A 35-year-old woman presents with three weeks of painful right proptosis, eyelid oedema, and diplopia. She is afebrile with normal thyroid function. Contrast CT shows diffuse enlargement of the lateral rectus muscle including its tendon. Her symptoms resolve almost completely within days of starting oral prednisolone. What is the diagnosis?

• A Thyroid-associated orbitopathy
• B Carotid-cavernous dural fistula
• C Orbital myositis secondary to sarcoidosis
• D Idiopathic orbital inflammatory disease (orbital pseudotumour) ✓

Explanation

Painful proptosis with dramatic steroid responsiveness is characteristic of idiopathic orbital inflammatory disease, a nonspecific lymphofibroblastic infiltration diagnosed after excluding infection, granulomatous disease, and thyroid orbitopathy. Involvement of the muscle tendon distinguishes it from thyroid-associated orbitopathy, which characteristically spares the tendon. Sarcoid myositis is rare and usually accompanied by systemic features, and a dural fistula would show arterialized conjunctival vessels rather than true myositis.

Reference: Parsons' Diseases of the Eye, 23rd ed.

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Written and medically reviewed by the StethoPrep medical team.