• Pornthep Kasemsiri, Sanguansak Thanaviratananich, and Warinthorn Puttharak.
• Department of Otorhinolaryngology, Faculty of Medicine, Khon Kaen University, Khon Kaen, Thailand.
• J Med Assoc Thai. 2011 Sep 1; 94 (9): 1122-6.

BackgroundThe Onodi cell (sphenoethmoidal cell) is an anatomical variation of the most posterior ethmoid air cell the pneumatizes laterally and/or superiorly to the sphenoid sinus and is intimately in contact with the optic nerve. If it is infected or goes unrecognized during surgery it may result in serious damage to the optic nerve. Nowadays, computed tomographic scans of paranasal sinuses (CT PNS) have been used to detect variations in paranasal sinus anatomy However there is a lack of data about the variations of Onodi cell.ObjectiveTo determine the prevalence and various patterns of Onodi cell.Material And MethodAxial, coronal, sagittal and sagittal oblique (parallel to the optic canal) CT scans of the paranasal sinuses, nasopharynx, neck and orbit performed at Srinagarind Hospital between January 1, 2004 and November 30, 2006 were reviewed A pilot study was carried out to investigate the inter-rater reliability of the identification of Onodi cell between the radiologist and rhinologist until the kappa value was 0.74. During the main study, the radiologist and rhinologist interpreted the CT scans independently. If there was a discordant opinion concerning the presence of Onodi cell, a consensus was reached by discussion between the rhinologist and the radiologist.Results187 CT scans (374 sides) were included Sagittal oblique view detected Onodi cell in 185 sides 49.5% (95% CI: 44.4-54.5). The patterns of Onodi cell were classified into three patterns. In the first pattern, the Onodi cell extended only superiorly to sphenoid sinus (46%; 95% CI: 38.9-53.1). In the second pattern, it extended only laterally to sphenoid sinus (1%; 95% CI: 0.3-3.9). The last pattern was a combined type, lateral and superior to sphenoid sinus (53%; 95% CI: 45.8-60.0).ConclusionThe prevalence of Onodi cell diagnosed by CT scans was 49.5% (95% CI 44.4-54.5) and the most common pattern was the combined type. This information may be useful for those who perform endoscopic sinus surgery.

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