Assessing the relationship of agger nasi pneumatization to the lacrimal sac: a dynamic computed tomography-dacryocystography analysis

Introduction

Primary acquired nasolacrimal duct obstruction (PANDO) disrupts tear drainage in adults, leading to discomfort and diminished quality of life (1). Endoscopic dacryocystorhinostomy (DCR) has gained favor for treating PANDO due to its minimally invasive approach, shorter recovery period, and high success rates (2). The success of this procedure critically depends on precise exposure of the lacrimal sac (3-6). Despite extensive research, a consensus on the optimal extent of lacrimal sac opening for consistent success remains elusive. However, prevailing opinions suggest that opening the lacrimal sac to the level of the common canaliculus is sufficient (7,8). Therefore, accurately determining the position of the lacrimal sac, particularly concerning critical nasal cavity landmarks such as the axilla of the middle turbinate, is essential.

One study used preoperative computed tomography-dacryocystography (CT-DCG) to better localize the common canaliculus (9). Recent studies have adopted this study’s findings, defining the plane 5 mm below the lacrimal sac’s fundus as the position of the common canaliculus (10,11). However, given racial variations, the limited size of sample, and the advancements in CT imaging technology providing increasingly detailed images in recent years, conducting a new study with a larger sample size to determine the position of the common canaliculus is necessary.

An agger nasi cell (ANC) refers to the pneumatization of the agger nasi, a ridge anterior to the attachment of the middle turbinate on the nasal surface of the frontal process of the maxillary bone, adjacent to the lacrimal sac (12,13). It serves as a critical anatomical landmark in endoscopic surgeries targeting the frontal recesses and sinuses (14). Recent approaches, diverging from previous methods that have simply categorized ANC as anterior ethmoid cells (4,11,14-16), focus on understanding the origin of the ANC and use three-dimensional CT (3D-CT) to improve accuracy in dynamic localization (12,13). In endoscopic DCR, determining the spatial relationship between the lacrimal sac and ANC is crucial. While some studies have endeavored to assess the potential overlap between the lacrimal sac and ANC, their methodologies often lacked the use of contrast agents to delineate the lacrimal sac’s structure (10,17). Additionally, their assumption that the plane 5 mm below the lacrimal sac’s fundus is the location of the common canaliculus may compromise the accuracy of their findings (10,17). Employing dynamic methodologies to identify the shape of the ANC and its potential overlap with the lacrimal sac would offer greater precision and accuracy. Unlike previous approaches that have concentrated on a single plane, dynamic methods that involve the examination of the lacrimal apparatus across various CT planes may provide a more comprehensive assessment.

Our study employed CT-DCG to examine the configuration of the lacrimal sac, emphasizing the precise localization of the common canaliculus. Through multiplanar CT imaging, we dynamically identified and classified agger nasi pneumatization, aiming to evaluate the potential overlap between the ANC and the lacrimal sac. These insights are crucial for refining DCR procedures and may ultimate lead to improved patient care and surgical outcomes. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-541/rc).

Methods

Patient enrollment

This retrospective study consecutively enrolled patients of Han Chinese ethnicity with unilateral PANDO who had undergone orbital CT-DCG scans before endoscopic DCR at the Eye, Ear, Nose, and Throat Hospital of Fudan University from January 2021 to June 2023. This study was conducted in a tertiary care setting, involving inpatient participants who were scheduled for DCR surgery under general anesthesia. The exclusion criteria included acute dacryocystitis, canaliculi obstruction, rhinitis, and secondary nasolacrimal obstruction due to conditions such as tumors, trauma, sarcoidosis, or Wegener granulomatosis. This study was conducted in accordance with the Declaration of Helsinki (as revised in 2013) and was approved by the Ethics Committee of the Eye, Ear, Nose, and Throat Hospital of Fudan University (No. 2023164). The requirement for individual consent in this retrospective analysis was waived. Ultimately, a total of 110 adult participants with obstructed nasolacrimal ducts aged from 25 to 85 years were included in this study (Figure 1).

Figure 1 Flowchart for the enrollment of patients. DCR, dacryocystorhinostomy; CT-DCG, computed tomography-dacryocystography.

