The prevalence of the Onodi cell - Most suitable method of CT evaluation in its detection

BackgroundThe Onodi cell (sphenoethmoidal air cell) is an anatomic variant of the most posterior ethmoid cell that pneumatised superiorly and laterally to the sphenoid sinus, and is in close relation to the optic nerve. The proper identification of the Onodi cell is essential during a pre-operative computer tomography (CT) examination, as the presence of that ethmoid cell variant makes sphenoid sinus surgery more risky. The bulging of the optic nerve to the ethmoid cell wall is well visualized during endoscopic examination, but there are no clearly defined criteria for a potentially dangerous Onodi cell type in CT examinations.ObjectiveTo determine the prevalence and types of Onodi cell in CT examination and find the most suitable CT scanning planes to identify it.Material and methodsThree plane (axial, coronal and sagittal) reconstructions of 196 paranasal sinuses were analyzed. The most posterior ethmoid cell was classified into four types, according to its position in relation to the sphenoid sinus and the optic nerve canal bulging into the lumen of the sphenoethmoid cell.ResultsThe Onodi cell was detected in 39.8% of cases, although in 55.6% of cases a direct contact between the most posterior ethmoid cell and the optic nerve was present. Bulging of the optic nerve canal was seen in 25% of cases. In two cases two posterior ethmoid cells were in direct contact with the optic nerve canal.ConclusionThe prevalence of the Onodi cell was higher than previously reported. Pre-operative paranasal sinuses CT examination should be evaluated in all three planes (axial, coronal and sagittal) to avoid missing or over-detection of the Onodi cell. Axial and sagittal planes are preferable for the detection of the Onodi cell.