Journal of Oral and Maxillofacial Radiology

: 2017  |  Volume : 5  |  Issue : 3  |  Page : 63--66

Pneumatization patterns of onodi cell on multidetector computed tomography

Vishwanath T Thimmaiah, C Anupama 
 Department of Radiodiagnosis, JSS Medical College and Hospital, JSS University, Mysore, Karnataka, India

Correspondence Address:
Dr. Vishwanath T Thimmaiah
Department of Radiodiagnosis, JSS Medical College and Hospital, JSS University, MG Road, Mysore - 570 004, Karnataka


Background: Onodi cell is the most common anatomical variant of posterior ethmoidal air cells. Pneumatization patterns of Onodi cell are important clinically due to its proximity to optic nerve and internal carotid artery canal. Multidetector computed tomography (MDCT) provides the best imaging modality for identification and pneumatization pattern of Onodi cell. Objectives: The present study is done to know the patterns of pneumatization of Onodi cell by 128 slice MDCT. Materials and Methods: This was a retrospective analysis of paranasal sinuses in 1080 cases by MDCT in adults. Axial images of paranasal sinuses were taken by 128 MDCT with further multiplanar reconstruction to identify Onodi cell and their pneumatization patterns. Statistical analysis was done using SPSS version 20, and 95% confidence interval (CI) values were calculated using a confidence Interval calculator for proportional values. Results: A total of 260 cases (24.07%, CI - 21.52%–26.62%) showed Onodi cell among the total 1080 cases included in this study. Males formed more cases than females with 62.69% and 37.31%, respectively, Type II was the most common pattern of pneumatization with 56.55% (CI - 50.12–62.18), followed by Type I with 41.53% (CI - 35.54–47.52) and Type III with 1.92% (CI - 0.25–3.59) of cases. Conclusion: Onodi cell is the most common anatomical variation of ethmoidal sinuses with higher prevalence among Asian populations. Their patterns of pneumatization are important clinically to avoid surgical damage to optic nerve and internal carotid artery canals. Onodi cell is better detected with MDCT and helpful for sinonasal surgeons for a better surgical outcome.

How to cite this article:
Thimmaiah VT, Anupama C. Pneumatization patterns of onodi cell on multidetector computed tomography.J Oral Maxillofac Radiol 2017;5:63-66

How to cite this URL:
Thimmaiah VT, Anupama C. Pneumatization patterns of onodi cell on multidetector computed tomography. J Oral Maxillofac Radiol [serial online] 2017 [cited 2018 Jan 22 ];5:63-66
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Full Text


Onodi cell or sphenoethmoidal air cell is a posterior-most ethmoidal air cell that pneumatize either superolateral, superior or lateral to sphenoid sinus.[1] Adolf Onodi was the first to describe the presence of Onodi cell in the year 1904.[2] Embryologically, sphenoid bone ossifies from the lower chondral ossification center and the upper ossification center merging with that of Ethmoid. Posterosuperior ethmoidal air cells may surround optic nerve, and when persist, it forms the Onodi cell.[3] Onodi cell if present has to be detected preoperatively, because of its very proximity to optic nerve and internal carotid artery canal so as to avoid injury during sinus surgeries. Multidetector computed tomography (MDCT) plays a very important imaging modality for detection of Onodi cell and to assess their patterns of pneumatization in relation to sphenoid sinus.

Aims and objectives

The study was undertaken to know the patterns of pneumatization of Onodi cell by 128 slice MDCT.

 Materials and Methods

Study design

A retrospective radiological study was undertaken in the Department of Radiology, JSS Medical College and Hospital, Mysore, Karnataka, India over a period of 6 months from January to June 2016.


A total of 1080 peripheral nervous system studies were analyzed during the study, out of which 260 cases satisfying inclusion criteria were included.

Inclusion criteria

All adults of age above 18 years and of both sexes were included in the study.

Exclusion criteria

Posttraumatic, infective, neoplastic, and postoperative cases that distorted the normal anatomy of paranasal sinuses were excluded from the study.

