Sphenoethmoidal air cell (Onodi air cell)

• Posterior ethmoidal air cell extending superolaterally and posteriorly beyond the typical ethmoidal sinus extent
• Aerated cell in close proximity to the optic nerve canal and medial aspect of anterior clinoid process
• Bony separation of <1 mm between cell wall and optic nerve or internal carotid artery

• The Onodi cell is the most posterior ethmoidal air cell that extends laterally, superiorly, and posteriorly into the region of the anterior sphenoid sinus, creating an aerated cavity distinct from typical ethmoidal cells.
• On coronal CT imaging, the Onodi cell appears as a well-aerated cell located in the posterosuperior ethmoid region, often extending toward or into the anterior clinoid process.
• The critical anatomic relationship is the close proximity to the optic nerve, with as little as 0.03 mm (median 0.08 mm) of bone separating the cell wall from the optic nerve canal.
• The cell may also have intimate contact with the internal carotid artery, which runs posteromedially, creating additional surgical risk.
• Aeration of the anterior clinoid process alone does not confirm Onodi cell presence, as recesses of the sphenoid sinus frequently pneumatize this region independently.
• A central Onodi air cell is a rare variant that extends superiorly above the sphenoid sinus rather than laterally.
• Most Onodi cells are asymptomatic incidental findings, but disease within the cell (mucocoeles, squamous cell carcinoma, acute rhinosinusitis) may lead to early optic nerve compression.
• The reported incidence varies widely from 3.4% to 60% depending on the imaging definition criteria used.

• Posterior ethmoidal sinus (normal anatomy) — distinguished by typical size and location within standard ethmoidal series without posterior or superior extension.
• Sphenoid sinus recess or aeration of anterior clinoid process — does not arise from the posterior ethmoid and lacks the characteristic lateral extension from the posterior ethmoidal cells.
• Sphenoidal mucocoele — presents as expansion and opacification of an Onodi cell with possible mass effect on adjacent structures; requires clinical symptoms.
• Squamous cell carcinoma of sphenoethmoidal cell — appears as expansile, sometimes destructive lesion within the cell with heterogeneous enhancement.
• Central Onodi air cell — rare superior extension variant that lies above rather than lateral to the sphenoid sinus.

When reporting an Onodi air cell, clearly specify its location (e.g., "posterolateral ethmoid with extension toward anterior clinoid"), describe any disease within the cell, explicitly note the intimate relationship with the optic nerve and internal carotid artery, and emphasize the minimal bony separation as a surgical consideration.

• Confusing anterior clinoid process aeration with Onodi cell presence—anterior clinoid aeration is often a sphenoid sinus recess, not an Onodi cell.
• Failing to recognize the extremely thin (sometimes <0.1 mm) bony wall between the Onodi cell and optic nerve, leading to underestimation of surgical risk.
• Overlooking a central Onodi air cell variant that extends superiorly rather than posterolaterally, which may be missed on sagittal review if coronal anatomy is not carefully assessed.
• Not distinguishing between asymptomatic incidental Onodi cells and those with secondary disease (mucocoele, infection, malignancy) that require urgent attention due to rapid optic nerve involvement.