Lateral optico-carotid recess as an anatomical landmark for optimal endoscopic optic nerve decompression

Introduction

Microsurgical transnasal optic nerve decompression, introduced in the early 1980s (1), has gained popularity since the late 1990s due to advancements in tools and fully endoscopic techniques (2,3). Initially used for Graves’ neuropathy and traumatic optic neuropathy (TON), its indications now include idiopathic intracranial hypertension, nerve inflammation, tumors in the optic nerve canal, and pre-radiotherapy preparation (4-10).

Despite its growing popularity, the literature shows significant variation in both the clinical utility and technical aspects of optic nerve decompression, particularly regarding the extent of bone removal. Some recommend removing the entire lateral bony wall of the sphenoid sinus along the optic nerve from the orbital apex to the optic chiasm (11-13). Given that anatomical studies (14,15) estimate the length of the medial wall of the optic canal to be on average 1 cm, this extent of decompression appears overly extensive. For this reason, others have suggested a more limited approach, removing only 1 cm of bone posterior to the anterior wall of the sphenoid sinus (2). In fact, the actual extent of optic canal wall resection has not been specified in many papers (16,17), while according to Hart et al., the discrepancy in reported results of optic nerve decompression may, in part, result from different exposures of the nerve during the procedure (18).

In consequence, it seems advisable to develop a standardized operational procedure model that not only allows for decompression with an optimal range but also facilitates future comparisons of the effectiveness of such procedures across medical centers.

Traditionally, the orbital apex, when viewed intranasally, is believed to align with the anterior wall of the sphenoid sinus, making it a useful starting point for measuring the length of decompression. However, radiological studies by Aujla et al. (19) have shown that the orbital apex can be located up to 3.5 mm further anterior to this point. Moreover, it is not an anatomical structure that can be easily and precisely located during endoscopic surgery because it is obscured by the flat bony wall of the sphenoid sinus or the posterior ethmoidal cell. Therefore, the lateral optico-carotid recess (LOCR), which can be identified both endoscopically and radiologically, seems to make a better reference point for determining the optimal extent of decompression.

Planning optimal extent of decompression (neither too long nor too short) based on standard computed tomography (CT) reconstructions—such as frontal, axial, and sagittal views—can be prone to error for at least two reasons. First, the walls of the optic nerve canal are not of equal length: the upper wall is longer than the lower, and the medial wall is longer than the lateral (18). Additionally, the canals slope slightly downward from the cranial cavity and deviate outward from the sagittal plane by approximately 30 degrees (18,20). Thus, it seems more logical and precise to determine the optimal position of the theoretical posterior decompression end (PDE) based on oblique cross-sectional CT planes—two parallel and one perpendicular to the course of the optic nerve within its bony canal.

In this study, we attempted to determine the optimal position of the PDE using oblique cross-sectional CT planes in relation to the LOCR, an anatomical landmark that is both endoscopically visible and radiologically detectable.

Methods

High-resolution CT (HRCT) scans of 30 patients’ heads (60 orbits) were used for this study: 13 women and 17 men, aged from 29 to 83 years, mean 57±13.2 years. All the patients were candidates for cochlear implantation; subjects with signs of chronic sinusitis, osteoneogenesis, posttraumatic and congenital deformations were excluded. The original CT examinations were taken from the University Clinical Hospital in Bialystok medical record database and anonymized before any further processing for the research purposes. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Bioethics Committee at the Medical University of Bialystok (document No. APK002/373/2024). Individual consent for this retrospective analysis was waived.

HRCT source imaging was obtained using multidetector-row spiral CT with a scan thickness of 0.5 mm. Given the oblique course of the optic canal relative to standard CT planes, a multiplanar reformation (MPR) was conducted to acquire scans perpendicular to the canal and in two additional planes along its axis: one horizontal and another vertical (Figure 1). This approach allowed for a comprehensive assessment of the entire circumference (360°) of the bony canal along its complete length. The RadiAnt DICOM Viewer software, Version 2022.1.1, facilitated the management of HRCT data.