Data collection

Axial images of paranasal sinuses were taken with Philips ingenuity 128 slice MDCT scanner (Netherland). The patient was positioned supine with head first, and axial sections were captured in helical format by fixing the tube current at 120 kVp and 117 mAs. High-resolution images were obtained with scan parameters of collimation 64 × 0.625, pitch of 0.39, field of view of 220 mm, filter at Y-sharp (YC), slice thickness of 1 mm, window centering at 200, and window width of 2000. Multiplanar reconstruction was done from axial images in a dedicated workstation to obtain coronal and sagittal images without any loss of volume data. Images were stored in hospital picture archiving and communication system and then the images were retrieved from hospital database system obtaining the inpatient number. Both sagittal and coronal planes were used to identify the presence of Onodi cell. Onodi cell was identified as a posterior most ethmoid cell that in present either superior and/or lateral to sphenoid sinus. Further on sagittal plane, Onodi cell was seen separate from the posterior group of ethmoid air cells. Their pneumatization patterns and relationship with sphenoid sinus were analyzed using coronal reformatted images. Pneumatization patterns of Onodi cell were further classified into three patterns in the present study for their easy recognition and to assess their relationship with sphenoid sinus by the following methodology. A horizontal line was drawn at the uppermost part of sphenoid sinus in coronal reformatted image at a plane where maximum size of Onodi cell was seen in contact with bony sphenoid sinus. If the cell was seen above and medial to sphenoid sinus, then it was labeled as Type I pattern of pneumatization, Type 2 if the air cell was seen both above and below the horizontal line and Type III if it was seen below the horizontal line. All the findings were reported by a single radiologist and analyzed retrospectively.

Statistical analysis

All the data were entered into Microsoft Excel 2013 sheet for analysis. Categorical variables were reported as proportions. The analysis was done using SPSS version 20 (Philips health care, Netherland). 95% confidence interval (CI) values were calculated using a confidence interval calculator for proportional values.


1080 MDCT paranasal sinuses were analyzed over a period of 6 months, out of which 260 cases showed the presence of Onodi cell [Table 1]. Males constituted 163 cases (62.69%) and females 97 cases [37.31%, [Table 2]. Type I [Figure 1]a were 108 cases (41.53%, CI - 35.54–47.52), Type II [Figure 1]b and [Figure 1]c 147 cases (56.55%, CI - 50.12–62.18), and Type III [Figure 1]d five cases [1.92%, CI - 0.25–3.59, [Table 3].{Table 1}{Table 2}{Figure 1}{Table 3}


Posterior ethmoid sinuses have complex anatomy, and their normal anatomical variations are frequently encountered. Onodi cell is one among the posterior ethmoid variations that need to be further evaluated because of its very proximity to optic nerve and internal carotid artery canal. Onodi cell has varying definitions that include at least one of the following criteria: Endoscopically identifiable optic canal bulge or the presence of air cell either lateral, superior or superolateral to sphenoid sinus and present in close approximation to optic nerve canal or Internal carotid artery canal.[4] Onodi cell has to be differentiated form overriding posterior ethmoid cell because of their anatomical resemblance and both being closely related to optic nerve canal.[5] Onodi cell is isolated air cell, that is completely separate from the posterior ethmoid air cells or sphenoid sinus, whereas overriding posterior ethmoid air cells are in continuous with that of posterior ethmoid air cells. The prevalence of Onodi cell varies according to their description and modalities used for identification. According to Driben et al., the prevalence of Onodi cell was 39% on endoscopic examination of cadavers.[6] The prevalence was 12%, 8%, and 25% according to Arslan et al.,[7] Unal et al.[8] and Nitinavakarn et al.[9] respectively in their CT scan study of Onodi cell. Identification of Onodi cell is very important clinically because of its proximity to optic nerve canal. There is increased the risk of injury to the optic nerve during sphenoid sinus surgeries and may be mistaken peroperatively resulting in incomplete clearance of the sinus disease. Onodi cell mucocele or pyocele can cause retrobulbaroptic neuritis, neuropathy, or loss of vision if not treated completely.[10] Onodi cells may interfere with surgical exposure of sellar floor and complete excision of sellar or parasellar mass lesions. Endoscopic sinus surgery has been widely accepted surgical option available for paranasal sinus diseases, and it has to be performed very safely with few complications. Anatomical variations of posterior ethmoid air cells are more common and the presence of Onodi cell in relation to optic nerve can have an increased risk of injury if adequate preoperative detection is not done.[11] Hence, improving the preoperative identification of these Onodi cell and their pattern of pneumatization by MDCT can decrease the risk of subsequent optic nerve injury.

