A 40-year-old patient presents with bilateral proptosis, lid retraction, and eyelid lag. Hertel exophthalmometry measures 23 mm in the right eye and 21 mm in the left (normal <20 mm). Thyroid function tests reveal hyperthyroidism. The specific sign of superior scleral show (white below upper limbus) is present. Which orbital finding on CT most specifically explains the proptosis in thyroid eye disease?
- A Enlargement of the lacrimal gland
- B Enlargement of EOM including the tendon insertions
- C Enlargement of the extraocular muscles (EOM) with tendon sparing ✓
- D Increased retrobulbar fat volume with normal muscle size
Explanation
Thyroid eye disease (Graves' orbitopathy) produces CT-characteristic enlargement of the extraocular muscle bellies with relative sparing of the tendon insertions — the 'fusiform' or 'Coca-Cola bottle' shape. This is because the inflammatory process (glycosaminoglycan deposition, T-cell infiltration) targets the muscle belly. The inferior rectus is most commonly and severely involved (causing diplopia), followed by medial rectus. The tendon-sparing pattern distinguishes TED from orbital myositis (which involves tendons) and lymphoma. The volume of enlarged muscles and increased retrobulbar fat pushes the globe anteriorly, causing proptosis. Retrobulbar fat volume also increases in TED but muscle hypertrophy is the more specific finding.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.