Computed tomography assessment of anterior ethmoidal canal dehiscence: An interobserver agreement study and review of the literature

Abstract

Purpose

The anterior ethmoidal artery can be injured in functional endoscopic sinus surgery. The ability of computed tomography (CT) to identify dehiscence of the anterior ethmoidal canal (AEC) has not been widely evaluated. The aim of this study was to evaluate the interobserver agreement in the CT assessment of AEC dehiscence.

Methods

We conducted a retrospective review of consecutive CT scans of the paranasal sinuses (PNS) between January 1, 2012, and December 31, 2012. Two neuroradiologists separately assessed the presence of AEC dehiscence, the presence of PNS opacification, and the best CT plane to evaluate the AEC. Statistical analysis included descriptive analysis and interobserver agreement (kappa coefficient).

Results

The AEC was below the skull base in 199 (22.3%) cases. Dehiscence of the AEC was found in 13.2% for reader 1 and in 7.3% for reader 2. The interobserver agreement for identification of AEC dehiscence was only fair (κ = 0.246). The interobserver agreement for the AEC dehiscence in cases with opacification of ethmoidal air cells was substantial (κ = 0.754).

Conclusion

The suboptimal interobserver agreement could potentially limit the usefulness of CT scans for routine assessment of AEC dehiscence. In patients with PNS opacification, CT scans could still add valuable information regarding AEC dehiscence.

Introduction

Functional endoscopic sinus surgery (FESS) is a common treatment option for patients with chronic rhinosinusitis refractory to medical treatment, with a 67–98% success rate.¹ It is also indicated for patients with neoplastic processes involving the skull base and orbits. The rate of complications associated with this procedure is about 5.6–22%, including cerebrospinal fluid leak, optic nerve injury, oculomotor deficits, nasolacrimal duct injury, anosmia, and perioperative hemorrhage.^2,3 The anterior ethmoidal artery (AEA) is one of the vascular structures that can be injured in FESS, causing significant intraoperative periorbital or intraorbital hematoma. This occurs when it is injured in close proximity to the lamina papyracea because the proximal stump of the artery retracts into the orbital cavity.^4,5

Normal and variant anatomy of the anterior ethmoidal canal

The AEA is a branch of the ophthalmic artery, which supplies the anterior ethmoidal cells and the frontal sinus.⁶ It also gives rise to meningeal vessels along its course in the olfactory fossa and descends to the nasal fossa to supply the anterior third of the nasal septum and the lateral wall of the nose.⁶ The AEA is divided into three segments: intraorbital, ethmoidal and intracranial.⁷ It enters the olfactory fossa through the lateral lamella of the cribriform plate along the anterior ethmoidal sulcus.⁶ The lamellas are vertical bony structures that develop within the cartilaginous olfactory capsule. These structures compartmentalize the ethmoidal labyrinthine complex. They course through the ethmoidal air cells and extend superiorly to the anterior skull base from the lateral nasal wall. From anterior to posterior the lamellas are: (a) the uncinate process, (b) anterior margin of the bulla ethmoidalis, (c) lamella of the middle turbinate (basal lamella), (d) lamella of the superior turbinate, and (e) lamella of the supreme turbinate. In cases when the supreme turbinate is absent, the anterior face of the sphenoid sinus is considered the fifth lamella.^8,9 The visualization of these structures is best on sagittal planes (Figure 1). Multiple dissection studies have shown that the AEA is located between the second and the third lamella in most cases.⁹ The AEA is usually centered behind the ethmoidal bulla. If the ethmoidal bulla does not extend to the skull base and there is a suprabullar recess, the artery can be exposed.¹⁰ Monjas-Cánovas et al.¹¹ reported that in 100% of cases, the AEC was between the second and third lamella. Moon et al.¹² reported that the AEC was located between the second and the third lamella in 87.1% of cases and within the second and the third lamella in 12.9% of cases. Contrary to these reports, Ferrari et al.⁴ reported that the AEC was located within the second lamella in 3.6% of cases, between the second and the third lamella in 32.1% of cases, and within the third lamella in 64% of cases. Therefore, the anatomic variations of the AEA are wide, and it is essential to be aware of these variants in the surgical field.

Figure 1.

Sagittal computed tomography (CT) image showing lamellar anatomy: (1) uncinate process, (2) bulla ethmoidalis, (3) basal lamella, (4) lamella of the superior turbinate, and (5) lamella of the supreme turbinate. SS: sphenoid sinus; MT: middle turbinate; IT: inferior turbinate.

