The Onodi Cell and Optic Canal as Variations Assessed with Paranasal Sinus CT

In 2014, the Onodi cell was defined as the most posterior ethmoid cell which is situated laterally and/or superiorly to the sphenoid cell. It is closely connected to the optic nerve. So, if it is not recognized during endoscopic sinus surgery, it may result in some optic nerve injury or failure to open the sphenoid sinus.The objective of the present study was to determine the prevalence of the Onodi cell and variations of the optic canal. Paranasal thin slice CT scans obtained from 237 patients in Osaka Kouseinenkin Hospital were analyzed. Patients with previous histories of facial trauma, paranasal sinus surgery or nasal cancer were excluded.The prevalence of the Onodi cell was 27.4% (130 sides). The optic canal exposure types were classified into 4 groups: A: posterior ethmoid only; B: Sphenoid sinus only; C: posterior ethmoid and sphenoid sinus; and D: no exposure. Group A comprised 97 sides, group B 300 sides, group C 49 sides, and group D 28 sides. The prevalence of a pneumatized anterior clinoid process was 21.7% (103 sides). There was a significant correlation between the Onodi cell and sphenoid sinus and paranasal sinus lesions in the Chi-square test. Our data should prove useful in endoscopic sinus surgery. We must take care to avoid the optic nerve injury in patients with Onodi cell and a pneumatized anterior clinoid process.