Proptosis is an outward displacement of the globe from its normal position. It may be a sign of an orbital mass or due to local infection or inflammation that causes an increased volume in the orbit. (The term exophthalmos also means outward displacement of the globe from its normal position but is typically reserved for proptosis secondary to thyroid ophthalmopathy.)

The direction in which the globe is displaced can help determine the likely etiology of proptosis. Retrobulbar lesions within the muscle cone, such as cavernous hemangioma, glioma, meningioma, hydatid cysts, metastases, and arteriovenous malformations, usually cause axial displacement of the globe. Maxillary sinus tumors cause superior displacement of the globe, while frontoethmoidal mucoceles, abscesses, osteomas, dermoid cysts, and lacrimal gland tumors cause inferior displacement. 

Idiopathic orbital inflammation can mimic an orbital tumor by causing proptosis. It is most likely associated with underlying autoimmune pathology. It typically presents with unilateral proptosis and responds well to treatment with systemic corticosteroids. 

Thyroid-related orbitopathy, another autoimmune disease, is the most common cause of both unilateral and bilateral proptosis in adults. White women between 30 and 50 years of age are most frequently affected. Here proptosis may be associated with eyelid retraction, abnormal thyroid levels, strabismus, and optic nerve compromise.