A 30-year-old woman has painful proptosis with diplopia. MRI shows enlargement of the right lateral rectus belly WITH involvement of its tendon insertion, unlike the typical pattern seen in thyroid eye disease. The MOST likely diagnosis is:
- A Carotid-cavernous fistula
- B Euthyroid Graves disease
- C Idiopathic orbital myositis (orbital pseudotumour) ✓
- D Metastasis to the extraocular muscle
Correct answer: C. Idiopathic orbital myositis (orbital pseudotumour)
Explanation
Idiopathic orbital inflammatory myositis inflames the entire muscle including the tendon and its insertion, producing pain exacerbated by eye movement and a myositic pattern on imaging. In thyroid eye disease the muscle belly alone is enlarged while the tendinous insertion characteristically SPARES, which is the single most useful imaging discriminator between the two. C carotid-cavernous fistula dilates the superior ophthalmic vein rather than a muscle.
Reference: Kanski's Clinical Ophthalmology, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.