaging (MRI) showed enhanced hypertrophy of the left inferior oblique muscle. Systemic work‐up for possible inflammatory diseases yielded negative results. Therefore, a presumptive diagnosis of idiopathic isolated myositis of the left inferior oblique muscle was made. The patient was treated with 60 mg of oral corticosteroid per day for the first week, and the dose was tapered for 1 month as the symptoms reduced. Two months later, the patient became free from any symptoms and follow-up orbital MRI showed a significant decrease in size of the left inferior oblique muscle. There have been no signs of recurrence for 7 months. Conclusions: A presumptive diagnosis of idiopathic isolated myositis of the inferior oblique muscle was made in a patient with swelling of the left lower lid and binocular diplopia based on orbital MRI and systemic work‐up. Good results were achieved with oral corticosteroid therapy.