orbital pseudotumor due to thyroid opthalmopathy v, myositis

Differential points:  left image, from internet, shows medial rectus hypertrophy which (along with inferior rectus) is characteristic of thyroidopthalmopathy.  This condition is also tendon sparing.  Right image is orbital pseudotumor which in this case affects lateral rectus and tendon.  The condition on right can be secondary to a number of different conditions including RA, orbital tumor, Crohn's disease, and others.