Burglars know there’s more than one way to skin a vault. – James Chiles The orbit is a cone‐shaped vault (Figs 218 and 221). At its apex are three orifices through which pass the nerves, arteries, and veins supplying the eye. Unlike the eye, in which most parts are amenable to direct visualization, evaluation of the orbit often requires the use of diagnostic tools such as CT scans (Figs 2, 3, 142, 224, 225, 227–229B, 234, 402, and 480) and MRI (Figs 120, 124, and 140). CT scans are usually the radiologic technique of choice to evaluate orbital diseases such as fracture (Fig. 231), foreign bodies, thyroid disease (Figs 2 and 3), and sinusitis (Figs 223 and 224). MRIs are preferred for soft tissue and neurologic conditions (Fig. 124). Fig 218 Side view of orbit; periosteum (periorbital) of the orbit (green), the orbit septum (red), and tarsal plate (blue) are continuous connective tissue membranes. This fibrous membrane then goes on to cover the optic nerve as it exits the orbit and is continuous with the dura mater covering the brain. CT scanning has made amazing contributions to medical diagnosis, but it is a large contributor to the six‐fold increase in diagnostic radiation in the last 30 years, because of overutilization. It is predicted that CT scans may be responsible for 1.5–2.0% of all future cancers in the USA and studies reveal that patients are not informed of this risk 90–95% of the time. Special caution should be given to children and pregnant women. Contrast materials are preferred for most CT and MRI procedures since it increases sensitivity. Usage of iodine‐based chemicals used in CT scans should be minimized with histories of iodine allergy and in pregnant women and children. MRI is more hazardous with foreign bodies of unknown origin and in pregnant women and patients with pacemakers, metallic cardiac valves, and other ferro‐magnetic implants. Gadolinium is used for contrast. Fig 219 Yellow subconjunctival fat. Source: Courtesy of University of Iowa, Eyerounds.org Vascular testing may be done with CT or MRI angiography (Figs 128, 146, and 147). CT‐A can detect 3–5 mm cerebral aneurysms and venous thrombosis. Either CT‐A or MR‐A is used to detect carotid artery stenosis. The yellow orbital fat may herniate through the orbital septum under the conjunctiva (Fig. 219) and shouldn’t be mistaken for an abnormal growth. Fig 220 Subcutaneous orbital fat often causes a desire for cosmetic excision, but caution patient about the risk of orbital hemorrhage (Fig. 235). The fat more commonly herniates under the skin of the lower lid. The apparent swelling shouldn’t be confused with lid edema. Unlike fluid, the fat may be rolled beneath the examiner’s fingertips (Figs 173 and 220). The orbit is surrounded on four sides by the periorbital paranasal sinuses, i.e., the maxillary, frontal, sphenoid, and ethmoid sinuses. Pain, described as deep, or behind the eye, is most often due to allergy or infection of these sinuses. Pressure applied to the skin overlying the inflamed frontal, maxillary, and ethmoidal sinuses may cause tenderness (Fig. 223). The sphenoid sinus is behind the globe and cannot be tested in this way (Fig 222). Fig 221 Front view shows the apex of the orbit. Note proximity to sinuses with its potential for causing infection and the significant number of blood vessels vulnerable to hemorrhage (Fig. 235). Fig 222 CT scan of sphenoid sinusitis with air‐fluid level (↑).
Chapter 5
The orbit
Imaging
Sinusitis
Clues that may indicate disease of the orbit
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