Prevalence of pneumatisation patterns of Onodi cells among the North-East Indian population

Abstract

Background

The posterior ethmoid sinuses have a complex anatomy and a highly variable normal structure. The posterior ethmoid Onodi cell needs careful assessment because of its proximity to the optic nerve and the internal carotid artery canal. Recognising the distinctions is essential for optimal pre-op preparation. Sphenoid sinus surgery can now be performed without worrying about compromising vital structures like the internal carotid artery or optic nerve. Preoperative radiological assessment is a quick and easy way to check for ethmoid cell pneumatisation differences.

Aim and objectives

To determine the pneumatisation patterns of Onodi cells among the North-East Indian population using 16-slice multidetector computed tomography (MDCT).

Results

Nine-hundred MDCT paranasal sinuses were analysed over a period of 0–2 years, out of which 220 cases showed the presence of Onodi cell. Type I were 89 cases (40.45%, CI 32.00–48.90), Type II were 128 cases (58.18%, CI 49.72–66.63), and 3 cases with Type III cell (1.36%, CI 0.18–2.54).

Conclusion

The most prevalent pneumatisation pattern is Type II. The pneumatisation patterns of Onodi cells can be best detected and evaluated by multiplanar MDCT imaging. Sinus surgeons need a firm grasp of pneumatisation patterns to avoid damaging the optic nerve and internal carotid artery canal during operations.

Background

Nasal cavities and paranasal sinuses differ greatly from person to person. Pneumatisation, the process of paranasal cavity enlargement, can result in a misaligned sinus structure. Pneumatisation of ethmoid air cells occurs bilaterally within the ethmoid labyrinth and may migrate to the adjoining paranasal sinuses, where it gives rise to Haller cells, agger nasi cells, and Onodi cells [1]. This study aims to assess the prevalence of an Onodi cell variation in people across a north-eastern state. Pneumatisation of the superolateral, superior, or lateral aspect of sphenoid sinus can occur due to posterior most ethmoidal air cell, also known as Onodi or sphenoethmoidal air cell [2]. This sort of cell was first identified by Adolf Onodi in 1904 [3]. During embryonic development, the sphenoid bone is formed by the union of the lower chondral ossification centre of the temporal bone with the upper ossification centre of the ethmoid bone. Posterior superior ethmoidal air cells have a potential to grow in the body of upper sphenoid and encircles the optic nerve [4]. Sphenoiditis is more common in patients with Onodi cells. However, Onodi cells cannot be singled out as the sole cause of this condition [5]. In order to avoid disastrous outcomes during sinus surgery, such as injury to the optic nerve or internal carotid artery canal, identifying Onodi cells in advance is vital. Imaging using multidetector computed tomography (MDCT) is critical for locating Onodi cells and assessing pneumatisation patterns in relation to the sphenoid sinus.

Aims and objectives

The aim of the study was to determine the variations in pneumatisation patterns of Onodi cells among the North-East Indian population using 16-slice MDCT.

Methods

Study design

A retrospective study was conducted at the Department of ENT and Radiodiagnosis at a zonal hospital in north-eastern state in India, serving to around 20,000 patients per year. The study was carried out during the period of January 2021 to December 2022.

Subjects

A total of 900 paranasal sinuses studies were analysed during the study (includes patients with NCCT (non-contrast computed tomography) head for headache evaluation, NCCT head and face for trauma evaluation), out of which 220 cases fulfilled the inclusion criteria.

Inclusion criteria

• All adults of age above 18 years

• Males and females were included in the study.

Exclusion criteria

The study excluded imaging done for posttraumatic, infective, neoplastic, and postoperative conditions that resulted in distortion of the normal anatomy of the paranasal sinuses.