The relationship between Onodi cell and optic nerve canal has been investigated previously and their actual prevalence varies considerably among different studies. Inconsistency in their prevalence may be due to different criteria's used to define Onodi cell or different modalities used to identify or due to racial factors. According to Yeoh and Tan, Onodi cell is more frequently present in Asian population compared to other races.[12] The prevalence was 42% according to Kainz J et al. using endoscopic dissection for identification of Onodi cell [13] and 14% using endoscopic surgeries by Weinberger et al.[14] Onodi cell has to be differentiated from overriding posterior ethmoid cell which can be coexisting in 36.8% of Onodi positive cell specimens.[15] Posterior ethmoid cells may expand beyond the ethmoid bone into the sphenoid bone usually superolaterally and may surround optic canal simulating Onodi cell in majority of the cases. Surgical approach will change once the Onodi cell is identified and it may be seen extending up to 1.5 cm beyond the anterior surface of the sphenoid sinus. The inferomedial floor of sphenoid sinus is the preferred route to enter Onodi cell so as to avoid any injury to optic nerve.

MDCT offers the best imaging tool to detect and recognize the pattern of pneumatization of Onodi cell. The advantage is multiplanar reconstruction and high-resolution bone reconstruction algorithm. Three patterns of pneumatization of Onodi cells are described in the present study with Type II (superolateral) being most common followed by Type I (superior) and Type III (Lateral). In our study, we do had a problem to differentiate Onodi cell and overriding posterior ethmoid air cell if both were coexistent. However, overriding posterior ethmoid cells did not have a complete lined bony septum, whereas Onodi cell has a completely lined bone septum which is separate from posterior ethmoid group of air cells.

Systematic evaluation of paranasal sinuses by CT scan is needed to detect Onodi cell and other posterior ethmoidal air cells so as to avoid any potential complications during endoscopic sinus surgeries.[16] Visualization of Onodi cells in all orthogonal plaines, i.e., axial, coronal and sagittal are necessary for their accurate identification and pneumatization patterns. Hoang et al. in his study showed that coronal plain is more useful than other planes for evaluation of ethmoidal sinus and its variations.[17] However, in the current study, multiplanar reconstructions were made in all three planes and Onodi cell was analyzed for their presence and patterns of pneumatization. Analyzing these ethmoidal air cells in any single plane would result in the under or over the interpretation of Onodi cell, thus requiring multiplanar imaging modality for their accurate detection and assessment of pneumatization. Further MDCT also has an advantage of faster scanning, reconstruction algorithm for bone and soft tissue and high spatial resolution. MDCT does have a limitation of radiation risk compared to other modalities, but with easy availability and noninvasiveness, MDCT can be used preoperatively to know the ethmoid sinus variations in all patients before undergoing sinonasal surgeries.


Onodi cell presence and their patterns of pneumatization are very useful for operating sinus surgeons for clearing inflammatory pathology involving paranasal sinuses. Inadvertent optic nerve and internal carotid artery canal damage can be avoided during surgeries with adequate preoperative knowledge about the paranasal sinus anatomy. Type II is the most common type of Onodi pneumatization pattern when compared to other types. MDCT offers the best imaging modality for Onodi cell detection and to assess their pneumatization patterns.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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