Diagnostic Criteria for Orbital Proptosis

Proptosis is defined as an abnormal protrusion of eyeball. 

Owing to the rigid bony structure of the orbit with only anterior opening for expansion, any increase in orbital contents taking place from the side or from behind will displace the eyeball forward.  Proptosis can be the result from varies disease processes including infections, inflammations, Tumours, trauma, metastases, endocrine lesions, vascular lesion, orbital - extra orbital osseous lesions.

Proptosis can be measured Clinically as well as Radiologically.

How to Measure Proptosis Radiologically ?

Both CT and MRI can be used.

Plane of the scan or axial sections must be parallel to the plane passing through the optic nerve head and lens.

Eyelids should be open with the patient looking straight ahead without eye movements. 

Interzygomatic line is drawn first, a straight line connecting the anterior margins of zygomatic processes, at the level of median portion of the globe or orbit. 

The distance from posterior sclera margin to Interzygomatic line (IZL) , Normal is 9.9mm +/- 1.7mm (Reference : Robert A.Nugent, Rod I.Belkin, Janet M. Niegel, Jack Rootman et al, Correlation of CT and clinical findings. Radiology, 177(3):675-82. Dec 1990)

Other measurements includes thickness of recti, the superior rectus muscle group (comprising of superior rectus muscle and the elevator muscle of the upper eyelid) and inferior rectus muscle best measured coronal and sagittal planes. The horizontal diameters of the lateral and medial recti are best measured on axial plane.

References: 

Thyroid ophthalmopathy revisited ; Karina Freitas Soares Machado; Marcelo de Mattos Garcia.

Szucs-Farkas Z, Toth J, Balazs E, et al. Using morphologic parameters of extraocular muscles for diagnosis and follow-up of Graves' ophthalmo pathy: diameters, areas, or volumes? AJR Am J Roentgenol. 2002;179:1005-10.