INCIDENCE OF THE ONODI (SPHENOETHMOID) CELL AND ITS CLINICAL SIGNIFICANCE

The Onodi (sphenoethmoid) cell, named after Onodi's report in 1903, has attracted attention in recent advances in CT and endoscopic sinus sugery. The paranasal sinus Anatomic Terminology Group in 1993 defined the Onodi cell as the most posterior ethmoid cell pneumatizing laterally and superiorly to the sphenoid sinus. Since the Onodi cell is intimately related to the optic nerve, this anatomic variant is considered clinically significant. The optic nerve is placed at risk during sinus surgery, and lesions of the Onodi cell may cause visual symptoms. Although the Onodi cell is generally accepted to occurs in about 10%, the reported incidence varies between 3.4% and 51%. We assessed the reliability of CT in detecting the Onodi cell to further understanding of this clinically relevant anatomic variation and the prevalence of affected Onodi cells. We studied CT findings in 100 randomly chosen patients, evaluating 79 with both coronal and axial CT scanning and 21 with axial CT scanning alone. Using a cadaveric head containing a typical Onodi cell, we studied gross and CT findings. We found the Onodi cell in 23 patients (23%) including 4 with affected Onodi cells, i. e., 2 with cystic lesions (1 with visual disturbance) and 2 with sinusitis. In 6 patients (6%), coronal CT images showed features such as the Onodi cell. Axial CT scanning showed that confusing features in 5 cases corresponded to those of sphenoid sinuses divided into 2 by a transverse septum and to that of a posterior ethmoid cell invading under the sphenoid sinus in 1 case. In 10 patients (10%), we could not determine the presence of the Onodi cell due to ethmoid cell complexity ethmoid cell and limited CT resolution. As suggested previously by correlative studies between CT and endoscopy, our results indicated that the Onodi cell is a more frequent anatomic variant than commonly appreciated, though CT is not always able to detect it. Serial axial CT imaging thus appears more useful than coronal CT imaging in detecting the Onodi cell. Pathological characteristics of Onodi cell lesions are required to be clarified in future studies.