Date Published: November 15, 2016
Publisher: Springer Vienna
Author(s): Yasushi Motoyama, Yasuo Hironaka, Fumihiko Nishimura, Pritam Gurung, Ryota Sasaki, Yasuhiro Takeshima, Ryosuke Matsuda, Kentaro Tamura, Ichiro Nakagawa, Young-Su Park, Hiroyuki Nakase.
The aim of this study was to identify the correlation between the location of the internal carotid artery (ICA) and the need for an orbitozygomatic approach (OZA) when approaching a basilar apex (BX) aneurysm.
By imaging the virtual trajectory to access the basilar artery (BA) through the ICA, the correlations among the height of the BX, the height and lateral breadth of the bifurcation of the ICA, and the need for removal of the orbital rim or zygomatic arch were investigated using three-dimensional computed tomography angiography (3DCTA) data of approximately 80 random samples not limited to BX aneurysms. Furthermore, the utility of 3D simulation to determine the need for the OZA was verified using data from five patients with BX aneurysms.
The height of the bifurcation of the ICA was inversely correlated and the height of the BX was positively correlated with the need for the OZA (both p < 0.017). Among patients undergoing surgery, clipping was successfully performed without the OZA in two patients in whom the distance from the simulated skull point on the extended line from the BX through the bifurcation of the ICA was more than 4 cm from the zygoma and orbital rim. It is necessary to determine the spatial relationship between the basilar artery and the ICA to decide whether the OZA is needed for surgery. Correlations of the height of the ICA and BX with the need for the OZA were not very strong individually, though they were significant. Therefore, simulation using 3DCTA appears to be important for planning the surgical approach for the treatment of BX aneurysms.
Basilar apex (BX) aneurysms, including basilar tip and basilar artery-superior cerebellar artery (BA-SCA) aneurysms, are difficult to treat because of their deep location. Although these aneurysms have been treated with endovascular coil embolization in many recent cases, some patients still require direct surgical intervention, including those with small aneurysms, who have a poor interventional access route, and those who have contraindications to receiving iodinated agents. Access routes to the interpeduncular or prepontine cistern for observation of the aneurysms include the pterional, subtemporal, and transpetrosal approaches; the specific approach is selected on the basis of aneurysm size, the projection of the dome, and the location of the neck of the aneurysm. The orbitozygomatic approach (OZA) has been useful in accessing BX aneurysms, especially in cases where it is in a high position, because this approach can facilitate upward and oblique viewing from below through the wide operative space [3, 8, 9, 16]. However, the OZA needs additional removal of the orbital rim and zygomatic arch, in addition to standard pterional craniotomy, which increases invasiveness, the risk of facial nerve palsy, temporal muscle atrophy, and deformity after surgery, and results in an extended operative time. Appropriate selection of the OZA requires indications that have yet to be established. The trajectory to BX aneurysms in the interpeduncular or prepontine cisterns has been suggested to be related to not only the height of the apex of the basilar artery (BA), but also the height and lateral breadth of the bifurcation of the internal carotid artery (ICA) . To access BX aneurysms across the ICA, it is better to simulate how the ICA blocks the operative field to appropriately observe the target. The use of the three-dimensional (3D) relationship between the ICA and the BA to help select the approach for BX aneurysms has not been well studied. Therefore, we used data from three-dimensional computed tomography angiography (3DCTA) to construct a virtual image of the trajectory needed to obtain an appropriate operative field in which the BX aneurysm is well visualized through the ICA. According to this image, we investigated how the BA and ICA are related to the need for the OZA, which is divided into two components, the orbital rim and the zygomatic arch, when approaching BX aneurysms.
Post-processing was performed on an Aquarius NET TM Workstation (TeraRecon, San Mateo, CA, USA). This software automatically provides 3D vessel images, as well as overlook, front, and rear views, together with a view of the skull base bony structure. In addition, three multiplanar reformation planes (sagittal, coronal, and axial) are shown for the location of the camera. Data obtained by scanning 3DCTA were immediately transferred to a computer server in our institution, and, within 5 min, the 3D images could be manipulated on any computer terminal, being always available for manipulation by surgeons using software for preoperative assessment.
We measured the following distances for evaluation of the need for the OZA for BX aneurysms through the ICA, which were calculated using measurement tools on the workstation (Figs. 1 and 2b).The height of the apex of the BA from the dorsum sellae: height of AThe height of the bifurcation of the ICA from the dorsum sellae: height of BThe lateral breadth of the bifurcation of the ICA from the midline: width of BThe distance from point C to the zygomatic arch: C-Z distance (inversely reflects the need for removal of the zygoma)The distance from point C to the lateral canthus: C-L distance (inversely reflects the need for removal of the orbital rim)
If point C is close to the zygomatic arch, zygotomy is necessary for the appropriate trajectory. If point C is near the lateral canthus, frontal orbitotomy is required. The distance between point C and the zygomatic arch (inversely related to the need for zygotomy) was measured on the workstation. The distance between point C and the lateral canthus (corresponds to the need for frontal orbitotomy) was calculated. We investigated the correlation between the C-Z and C-L distances with the height of A and the height and width of B. The patients were categorized according to age for sub-analyses. Measurements are expressed as means ± standard deviation (SD) or medians. The relationships between the distances from point C and measurements of points A and B obtained from the workstation were investigated using Pearson’s correlation. A P value of 0.05 or lower was considered to indicate significance. The Bonferroni method was used to correct for multiple comparisons as appropriate.