Figure 1 Optic canal visualization in three reformatted HRCT planes. (A) Plane perpendicular to the optic canal. (B) Horizontal plane running along the canal axis. (C) Vertical plane running along the canal axis. RA, right anterior; LP, left posterior; I, inferior; WL, window level; WW, window width; MPR, multiplanar reformation; A, anterior; R, right; L, left; PR, posterior right; AL, anterior left; HRCT, high-resolution computed tomography.

The extent of necessary decompression was defined as the distance from the orbital apex [anterior decompression end (ADE)] to the point where intracranially the optic nerve was surrounded by bone in only half of its circumference (180° of the bony canal circumference)—the PDE. The specific locations of ADE and PDE of this “theoretical optimal” decompression are depicted in Figure 2.

Figure 2 Reformatted HRCT scans showing the extent of optic canal decompression. Lines designate the ADE and the PDE. (A) Line designating the ADE visible in the plane reconstructed vertically, along the axis of the optic canal. (A1) The ADE as seen on a plane transversing the canal along the line on (A)—the most anterior scan where the canal is visible as a fully closed ring (arrow). (B) Line designating the PDE, seen on the plane reconstructed vertically along the axis of the optic canal. (B1) The PDE as seen on a plane transversing the canal along the line on (B)—the most anterior scan where only about half of the circumference of the bony canal is present (arrow). HRCT, high-resolution computed tomography; ADE, anterior decompression end; PDE, posterior decompression end.

The extent of the necessary decompression was also assessed relative to the medial edge of the LOCR. The entry into the LOCR on the lateral wall of the sphenoid sinus manifests as a more or less triangular-shaped plane between the deepest protrusion of the internal carotid artery and the optic nerve. The posterior apex of this triangle—medial edge of LOCR is formed by the junction of the optic nerve and the anterior bend of the internal carotid artery, while the base of the triangle faces the anterior sphenoid wall (21). The distance between the medial edge of the LOCR and the theoretical PDE was measured.

In some cases, when the optic strut and anterior clinoid process have minimal pneumatization, the recess is not formed. In these instances, measurements were taken in relation to the dimple point at the junction of the optic nerve and the internal carotid artery, if visible.

Figure 3 shows the anatomical relations between the optico-carotid recess, the optic canal, carotid artery, ADE and PDE as seen on a three-dimensional (3D) display of HRCT.

Figure 3 Anatomical relationships of the optico-carotid recess entrance (marked in blue) and key anatomical structures as seen on a 3D display of HRCT, in a lateral view of the sphenoid sinus wall. Vertical lines represent the theoretical anterior (solid line—ADE) and posterior (dashed line—PDE) decompression ends. Note that the dashed line runs near the junction of the carotid artery and optic nerve (medial edge of the lateral OCR). The solid line marks the region of the orbital apex. ON, optic nerve (marked in white); CA, carotid artery (marked in red); OCR, optico-carotid recess; ST, sella turcica; OR, orbit; ASW, anterior wall of the sphenoid sinus; SS, sphenoid sinus septum; 3D, three-dimensional; HRCT, high-resolution computed tomography; ADE, anterior decompression end; PDE, posterior decompression end.

Results

According to the conventional Hammer and Radberg classification for sphenoid sinus pneumatization (22), the sellar type was observed in the majority of our cases—56 (93%), while the presellar type was found in 4 (7%) cases, and the conchal type was not observed.

Poor pneumatization of the sphenoid sinus (presellar type), combined with the presence of asymmetrical bony septations within the sphenoid sinus that prevented the formation of the lateral opticocarotid recess, and a lack of pneumatization adjacent to the optic nerve, made it impossible to localize the recess in 9 (15%) cases.

