The ethmoid air cells are a labyrinth of 3-18 thin-walled air-filled cells within the ethmoid bone between the nasal cavity and the orbit. They are divided into anterior cells (draining into the middle meatus) and posterior cells (draining into the superior meatus). The paper-thin medial orbital wall (lamina papyracea) forms the lateral wall of the ethmoid labyrinth. The skull base (fovea ethmoidalis) forms the roof.
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| Ossification | Present at birth and pneumatise the ethmoid labyrinth further through childhood, reaching adult size at about 12-14 years. |
Ethmoid sinusitis is the most common source of orbital complications: preseptal and orbital cellulitis from direct spread through the lamina papyracea is the most frequent orbital infection in children. The ostiomeatal complex, where the anterior ethmoid, maxillary and frontal sinuses drain into the middle meatus, is central to chronic rhinosinusitis, and FESS targets the ethmoid cells as the hub of sinus drainage. The agger nasi cell is the consistent anterior ethmoid landmark for identifying the frontal recess. Breaching the lamina papyracea during ethmoidectomy exposes orbital fat and risks orbital haemorrhage. Damage to the fovea ethmoidalis, especially when it is low (Keros type 3), causes CSF leak and intracranial injury.
Acute ethmoid sinusitis spreads through the paper-thin lamina papyracea to produce subperiosteal abscess and orbital cellulitis, causing periorbital swelling, proptosis, chemosis, and ophthalmoplegia; managed by IV antibiotics with surgical drainage for subperiosteal abscess causing optic nerve compromise.
Instrumentation through the ethmoid cells at or near the fovea ethmoidalis during FESS penetrates the thin skull base, causing iatrogenic CSF rhinorrhoea that appears as clear watery nasal discharge, confirmed by beta-2 transferrin assay and managed by endoscopic skull base repair.
Penetration of the thin medial orbital wall during ethmoid surgery, letting orbital fat herniate and causing periorbital bruising or orbital haemorrhage; recognising it stops further dissection at that site.
