In thyroid eye disease (Graves' orbitopathy), the MOST commonly affected extraocular muscle, and its characteristic pattern of CT involvement, is:

• A Lateral rectus — tendon insertion involvement
• B Inferior rectus — muscle belly enlargement sparing the tendon ✓
• C Medial rectus — diffuse fibrosis including the tendon
• D Superior oblique — isolated belly thickening

Explanation

In thyroid eye disease, muscle belly enlargement sparing the tendinous insertion is the hallmark CT finding that distinguishes it from inflammatory orbital myositis (which involves the tendon). The inferior rectus is the most commonly affected muscle (52–60% of cases), followed by the medial rectus, superior rectus, and lateral rectus in decreasing order. Inferior rectus restriction causes limitation of upgaze and hypotropia. The mnemonic is 'I'M SLow' — Inferior, Medial, Superior, Lateral.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.