Data collection

A 16-slice CT scanner made by Siemens, called the SOMATOM Emotion, was used to acquire the axial pictures of the paranasal sinuses. The patient was laid out flat on his back. Then, using a constant tube current of 130 kVp and effective mAs of 35, the axial sections were acquired in spiral mode. With these settings (collimation 16 × 0.6, pitch 0.8, FOV 180 mm, HCE image filter, slice thickness 5 mm, window centering 400, and window width 4000), we were able to capture images with a high degree of detail. Axial pictures were acquired, and then coronal and sagittal images were created using multiplanar reconstruction from the axial images on a specialised workstation, with no volume data lost in the process. Using the patient’s identification number, the photographs were located in the hospital’s database and retrieved from the picture archiving and communication system. The sagittal and coronal planes were rich with Onodi cells. The Onodi cell sits atop or to the side of the sphenoid sinus and is the most posterior ethmoid cell. It is clear in the sagittal plane that the Onodi cell is situated distant from the posterior cluster of ethmoid air cells. The pneumatisation patterns of Onodi cells and their relationship to the sphenoid sinus were studied using coronal-reformed images. The pneumatization patterns of the Onodi cells were further classified into three groups so that they could be more easily recognised and evaluated in relation to the sphenoid sinus. To determine the pneumatization pattern, a horizontal line was drawn in the coronal reformatted image at the highest point of the sphenoid sinus on a plane where the largest Onodi cell was in contact with the bony sphenoid sinus (Figs. 1, 2). If an air cell was found to be situated above and medial to the sphenoid sinus, it was labelled as having a Type I pneumatisation pattern. Some air cells, designated Type II, were visible both above and below the horizontal line, while others, designated Type III, were visible just below the line (Figs. 3, 4, 5, 6). The retrospective analysis and all findings reporting were done by a single radiologist.

Fig. 1

Reference line is drawn along the superior most part of left sphenoid sinus. Type II Onodi cell is seen above and below the line

Fig. 2

Reference line drawn along the superior most part of left sphenoid sinus. Type III Onodi cell is seen below the line

Fig. 3

Coronal reformatted MDCT image showing Type I Onodi cell on right side with optic nerve indenting along the superolateral wall

Fig. 4

a and b Type II Onodi cell in coronal and sagittal MDCT image

Fig. 5

Type II Onodi cell on left side, with optic nerve along its lateral wall

Fig. 6

Coronal reformatted MDCT image showing Type III Onodi cell

Statistical analysis

In order to analyse the information, it was entered into a Microsoft Excel 2013 spreadsheet. Proportions were reported for categorical variables. SPSS version 20 was used for the analysis. Confidence interval (CI) values for proportions were determined using a confidence interval calculator.

Results

Nine-hundred MDCT paranasal sinuses were analysed over a period of 0–2 years, out of which 220 cases showed the presence of Onodi cell. Males constituted 146 cases (66.36%) and females 74 cases (33.63% (Table 1)). Type I (Fig. 1) was 89 cases (40.45%, CI 32.00–48.90), Type II (Figs. 2a, b & 3) 128 cases (58.18%, CI 49.72–66.63), and Type III (Fig. 4) 3 cases (1.36%, CI 0.18–2.54) (Table 2).

Table 1 Total no. of Onodi cases with sex distribution in study

Table 2 Case distribution as per type of Onodi cells

Discussion

Abnormalities to the normal form of the posterior ethmoid sinuses are common due to their intricate design. The Onodi cell should be radiographically evaluated because of its closeness to the optic nerve and internal carotid artery. The word ‘Onodi cell’ can refer to a number of different things. A bulge in the optic canal that can be seen with an endoscope, or the presence of an air cell laterally, superiorly, or superolaterally positioned near the sphenoid sinus and next to the canal of the optic nerve or internal carotid artery [6, 7], are all required. Distinguishing the Onodi cell from the dominant posterior ethmoid cells is essential because of their morphological similarities and proximity to the optic nerve canal [7].

