Enlarged extraocular muscle bellies sparing the tendons (inferior and medial first), increased orbital fat and proptosis; the report must say whether the orbital apex is crowded, because compressive optic neuropathy needs urgent decompression.
Orient first
- Muscle involvement order classically: inferior, medial, superior, lateral (the “I’m SLow” mnemonic — verify).
- Tendon sparing separates it from orbital myositis (which involves the tendon).
- On MRI, oedema in the muscles on fluid-sensitive sequences indicates active inflammation — relevant for steroid or radiotherapy decisions.
Acquire the study
- Non-contrast CT orbits, 1 mm, axial and coronal reformats.
The manoeuvre
- Coronal reformats: muscle belly thickness per muscle; tendon sparing on axial images.
- Orbital apex crowding: effacement of fat around the optic nerve on coronal slices.
- Proptosis: globe position relative to the interzygomatic line in mm.
- Lamina papyracea bowing; stretched optic nerve.
What confirms it
- Bilateral tendon-sparing muscle enlargement with a thyroid history.
What licenses you to exclude it
- Tendon involvement, a mass or unilateral single-muscle disease prompts other diagnoses.
The classic misread
- Missing apical crowding by looking only at the mid-orbit.