A 45-year-old woman has bilateral exophthalmos, lid lag, and injection over the lateral rectus insertion on slit-lamp examination. Orbital MRI shows bilateral fusiform enlargement of extraocular muscles sparing the tendons. The MOST likely diagnosis is:

• A Orbital pseudotumour (idiopathic orbital inflammation)
• B Cavernous sinus thrombosis
• C Thyroid-associated orbitopathy (Graves' disease) ✓
• D Lymphoma of the orbit

Explanation

Thyroid-associated orbitopathy (TAO) characteristically shows fusiform enlargement of the extraocular muscle BELLIES with sparing of the tendons on CT/MRI — a key differentiating feature. The order of muscle involvement by frequency is: Inferior rectus > Medial rectus > Superior rectus/levator complex > Lateral rectus (mnemonic: I'M SLow or I'M SLagging). Bilateral proptosis, lid lag, chemosis of lateral rectus insertion, and systemic thyroid disease are additional features. Orbital pseudotumour involves tendons and muscles together. Lymphoma shows a moulded, 'cast-like' soft tissue mass.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.