Since various diagnostic criteria are applied, the percentage of Onodi cells varies. At least one Onodi cell was found in 39% of endoscopically inspected cadavers, as reported by Driben et al. [8]. The prevalence of Onodi cells on CT scans varies widely, from 12% in a study by Arslan et al. [9] to 8% in a study by Unal et al. [10] and to 25% in a study by Nitinavakarn et al. [11]. Additionally, clinical identification of Onodi cell is quite important because of its proximity to vital structures. Misdiagnosis prior to surgery increases the likelihood of insufficient clearance of the sinus disease during a sphenoid sinus treatment, which can cause permanent damage to the optic nerve. Retrobulbar ocular neuritis, neuropathy, and visual loss can result from untreated Onodi cell mucocele or pyocele [12, 13]. The presence of Onodi cells increases the risk of surgical exposure of the sellar floor, making it more difficult to achieve a complete excision of sellar or parasellar mass lesions. Because of its superior effectiveness in treating paranasal sinus diseases, endoscopic sinus surgery must be performed with great care.

The posterior ethmoid air cells are the most frequently discussed area of morphological diversity. Optic nerve injury is more likely if an Onodi cell is near the nerve if preoperative detection fails [14]. Preoperative discovery of Onodi cells by MDCT and understanding of their pattern of pneumatization can lessen the risk of optic nerve injury. Previous studies have investigated the potential role of Onodi cells in the optic nerve canal, but their findings have been contradictory. The observed variation in prevalence may be attributable to a number of factors, including differences in diagnostic criteria, methodology, and even ethnicity.

Yeoh and Tan [15] found that Onodi cells were more prevalent in Asians than in those of any other race or ethnicity. Endoscopic dissection by Kainz et al. [16] revealed a 42% prevalence of Onodi cells, while endoscopic surgery by Weinberger et al. [17] revealed a prevalence of 14%. It is important to distinguish between Onodi cells and dominant posterior ethmoid cells since the latter can coexist with the former in 36.8% of Onodi-positive cell specimens [18]. Expansion of posterior ethmoid cells into the superolateral region of the sphenoid bone can simulate the Onodi cell. The identification of the Onodi cell may require a shift in surgical technique due to the fact that it can extend up to 1.5 cm beyond the front border of the sphenoid sinus. The Onodi cell is often reached via the inferomedial floor of the sphenoid sinus so as to avoid injuring the optic nerve.

MDCT is the ideal imaging modality for recognising and analysing the pneumatization pattern of the Onodi cell due to its high-resolution bone reconstruction technique and multiplanar reconstruction capabilities. Pneumatization of Onodi cells was shown to occur in one of three patterns, with Type II (superolateral) pneumatization being the most prevalent. Type I (superior) and Type III (lateral) pneumatization were the least common. Type II pneumatized Onodi cells are the norm, as reported by Thimmaiah V. T. et al. [19].

To avoid complications during endoscopic sinus procedures, Onodi cells and other posterior ethmoidal air cells should be systematically assessed using CT scan [20]. Imaging Onodi cells in many planes (axial, coronal, and sagittal) is required for correct detection and evaluation of pneumatization pattern changes. A study by Hoang et al. [21] found that the coronal plain is the best for assessing ethmoidal sinuses and their variants. This research emphasises the value of employing multiplanar reconstructions for assessing pneumatization presence and patterns in Onodi cells. Since the Onodi cell can be misinterpreted when analysing these ethmoidal air cells in only one plane, a multiplanar imaging modality is required for precise identification and evaluation of pneumatisation. Faster scanning, a separate reconstruction technique for bone and soft tissue, and excellent spatial resolution all contribute to MDCT’s utility as an imaging tool. However, compared to other modalities, MDCT does have a radiation risk limitation. Nonetheless, MDCT can be utilised preoperatively to evaluate ethmoid sinus abnormalities in all patients prior to sinonasal operations because of its widespread availability and lack of invasiveness.

Conclusion

Endoscopic sinus surgeons need to be aware of the existence and pneumatization patterns of Onodi cells to prevent injury to the optic nerve and internal carotid artery canal. The most reliable method for detecting and evaluating pneumatization patterns of Onodi cells is multiplanar MDCT imaging, with Type II being the most frequently detected pattern. Using MDCT imaging technique with the r, doctors can get a clearer picture of the paranasal sinuses and reduce the risk of problems during sinonasal procedures.