Imaging description

The evaluation of an orbital infection should seek to define the extent of infection, the source of infection, and the presence of complications.

An imperative distinction is preseptal (periorbital) versus postseptal (orbital) cellulitis. The orbital septum is a thin fibrous layer of the eyelids that blends with the periosteum of the bony orbit. The septum cannot be specifically delineated on conventional imaging, but its position can be inferred. Inflammatory changes posterior to the septum, such as fat stranding, muscle enlargement, and abscess formation, imply the presence of orbital cellulitis (Figures 15.1 and 15.2). Inflammatory changes entirely anterior to the septum are classified as periorbital cellulitis [1].

The most common cause of an orbital infection is sinusitis, especially of the ethmoid sinus. Therefore, the sinuses should be scrutinized for signs of inflammation (Figure 15.1). Infection spreads through the bony walls via perivascular routes [2]. Other sources of infection include orbital foreign objects, adjacent dermal infection, and septicemia. An uncommon source is odontogenic infection (Figures 15.3 and 15.4), which usually arises from the maxillary dentition and spreads through the paranasal sinuses, premaxillary tissues, or infratemporal fossa. The most common signs of dental infection include periapical lucency, indistinctness of the lamina dura, and widening of the periodontal ligament space [3].