The septum divides a harmless preseptal cellulitis from a sight- and life-threatening postseptal one; imaging answers whether infection is behind the septum, whether there is a subperiosteal or orbital abscess, and whether it has reached the cavernous sinus.
Orient first
- Most postseptal infection spreads from the ethmoid sinuses through the lamina papyracea — subperiosteal abscess sits medially.
- Chandler-type staging: preseptal → orbital cellulitis → subperiosteal abscess → orbital abscess → cavernous sinus thrombosis (verify the scheme you report against).
- Proptosis, ophthalmoplegia and reduced acuity clinically predict postseptal disease.
Acquire the study
- Post-contrast CT orbits and paranasal sinuses, 1 mm axial with coronal reformats; soft-tissue and bone windows.
The manoeuvre
- Coronal and axial soft-tissue window: is the fat stranding anterior to the orbital septum only, or in the extraconal/intraconal fat?
- Medial orbital wall: a lentiform rim-enhancing collection beneath the periosteum = subperiosteal abscess; measure its volume or width in mm.
- Bone window: ethmoid opacification and lamina papyracea erosion.
- Superior ophthalmic vein calibre and filling; cavernous sinus enhancement asymmetry.
- Globe position (proptosis measured from the interzygomatic line) and optic nerve stretching (tenting of the posterior globe).
What confirms it
- Postseptal fat stranding with or without a rim-enhancing subperiosteal or intraorbital collection, usually beside opacified ethmoid cells.
What licenses you to exclude it
- Stranding confined anterior to the septum with normal postseptal fat and extraocular muscles means preseptal cellulitis only.
The classic misread
- Calling phlegmon an abscess — a collection needs a rim and fluid centre on the post-contrast series.