In 51 orbits (85%), the identification of the medial edge of the LOCR (as a point corresponding to the posterior apex of the triangle formed by the junction of the optic nerve and the anterior bend of the internal carotid artery) was achievable on CT imaging. In these cases, depending on the pneumatization of the optic strut and anterior clinoid process, a clearly defined recess or an indentation on the lateral sphenoid sinus wall, or at least a dimple point just in front of the junction of the carotid artery and optic nerve was observed.

The measurement of the distance from the orbital apex (ADE) to the point where the optic nerve intracranially is surrounded by bone in only half of its circumference (PDE) varied from 4.8 to 14.4 mm, with a mean of 8.7±2.5 mm (left 8.7±2.7 mm, right 8.7±2.5 mm).

According to our measurements PDE was identified either posteriorly or anteriorly to the medial edge of the LOCR. Consequently, a positive value was assigned to the distance measurement (in mm) when PDE was situated posterior to the medial margin of the LOCR, and a negative value was assigned when the PDE was located anterior to the medial edge of the LOCR (Figure 4).

Figure 4 Anatomical relations of the LOCR and the optic canal. A white line crosses the medial edge of LOCR, establishing the zero point for measuring the distance to the posterior end of the decompression. The white arrow represents positive values of the distance, and the black arrow indicates negative values of the distance. OR, orbit; OCR, optico-carotid recess; ICA, internal carotid artery; LOCR, lateral optico-carotid recess.

The measured distance between the medial margin of LOCR and the PDE ranged from −2.5 to 1.6 mm, with a mean of −0.4±1.3 mm (left −0.4±1.3 mm, right −0.4±1.3 mm).

Discussion

The lack of clear guidelines on the optimal extent of optic nerve decompression within its bony canal led us to develop a model for preoperative planning based on the canal’s position relative to anatomical landmarks visible during endoscopic sphenoid sinus examination.

We selected the LOCR as the reference point for our measurements because it is a relatively stable and identifiable structure located in direct contact with the optic nerve canal. Radiological studies do report that the presence of pneumatization of the optic strut and anterior clinoid process, forming a sufficiently deep depression to be defined as a recess, is observed in only about 40% of cases (21,23). Nonetheless, any indentation or slight depression at the junction of the internal carotid artery and optic nerve, which does not necessarily have to be a true recess, can be identified in over 80% of cases (24). Other anatomical studies have reported identifying the recess in over 90% (25) or even in 100% of cases (26,27). This discrepancy is likely due to varying criteria for the depth of the depression between optic nerve and carotid artery considered to be a recess.

The junction between the protrusions caused by the optic nerve and the carotid artery forms the posterior apex of the triangle-shaped entrance to the variably pneumatized LOCR. This exact site was used as the reference point for our measurements and, in our study, we were able to identify its location in 85% of cases.

Several obstacles to radiological and endoscopic identification of this point are reported in the literature. Beyond obvious issues such as poor sphenoid sinus pneumatization (conchal type of sphenoid sinus) or asymmetrical bony septation attaching to the lateral sinus wall, challenges include the lack of carotid artery exposure in the sphenoid sinus (28) and the absence of sphenoid pneumatization adjacent to the optic nerve (29).

Our study has shown that the location of the PDE can in most cases be precisely determined preoperatively, helping to avoid performing the decompression either too short or excessively long—extending to the optic chiasm. During the operation, decompression can be stopped at this preplanned point, which is situated very close to the easily identifiable medial edge of the LOCR. The proximity of the PDE to the LOCR reduces the likelihood of errors in distance assessment compared to using the orbital apex as a reference. This is because the LOCR is much closer and easier to locate during endoscopy than the more distant orbital apex.

In optic canal decompression surgery, even a few millimeters matter due to the small size of the optic canal, where nerve compression typically occurs in the narrow anterior segment, which then widens posteriorly (20,30). The canal’s bony walls are also thinner at the front and thicken towards the back, making drilling behind the posterior end of the canal both risky and unnecessary. Excessive bone drilling in confined space of the sphenoid sinus can cause dangerous temperature increase, risking thermal injury to the dura, pia and optic nerve, especially in patients with a pre-fixed (anteriorly positioned) optic chiasm, a variant present in about 7% of patients (31-33).

