A 35-year-old woman has painful diplopia with restriction of abduction. MRI shows enlargement of the left lateral rectus including its tendon. Which feature distinguishes this condition from thyroid eye disease?

• A Association with suppressed TSH levels
• B Preferential involvement of the inferior rectus
• C Sparing of the muscle belly with isolated tendon thickening
• D Enlargement of both the muscle belly and the tendon ✓

Explanation

Idiopathic orbital inflammation of the myositic type enlarges both the muscle belly and its tendon, and characteristically causes severe pain exacerbated by eye movement. In thyroid eye disease the tendon is characteristically spared, only the muscle belly being enlarged, with a typical order of involvement of inferior rectus, medial rectus, superior rectus, then lateral rectus. Tendon sparing is therefore the single most useful imaging discriminator between the two conditions.

Reference: Kanski's Clinical Ophthalmology, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.