A 35-year-old woman with Graves' disease develops proptosis with lid lag, lid retraction, and periorbital edema. CT orbit shows enlargement of extraocular muscles with sparing of the tendinous insertions. Which extraocular muscle is MOST commonly and FIRST affected in thyroid eye disease (TED)?

• A Superior rectus
• B Inferior rectus ✓
• C Lateral rectus
• D Medial rectus

Explanation

In thyroid eye disease (Graves' orbitopathy), the inferior rectus is the most commonly enlarged muscle, followed by the medial rectus, superior rectus, and lateral rectus (mnemonic: I'M SLow or IMSL). Enlargement occurs in the muscle belly with sparing of the tendons — a key feature distinguishing TED from orbital myositis (which involves the tendon). Inferior rectus fibrosis leads to hypotropia and restricted upgaze, often requiring corrective surgery.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.