Proptosis is forward displacement of the eyeball, an exaggerated degree of exophthalmos. There may be lower lid retraction. Proptosis may be assessed clinically by standing directly behind the patient and gradually tipping the head back, observing when the globe of the eyeball first comes into view; this is most useful for asymmetric proptosis. An exophthalmometer may be used to measure proptosis. Once established, it is crucial to determine whether the proptosis is axial or nonaxial. Axial proptosis reflects increased pressure within or transmitted through the cone of extraocular muscles (e.g., thyroid ophthalmopathy, cavernous sinus thrombosis), whereas nonaxial proptosis suggests pressure from an orbital mass outside the cone of muscles (e.g., orbital lymphoma, pseudotumor, mucocele). Pulsatile axial proptosis may occur in carotico-cavernous fistula, in which case there may be a bruit audible by auscultation over the eye. Venous angioma of the orbit may cause an intermittent proptosis associated with straining, bending, coughing or blowing the nose.
Dedicated orbital CT or MRI, the latter with fat-suppression sequences and intravenous gadolinium contrast, may be required to detect intraorbital masses.
Middle fossa tumors may cause pressure on the veins of the cavernous sinus with secondary intraorbital venous congestion causing a "false-localizing " proptosis.