CT-DCR scanning procedure

CT imaging was performed with a 128-slice dual-source high-resolution scanner (SOMATOM Definition Flash, Siemens Healthineers, Erlangen, Germany). The orbit was scanned with contiguous 0.75-mm axial sections aligned parallel to the bilateral infraorbitomeatal line under a bone window algorithm (width: 4,000; level: 700). Coronal and sagittal scans were acquired spontaneously, with patients positioned in a supine orientation. Prior to any intervention, a baseline CT scan was performed. Subsequently, each canaliculus underwent probing and irrigation with ioversol, a water-soluble iodinated contrast agent with a concentration of 320 mg/mL. After irrigation, a second CT scan was performed following a 5-minute interval to evaluate the distribution and flow dynamics of the contrast agent within the lacrimal drainage system. Upon completion of the scans, a saline solution was used to flush out the ioversol. Multiplanar reformatting (MPR) was applied to the axial, coronal, and sagittal planes of all images. The Syngo.via software was used for processing the CT data.

CT-DCR scan analysis

The shape and position of the lacrimal drainage system were initially examined. Analysis was performed on axial planes, using the entrance of the bony nasolacrimal canal (BNLC) as the reference point (Figure 2A). The height of the common canaliculus was determined by measuring the distance from the BNLC entrance to its insertion into the lacrimal sac (Figure 2B). Similarly, the distance from the BNLC entrance to the axilla of the middle turbinate was measured to ascertain the height of the axilla. Employing the same methodology, we determined the height of the lacrimal sac by measuring the distance between the uppermost part of contrast and the entrance of the BNLC (Figure 2C). Additionally, the distance between the common canaliculus and the axilla of the middle turbinate was also determined.

Figure 2 Axial sections of computed tomography scans illustrating three distinct levels. (A) The entrance of the BNLC is observed as the lowest section, where the lacrimal sac transitions into the nasolacrimal duct, forming a bony circle (*). The positions of the UP and MT are indicated. (B) The common canaliculus level (arrow) is discernible by the contrast agent junction between the canaliculus and the lacrimal sac. (C) The apex of the lacrimal sac is defined as the highest point where the contrast agent is present (triangle). UP, uncinate process; MT, middle turbinate; BNLC, bony nasolacrimal canal.

Subsequently, the ANC was dynamically located and categorized. Initially, the axilla of the middle turbinate in the sagittal plane was identified, and then a lateral review was completed to localize the origin of ANC, with a focus given to the pneumatization of the mound. MPR was then used to clarify the morphology of the ANC. The morphology of the ANC and its connections with other ostia such as the frontal, ethmoidal, and maxillary sinuses were classified and visualized.

To investigate the relationship between the lacrimal sac and ANC, a dynamic measurement approach was employed. Unlike conventional methods that focus solely on defining the axial plane of the common canaliculus, our approach involved a broader perspective. We established the upper boundary as the common canaliculus and the posterior boundary as the posterior aspect of the lacrimal sac. Within this defined spatial framework that covered the entire 3D space, any intersection between the lacrimal sac and ANC was classified as an overlap. The relevant area of the lacrimal sac is displayed in Figure 3, with Panel A showing the frontal view and Panel B the lateral view. The shaded red area in the figure denotes the lacrimal sac cavity that should be opened during DCR surgery, with the upper boundary being the common canaliculus plane, the lower boundary being the opening of the BNLC, and the posterior and lateral boundaries representing the walls of the lacrimal sac. After the the MPR in axial, coronal, and sagittal planes was reviewed, the presence of an ANC within the shaded red area was considered to indicate an overlap, as demonstrated in Figure 4. The star-labeled area in Figure 4 corresponds to the shaded red area in Figure 3. Conversely, if the ANC was completely outside the shaded red area, no overlap was considered to be present. All of the examinations were conducted by the same ophthalmologist, an attending physician with 10 years of experience in the field.

Figure 3 The illustration of the opening of the lacrimal sac during dacryocystorhinostomy. This is an illustration of the lacrimal duct structure. Panel A represents the frontal view, while Panel B depicts the lateral view. Directions of “lateral” and “nasal” in Panel A, as well as “anterior” and “posterior” in Panel B, are indicated. In both panels, asterisks denote the position of the nasolacrimal duct, “MT” represents the middle turbinate, and the red dots indicate the axilla of the middle turbinate. The shaded red area in the figure represents the lacrimal sac cavity that should be opened during DCR surgery, with the upper boundary being the nasolacrimal duct plane, the lower boundary being the opening of the BNLC, and the posterior and lateral boundaries being the walls of the lacrimal sac. If an ANC is present within the shaded red area, it is considered an overlap. Only when the ANC is completely outside the shaded red area is the absence of overlap considered. MT, middle turbinate; DCR, dacryocystorhinostomy; BNLC, bony nasolacrimal canal; ANC, agger nasi cell.