The mean vertical height and occipitofrontal distance from point C to the external auditory canal were 62.6 ± 10.1 mm (range, 4.2–76.7 mm) and 31.6 ± 16.3 mm (range, 45–95.7 mm), respectively. The scatter diagram shows a wide distribution of point C, which is located at least 4 cm anterior and just superior to the external auditory canal (Fig. 2a).
A 36-year-old female with a history of subarachnoid hemorrhage (SAH) because of a ruptured MCA aneurysm on the right side had been treated with direct clipping 7 years earlier. Follow-up magnetic resonance angiography (MRA) demonstrated de novo formation of an aneurysm in the BA-SCA with rightward projection. The aneurysm was 7 mm in diameter, and treatment was indicated to reduce the risk of SAH recurrence. The tortuosity of her vertebral artery was remarkable, and the aneurysm projected laterally and had a relatively wide neck. Direct clipping was chosen instead of coil embolization based on these anatomical features, as well as her young age. The apex of the BA was located just above the level of the dorsum sellae, but the distal portion of the ICA on the right side was short, and the level of the bifurcation of the ICA was low-set from the dorsum sellae. The images of the simulation by 3DCTA showed that the trajectory from the bifurcation of the ICA to the aneurysm was at a lower angle than that estimated according to the height of the BX. Point C was measured 2.3 cm above the zygomatic arch and 2.9 cm from the lateral canthus (Fig. 5a).Fig. 5A 36-year-old female with a history of subarachnoid hemorrhage due to a ruptured middle cerebral artery aneurysm on the right side presented with de novo formation of an aneurysm in the BA-SCA with rightward projection. The images of the simulation by 3DCTA show the trajectory through the bifurcation of the ICA, with point C measured at 2.3 cm above the zygomatic arch and 2.9 cm from the lateral canthus. The 3D images along the trajectory of the ABC line demonstrate the spatial relationship between the aneurysm and the distal part of the ICA indicating the necessity of the OZA (a). Upper figures show 3D images along the simulated trajectory on each point from A through C. Lower figures show the intraoperative view in real surgery relevant to the points from A through C. The 3DCTA of patients after surgery shows obliteration of the aneurysms with multiple clips via the OZA (b)
A wide variety of approaches for BX aneurysms has been reported previously, including the pterional approach, the subtemporal approach, the translaminar terminalis approach, and the OZA [5, 7, 15, 16, 19]. Selection of the OZA for BX aneurysms is often dependent on the height of the BX from the dorsum sellae. However, the approach to BX aneurysms is sometimes dependent on the position of the intracranial portion of the ICA, because the ICA can limit the trajectory to a BX aneurysm . Many studies have described how to mobilize the ICA to establish a wider operative field [8, 11, 18]. Even though mobilization of the ICA can be done by a variety of methods, such as anterior clinoidectomy, dissection of the distal dural ring, and division of the posterior communicating artery (PcoA), it can be obtained only in the proximal part of the intracranial ICA [3, 10, 18]. When a short ICA blocks the access route to a BX aneurysm, the OZA has been reported to be helpful . In fact, the spatial relationship between the ICA and the BA is thought to be a very important point to consider with regard to the surgical approach. However, definitive standards have not yet been established.
When using 3DCTA simulation, we should consider the invisible structures on the images made from CT scans, such as the oculomotor nerve, tentorium, and brain parenchyma, which are all important components that can limit the operative field. In this study, we randomly sampled 3DCTA data from 80 patients with various lesions, including cerebrovascular disease, tumors, and traumatic conditions. Among these enrolled patients, there were no patients with BX aneurysms. The number of patients with BX aneurysms undergoing real surgery is very small. Therefore, the tendencies and findings that were seen in this 3D simulation study cannot be generalized to patients with BX aneurysms. A variety of factors influence the creation of the operative field, such as swelling of the brain parenchyma or the thickness of the temporal muscle . Therefore, an examination is necessary in each individual case. In real aneurysmal surgery, especially for BX aneurysms, not only the approach, but also the characteristics of the aneurysm are extremely important for clipping. They include the size of the aneurysm and its neck, the shape and direction of the dome, atherosclerotic change of the wall of the aneurysm, and surrounding perforators, which strongly affect the difficulty of clipping. In this study, it was not possible to evaluate these factors in BX aneurysmal surgery. We should always consider all factors, including the trajectory and the characteristics of the aneurysms, in each individual case.
While imaging the virtual trajectory of the BX through the ICA, the height of the BX and the height of the bifurcation of the ICA were significantly correlated with the need for an OZA. However, the correlations of the heights of the ICA and BX with the need for OZA were not very strong individually. It was necessary to determine the spatial relationship between the BA and the ICA in order to decide whether an OZA is needed in real surgery. We proposed the point C, projected on the cranium by the extended line connecting BX (point A) with the bifurcation of ICA (point B) made by data from 3DCTA, as a potentially useful reference to determine the need for an OZA when accessing BX aneurysms.