The anterior ethmoidal canal (AEC), which houses the AEA, is usually embedded in the ethmoidal roof.¹³ Alquezar et al.⁹ found the AEA at the skull base in 66.6% of cases and below the skull base in 34.4% of cases. Lannoy-Penisson et al.¹⁴ described three anatomical variations of the AEC based on its relation to the skull base: grade I AEC within the ethmoidal roof, grade II AEC under the roof and considered as prominent, and grade III AEC distant from the ethmoidal roof (Figure 2). In grade III, a mesentery connects the canal to the roof of the ethmoid sinus, and there may be a space of up to 5 mm between the AEC and the roof.⁶ In cases of prominence of the AEC or when it is below the roof, the risk of AEA injury increases, since the anterior ethmoidal roof is the landmark and outer limit for endonasal surgery.¹⁴ Additionally, the normal bony covering of the AEC may be absent inferiorly, and the artery is suspended in a mucous membrane mesentery, which increases the risk of injury during surgery, as it becomes difficult to distinguish the AEA from the adjacent paranasal sinus (PNS) opacification with thickened mucosa and polyps (Figures 3 and 4).⁸ The prevalence of dehiscence of the AEC varies significantly in the literature, ranging from 6% to 66%.^9,12,15

Figure 2.

Sagittal CT images showing anatomical variations of the anterior ethmoidal canal (AEC): (a) grade I: within the roof; (b) grade II: under the roof, considered as prominent; and (c) grade III: distant from the ethmoidal roof (arrows).

Figure 3.

Sagittal CT images showing: (a) normal bony covering of the AEC and (b) AEC dehiscence (arrows).

Figure 4.

Sagittal (a) and coronal (b) CT images showing AEC dehiscence (arrows).

Computed tomography (CT) of the PNS has been used in the preoperative assessment to evaluate anatomic variations and possible lesions. Anatomical landmarks for locating the AEA on coronal CT include a small notch in the medial wall of the orbit and the anterior ethmoidal groove¹⁶ (Figure 5). Souza et al.⁶ found these landmarks reliable, as they described on coronal CT the medial notch of the orbit in 100% of cases and the anterior ethmoidal groove in 98% of cases. The most frequent anatomic landmark to localize the AEC is the notch in the medial wall of the orbit, which corresponds to the exit of the artery between the superior oblique and the medial rectus muscle.¹¹ This landmark makes the coronal reformats on CT the most reliable plane to localize this anatomical reference.

Figure 5.

Coronal CT images showing: (a) bony notch on the medial wall of the orbit and (b) anterior ethmoidal groove on the lateral walls of the olfactory fossae (arrows).

Most of the previously published literature describes the prevalence and the landmarks for CT identification of the AEA canal.^6,12,15,16 Identification of AEC dehiscence is important for preoperative planning. However, the ability of CT to identify dehiscence correctly has not been widely evaluated, with one study suggesting it is poor.⁴ The aim of this study was to evaluate interobserver agreement in the CT assessment of AEC dehiscence.

Methods

Our institutional research ethics committee approved this study. We conducted a retrospective review of consecutive CT scans of the PNS on a picture archiving and communicating system between January 1, 2012, and December 31, 2012. Cases with the AEC in the ethmoidal roof/skull base, postoperative cases with alteration of the anatomy, or pathology causing bony destruction were excluded. Two neuroradiologists with more than 10 years of experience separately read all the cases after an initial training session. Each neuroradiologist assessed the presence of AEC dehiscence, the presence of PNS opacification, and the perceived best CT plane to evaluate the AEC. PNS opacification was defined by the presence of soft-tissue thickening within the ethmoidal air cells. Dehiscence was defined by loss of continuity of the bone in the AEC.

CT was performed with a 64-channel multidetector CT scanner (Aquilion or Asteion, Toshiba Medical Systems, Markham, Canada; Discovery 750 or Lightspeed VCT, GE Healthcare, Chicago, IL; Somatom Sensation 64, Siemens Healthcare, Erlangen, Germany; HiSpeed QX1 64, Phillips Healthcare, Markham, Canada) and with a 320-channel multidetector CT scanner (Aquillion, Toshiba Medical Systems). Images were displayed on the axial plane with a section thickness of 2 or 3 mm, and on the coronal and sagittal planes with a section thickness of 2 mm.

Statistical analysis included descriptive analysis as well as interobserver agreement (kappa coefficient), with κ-values of 0–0.2 indicating poor agreement, 0.21–0.40 fair agreement, 0.41–0.60 moderate agreement, 0.61–0.80 substantial agreement, and 0.81–1.0 almost perfect agreement. All data were analyzed using MedCalc v12 (MedCalc Software, Ostend, Belgium).