Figure 4 Overlap of agger nasi cell with the lacrimal sac in axial and vertical positions. (A) The axial plane showing the anterior overlap of the agger nasi cell with the lacrimal sac. The dotted line denotes the plane of the posterior wall of the lacrimal sac, with the star symbols above indicating the area of anterior overlap by the agger nasi cell. (B) MPR image demonstrating the inferior overlap of the agger nasi cell with the lacrimal sac. The dotted line represents the plane of the common canaliculus, and the star symbols indicate the portion of overlap between the agger nasi cell and the lacrimal sac. MPR, multiplanar reformatting.

Statistical analysis

Patient ages and measured values are presented as the mean ± standard deviation, were measured three times by the same observer, and were averaged for statistical analysis.

Results

Among the 256 inpatient participants aged 18 years and above who were admitted to our hospital for endoscopic DCR surgery between January 2021 and June 2023, 256 underwent preoperative CT-DCG. After initial screening, 94 patients were excluded due to bilateral lacrimal obstruction. This left 162 patients eligible for further evaluation. Subsequently, an additional 52 patients were excluded due to the following reasons: 29 had canaliculi obstruction, 9 had acute dacryocystitis, 7 had rhinitis, 5 had a history of previous lacrimal trauma, and 2 had lacrimal tumors. Ultimately, 110 patients with aged between 25 and 85 years participated in this study, with the mean age being 57.05±13.31 years. The majority of participants were female (89.1%), and the distribution included 49 right-sided and 61 left-sided cases of nasolacrimal duct obstruction. The enrollment of patients is shown in Figure 1.

The location of common canaliculus

In this study, the common canaliculus was detected in 98 patients. For the remaining 12 patients who exhibited separated canaliculi, we considered the insertion point of the lower canaliculus into the lacrimal sac as the location of the common canaliculus. The average height of the lacrimal sac was 13.55±1.96 mm. Additionally, the average distance from the common canaliculus to the lacrimal sac’s fundus was 3.74±1.08 mm. Moreover, the average position of the axilla of the middle turbinate was approximately 4.59±2.55 mm above the entrance of the BNLC and 5.23±2.41 mm below the level of the common canaliculus.

Prevalence and classification of the ANC

In the 110 CT scans enrolled in this study, an origin of the ANC from the mound was observed in 100 cases, representing a high prevalence rate of 90.9%. The classification of various ANC types is detailed in Table 1. Upon meticulous examination, approximately 42.7% of the patients (47 out of 110) exhibited a discrete ANC (Figure 5A). In this classical presentation, the pneumatization of the agger nasi manifested as a discrete ostium situated in the posterior midportion, with its pathway terminating in the superior middle meatus. Notably, our observations revealed that 48.2% of the total cases (53 out of 110) were associated with adjacent ostia, including the maxillary, ethmoidal, or frontal sinuses, contributing to the complexity of ANC morphology (Figure 5B-5D). These diverse ANC patterns, along with their interconnections, are delineated in Table 1.

Figure 5 Classification and communication of agger nasi cells with adjacent sinuses. (A) MPR image depicting a discrete agger nasi cell located in the posterior midportion, ending in the superior middle meatus. (B) MPR image illustrating an agger nasi cell communicating with the frontal sinus. (C) MPR image showing an agger nasi cell communicating with the ethmoidal sinus. (D) MPR image displaying an agger nasi cell communicating with the maxillary sinus. Star symbols indicate the presence of agger nasi cells, while arrows depict communication pathways between agger nasi cells and the respective adjacent sinuses. MPR, multiplanar reformatting.

Relationship of the ANC with the lacrimal sac

During our spatial relationship analysis, it was observed that the majority of ANCs were positioned higher and posterior to the lacrimal sac. On average, the bottom of the ANC was located 1.96±3.04 mm below the common canaliculus and 5.69±3.06 mm below the lacrimal sac fundus in a vertical orientation. However, it is worth noting that among the 70 cases (63.6%) in whom the ANC was not positioned higher than the common canaliculus vertically (Figure 4B), 57 of these cases (51.8%) exhibited an anterior orientation relative to the lacrimal sac anteroposteriorly (Figure 4A), indicating an overlap between the ANC and the lacrimal sac. This overlap accounted for 51.8% of the ANCs and could have potentially affected endoscopic DCR outcomes. Interestingly, in five cases (4.5%), the overlap between the two structures was not detectable at the level of the common canaliculus in axial CT scans and would be considered nonoverlap if assessed under the previously described methods. However, the overlap was observed a few millimeters below this level, underscoring the significance of employing a dynamic detection method to avoid missing such cases.

