Orbital pseudotumor is an idiopathic inflammatory lesion of the orbit most commonly involving the extra-ocular muscles (most commonly the lateral rectus muscle), but can involve any of the orbital contents including the retro-orbital fat and the lacrimal gland.
The differential diagnosis of orbital pseudo-tumor of the orbit is:
Thyroid orbitopathy (Grave's disease): usually sparing the anterior tendon of the extra-ocular muscles and is limited to the muscle belly. The inferior and medial rectus muscles are commonly involved followed by the superior and lateral rectus & oblique muscles (I'M SLOw!)
Orbital lymphoma: homogeneously enhancing intraorbital mass usually separable from the extra-ocular muscles. Painless swelling with proptosis.
Tolosa-Hunt syndrome: the orbital inflammatory lesion involves the orbital apex with extension into the cavernous sinus.