Results

From January 1, 2012, to December 31, 2012, 1008 CT scans of the PNS were performed. We excluded 107 patients due to incomplete imaging (absent sagittal or coronal reformats, n = 56; trauma, n = 1; tumor, n = 2; motion artifact, n = 4; and prior surgery, n = 44). The remaining 901 patients formed the basis of the current study.

The AEC was found at the skull base (grade I and II) in 76.8% (n = 692) of cases. In 27% (n = 190) of these cases, the AEC was prominent within the skull base (grade II). The AEC was below the skull base (grade III) in 209 (23.1%) cases. Ten cases from this group were excluded due to thicker slices used for the scan (3 mm). The remaining 199 (22.3%) cases were scanned with a slice thickness of 2 mm. Within this group, the AEC was below the skull base bilaterally in 102 (11.4%) patients, 44 (4.8%) cases were only on the right side, and 53 (5.9%) cases were only on the left side.

A total of 199 patients with 301 AEC below the skull base were assessed by each neuroradiologist (146 on the right and 155 on the left). Dehiscence of the AEC was found in 41 (13.2%) cases for reader 1 and in 22 (7.3%) cases for reader 2. Dehiscence of the AEC with opacification of ethmoidal air cells was found in 12 (3.98%) cases for reader 1 and in six (5.31%) cases for reader 2. Dehiscence of the AEC without opacification of ethmoidal air cells was found in 29 (9.63%) cases for reader 1 and in six (1.99%) cases for reader 2. The interobserver agreement for identification of AEC dehiscence was only fair (κ = 0.246).

Opacification of the PNS was found in 59 (19.6%) cases for reader 1 and in 77 (25.5%) cases for reader 2. The interobserver agreement for the AEC dehiscence in these cases with opacification of ethmoidal air cells was substantial (κ = 0.754).

The best plane to assess the AEC for reader 1 was the coronal plane (53.8%; n = 162), while for reader 2, the best plane was the sagittal plane (77.7%; n = 234). The interobserver agreement for the best plane for AEC assessment was poor (κ = 0.092).

Discussion

Our results showed that there was poor interobserver agreement in the assessment of the dehiscence of the AEC on CT. To the best of our knowledge, the interobserver agreement on assessment of AEC dehiscence on CT has not been previously published. Evaluation of the interobserver agreement has been previously performed in the identification of the AEC on coronal planes¹⁷ and in the assessment of the anatomic landmarks of the AEA on coronal CT.¹⁸

The accuracy of CT in identifying AEC dehiscence has been previously estimated by Ferrari et al.,⁴ who described poor accuracy of cone-beam CT (slice thickness 2.0–2.5 µm) due to inadequate spatial resolution, making anatomical dissection the best modality to assess this feature.

The reported incidence of AEC dehiscence is variable in the literature: 11% by Moon et al.¹² and 16% by Floreani et al.,¹⁹ both of which are comparable to the incidence in our study (7.3–13.2%). However, a higher incidence of AEC dehiscence has been reported in other studies, ranging between 40% and 67%.^4,17,20,21 The discrepancies between the prior studies and our results can be explained by racial differences or due to the points where the nerves pass through the canal and cause focal bony dehiscence.^9,11 In addition, some of the studies where the incidence was higher were anatomical dissection studies that had the advantage of magnification provided by microscopes or endoscopes and subsequently higher sensitivity.⁴ On the other hand, in the study where the dehiscence was assessed on CT, the slice thickness used was 1 mm, and the assessment was performed on the sagittal plane. Thus, differences in study methodologies may also contribute to these discrepancies.

The effect of the PNS opacification in the detection of the AEC dehiscence has not been described in the literature. Only Lannoy-Penisson et al.¹⁴ mentioned that the presence of sinus pathology does not affect the identification and location of the AEC. However, its relationship with the assessment of dehiscence of the AEC has yet to be fully understood. The substantial agreement seen in our study on assessment of AEC dehiscence in cases with ethmoid opacification suggests that PNS opacification improves the delineation of the AEC bony contours and facilitates assessment for its dehiscence compared to AEC in well-aerated ethmoid air cells (Figure 6).

Figure 6.

Sagittal CT image showing AEC dehiscence in a case with complete opacification of the ethmoid and frontal sinus. Note the better delineation of the AEC bony contours (arrow).