Our measurements made in a plane perpendicular to the canal demonstrated that, in all cases, its lateral and upper walls terminated no further than 2.5 mm anterior or 1,6 mm posterior to the medial edge of LOCR, with a bilateral standard deviation of only 1.3 mm. In contrast, when measured from the orbital apex, the sought distance varied much more: from 4.8 to 14.4 mm. Nevertheless the advantage of the measurement based on the LOCR lies not only in the incomparably smaller spread of values. While the orbital apex can be easily localized on CT imaging, its position cannot be as precisely determined by the endoscopic surgeon and can be discovered only after removal of the adjacent bone and exposing the ring of Zinn.

Another advantage of our methodology is reconstruction of CT images in atypical oblique planes to allow analysis of the cross-sections of the channel in a plane perpendicular to its course. This approach enabled a more accurate assessment of the exact position where the bone covers only 180° of the nerve circumference (PDE) in relation to the endoscopically visible anatomical landmark—LOCR—on the lateral wall of the sphenoid sinus. Analysis based on standard reconstruction planes—axial, frontal, and sagittal—can be affected by individual variations in the canal’s course relative to these planes.

One might argue that neuronavigation is a clear solution, especially when LOCR is absent, and generally in skull base decompressive surgery. While neuronavigation is essential for endoscopic procedures to aid in overall orientation, its use should be carefully evaluated when targeting small anatomical structures of millimetric size. The accuracy of frameless neuronavigation systems has been measured at 2.9±3.3 to 4.4±1.8 mm (34). In ear, nose, and throat (ENT) surgery, where magnetic systems are often used, the error may be greater. Additionally, a “projecting error” should be considered since the target is deep within the complex skull base anatomy, located behind the facial structures used for system registration.

In a scenario where anatomical reference points are lacking, a “traditional” and more extensive decompression may still be a valid solution. This involves commencing the procedure from the lamina papyracea at the anterior aspect and progressing posteriorly towards the orbital apex. The identification of the orbital apex can be achieved by observing the appearance of the transverse fibers of the annulus of Zinn. The decompression is then extended up to 15 millimeters posteriorly to encompass the maximum extent of the optic canal, or even up to the midline.

A limitation of this study is the relatively small sample size. The study group did not encompass all possible anatomical variations such as those pertaining to the sphenoid bone pneumatization of the positioning of the optic nerve and internal carotid artery. Additionally, the study used CT scans from patients without pathologies in the sphenoid sinus area, such as meningiomas, that could alter normal anatomical conditions. Therefore, our model is more applicable to cases of nerve edema, such as Graves’ neuropathy, TON, idiopathic intracranial hypertension, or post-radiotherapy nerve inflammation. This model is also not suitable for tumors that compress the optic nerve from the outside, as the decompression range proposed in our study may be too limited, potentially leading to a fulcrum effect and persistent nerve compression. Therefore, our approach, which is based on measuring the optic canal along its actual axes and determining the posterior decompression endpoint from the LOCR rather than from the orbital apex, though theoretically sound, must be validated in a real clinical scenario.

Conclusions

The junction point of optic nerve and internal carotid artery (medial edge of LOCR), being directly discernible by the endoscopic surgeon was identifiable on reconstructed high-resolution computed tomography (HRCT) scans in 85% of subjects. This structure can provide a reliable landmark for determining the necessary extent of decompression of the optic canal. In contrast, measurements related to the orbital apex were found to be less credible.

By adhering to the precise extent of bony removal, one can mitigate the inherent risks associated with optic nerve decompression and prevent unnecessary bone drilling in the vulnerable proximal segment of the optic nerve canal.

Acknowledgments

Funding: None.

Footnote

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-1125/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. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Bioethics Committee at the Medical University of Bialystok (document No. APK002/373/2024). Individual consent for 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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