The ethmoid sinuses are a labyrinth of multiple small air cells within the ethmoid bone, divided into anterior (draining into the middle meatus via the infundibulum) and posterior (draining into the superior meatus) cells by the basal lamella of the middle turbinate. The lateral wall of the ethmoid (lamina papyracea) separates the ethmoid cells from the orbit; the roof (ethmoid roof, fovea ethmoidalis) separates them from the anterior cranial fossa at the cribriform plate level.
The ethmoid sinuses are central to chronic rhinosinusitis pathophysiology: the ostiomeatal complex (uncinatus, ethmoid infundibulum, anterior ethmoid cells) is the common drainage pathway of the anterior sinuses. FESS targets the anterior ethmoid cells and ostiomeatal complex for access and drainage. The lamina papyracea is easily violated during ethmoidectomy, causing orbital fat herniation and ecchymosis. The ethmoid roof (fovea ethmoidalis) perforation produces CSF rhinorrhoea and intracranial penetration. The olfactory cleft adjacent to the ethmoid roof contains the olfactory neuroepithelium at risk during aggressive anterior ethmoidectomy.
Inadvertent penetration of the thin lamina papyracea during FESS allows orbital fat to herniate into the ethmoid cavity, producing periorbital ecchymosis and potentially orbital haemorrhage; endoscopic recognition of orbital fat herniation should halt the dissection and any retropulsion of the herniated fat into the orbit should be avoided to prevent compartment syndrome.
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