There was poor interobserver agreement on the best plane for AEC assessment in our study. One of the observers found the coronal plane to be the best plane for AEC assessment in 53.8% of cases, in agreement with Cankal et al.,¹⁵ who described that the AEC can be identified on 3 mm coronal CT images in 68%, and Souza et al.,⁶ who found that the notch in the medial wall of the orbit is a reliable parameter for locating the AEA in 100% of the cases on coronal plane CT. Both these studies used slices 3 mm thick for their assessment. These data have also been corroborated by other studies.^16,18 On the other hand, the other reader found the sagittal plane to be the best plane to localize the AEC in 77.7% of cases, which is similar to the results described by Lannoy-Penisson,¹⁴ who found that the 2 mm sagittal plane is the best plane for the analysis of the three courses of the AEA in 72.7% of cases. The variability of these results may be explained by differences in slice thickness. In addition, if the artery is embedded in thick cortex or if the artery is very thin, it may also affect the visualization, even in images 1 mm thick.¹⁵

The course of the AEA and the relationship of the AEC to the skull base are also important parameters to assess on preoperative CT scans. In our study, the AEC was at the skull base (grades I and II) in 76.8% of cases, with the AEC within the skull base (grade II) in 27% of cases. The AEC was located below the skull base (grade III) in 23.1% of cases. These findings are comparable to the reports of AEC at the skull base in 72% of cases by McDonald et al., in 85.7% of cases by Moon et al.,¹² and in 57% of cases by Erpek et al.²² However, other studies have reported a lower incidence of AEC at the skull base, for example 16% in the study by Cankal et al.¹⁵ and 47.2% in the study by Yenigun et al.²³ The incidence of grade II AEC was 27% in our study. Most prior studies only described the incidence of the AEC below or at the skull base, but the isolated incidence of the grade II AEC has not been clearly described, which is also an important parameter to consider due to the risk of accidentally injury during skull-base surgery. Other authors^15,17,24 have described a higher incidence of the AEC lying below the skull base (about 80–84%). The discrepancy in terms of frequencies may be related to racial differences, study technique, and analysis method. However, although some studies have shown that the AEC is more frequently at the skull base, it must be considered that there is a probability of almost 40% finding it below the skull base.

Moreover, our results show that the AEC can be located below the skull base unilaterally (slightly more on the left (5.9%) compared to the right (4.8%)), which was not clearly described before. This finding highlights the importance of scrutinizing each side separately when required to assess the location of the AEC and its dehiscence. Some studies have reported an incidence of absent AEA of around 4–16%.^15,20,25,26 We did not evaluate this parameter, which is also an important variant to consider in preoperative CT and surgery planning.

Regarding slice thickness, in our study, the slice thickness was 2 mm. Some studies have used slice thicknesses between 1 and 3 mm on coronal CT to assess the AEC.^6,15,18 Lannoy–Peninson et al.¹⁴ used a slice thickness of 2 mm on sagittal planes, and Cankal et al.¹⁵ used sections 1, 2, and 3 mm thick. The differences in slice thickness among the multiple studies demonstrates that the rate of identification of the AEC improves with increase in the thickness sections, as the artery may not be well visualized when 1 mm slice thickness is used, especially in cases when the artery is embedded in thick cortex or when the artery is very thin.¹⁵ On the other hand, the variations in slice thickness, the bony reconstruction algorithm, the mA and KV used, and the differences between acquired versus reconstructed coronal planes may be some of the reasons for the differences in frequencies described in the multiple studies.

Our study has some limitations. It is a retrospective study of patients from a single hospital. We did not correlate with intraoperative findings for patients who proceeded to surgery, and hence our results cannot be used to estimate the diagnostic accuracy of CT in identification of AEC dehiscence. We had to exclude some of the patients whose AEC was below the skull base due to the slice thickness used for scanning. However, these cases were relatively few and unlikely to affect our results statistically.

Conclusion

Awareness of the anatomical variants and the radiological landmarks of the AEA are important parameters in the assessment of the preoperative CT. The suboptimal interobserver agreement demonstrated in this study could potentially limit the usefulness of CT scans for routine assessment of AEC dehiscence. However, given the substantial agreement seen in patients with PNS opacification, CT could still add valuable information regarding the AEC dehiscence in the preoperative setting in patients with opacification of ethmoid air cells. Future studies are required to compare the diagnostic accuracy of CT assessment versus direct intraoperative visualization to identify AEC dehiscence.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Angela Guarnizo https://orcid.org/0000-0003-1343-541X