Discussion

Successful endoscopic DCR relies heavily on the creation of a bony ostium that is not only appropriately sized but also precisely located to ensure effective exposure and marsupialization of the lacrimal sac (18). Previous studies have advocated positioning the upper boundary for the ostium at the level of the common canaliculus to ensure the proper opening of the lacrimal sac in endoscopic DCR (3,4,9). In our study, we conducted a detailed examination of the location of the common canaliculus using CT-DCG. We observed an average distance of 3.74±1.08 mm between the common canaliculus and the lacrimal fundus, situating the common canaliculus in the upper 30% of the sac. Additionally, we found that the common canaliculus lay 5.23±2.41 mm above the axilla of the middle turbinate, offering valuable guidance for DCR procedures.

The prevalence of ANC varies widely in literature, a discrepancy largely attributed to differences in anatomical definitions, terminologies, and methods of analysis (4,11,14). Recent research emphasizes the importance of focusing on the cell’s origin from the mound (13). Our study employed a dynamic approach in evaluating the ANC, specifically focusing on its origin and using 3D MPR to detail its morphology. The observed high prevalence rate of 90.9% in our cohort supports the clinical relevance of ANCs.

In our investigation, the 3D MPR provided insightful details. Not only did the MPR vividly illustrate the drainage pathway of discrete ANCs (Figure 5A), as observed in other limited studies using nasal endoscopy (19), but it also revealed connections between the ANC and adjacent ostia, including the maxillary, ethmoidal, and frontal sinuses (Figure 5B-5D). This interaction, to our knowledge, has not been previously documented. As with other nasal ostia, the ANC exhibited considerable variation with some being connected with more than one other ostium. This observation enhances our understanding of ANC morphology; however, it also prompts consideration of whether the decision to open the ANC during DCR surgery might impact other nasal ostia.

The pneumatization of the ANC influences the surgical access to the lacrimal fossa during endoscopic procedures, potentially obstructing the path to the lacrimal sac or causing confusion due to its close proximity to the medial surface of the lacrimal sac. Previous studies have suggested a high likelihood of ANC obstruction occurring at a level 5 mm below the lacrimal fundus, necessitating its removal (10,17). Our study, which used dynamic observation with 3D CT-DCG, revealed a 51.8% overlap of the ANC with the lacrimal sac during DCR. As there were five cases that would have been missed with previous methods, we believe this number to be more accurate and reflective of reality. Combined with our data showing that the bottom of the ANC was, on average, 2 mm lower than the common canaliculus, there appears to be a possibility of opening the ANC during endoscopic DCR. While previous recommendations (10) include removing the anterior wall of the ANC to ensure a sufficiently wide sac opening into the nasal cavity at the level of the common canaliculus, our study raises certain questions. Given the ANC’s communication with other nasal sinuses, we must evaluate whether minimizing ANC disruption is advisable in DCR surgery. This approach aims to prevent unintended communication between the ostium and other nasal sinuses, potentially reducing the risk of surgical failure.

Our study involved certain limitations that should be mentioned. First, the individual variations in sinus or lacrimal anatomy could have significantly influenced surgical outcomes. Our proposed figures offer a general guideline and may not fully capture the complexities of each patient’s unique anatomy. Detailed preoperative CT analysis in complexed individual patient and careful intraoperative assessments are both crucial for enhancing the precision of surgery. On the other hand, our study is based on the Chinese population which limits the generalizability of our results, given the known racial differences in anatomical structures. Future research should aim to include a diversity of racial populations to better understand and account for these variations.

Conclusions

In conclusion, our study of 110 patients emphasizes the importance of precise bony ostium placement in endoscopic DCR surgery. The close proximity of the common canaliculus to the lacrimal sac fundus supports its role as a key reference point for ostium creation. The high prevalence and varied morphology of the ANC are crucial considerations in surgical planning. Dynamic 3D CT-DCG revealed a significant ANC overlap with the lacrimal sac during DCR. However, ANC disruption should be weighed against potential complications related to unintentional sinus communication. These findings emphasize the importance of tailored surgical approaches in optimizing the outcomes and minimizing the risks in DCR procedures.

Acknowledgments

Funding: None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-24-541/rc

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-541/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki (as revised in 2013) and was approved by the Ethics Committee of the Eye, Ear, Nose, and Throat Hospital of Fudan University (approval No. 2023164). The requirement for individual consent in this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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