The relationship of the basilar artery bifurcation and dorsum sel lae

Overview

Aims:
The aim of the study was to as­sess the position of the terminal basilar artery bifurcation (BAB) in relation to the dorsum sel­lae (DS) based on CT scans of the head. The BAB is, in most cases, formed by two P1 segments of the posterior cerebral arteries at the level of the interpeduncular fos­sa, in close proximity to the DS. This type of division is cal­led bifurcation. Numerous pathologies may develop in this region, includ­­ing BAB aneurysms, which account for 5– 8% of all intracranial aneurysms. The literature states that the position of an aneurysm in relation to the DS plays a significant role in plan­n­­ing surgical strategies for BAB aneurysms.

Patients and methods:
The study involved 100 CTA scans of 54 women and 46 men in a range of 18– 88 years of age (mean age 52.49 years). Multiplanar reconstructions were used. The position of the BAB in relation to the sagittal midline plane (MP) and the lowest DS point (LDSP) in the transverse plane (TP) was analysed in the coronal section. The distance between the BAB and the DS was measured in the sagittal section.

Results:
In the studied patients (n = 100), the BAB was located on the right of the MP in 57% of the cases, on the left of the MP in 41% of the cases and in the midline or median position in 2% of the cases. The BAB was located above the LDSP (TP) in 47% of the cases, and below it in 53% of the cases. There were no cases in which the BAB was located in the TP. The average distance between the BAB and the MP was 0.35 m­m; SD 1.91 m­m, and the average distance between the BAB and the TP was 1.01 m­m; SD 4.47 m­m. Moreover, the average distance between the BAB and the DS was 9.34 m­m; SD 2.61 m­m.

Conclusions:
The study revealed no statistical­ly significant dif­ferences in the BAB position depend­­ing on gender. However, a highly significant dif­ference and higher location of the BAB in relation to the TP (in relation to the DS) was observed in persons above 45 years of age.

Key words:
basilar artery bifurcation – basilar artery tip – basilar artery aneurysm – dorsum sellae – computed tomography

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Chinese summary - 摘要

基底动脉分叉与背鞍的关系

目的：

该研究的目的是根据头部的CT扫描评估末端基底动脉分叉（BAB）相对于鞍背（DS）的位置。 在大多数情况下，BAB由位于椎间窝水平的两个大脑后动脉的P1段形成，紧邻DS。 这种划分称为分叉。 该区域可能出现许多病变，包括BAB动脉瘤，占所有颅内动脉瘤的5-8％。 文献指出动脉瘤相对于DS的位置在规划BAB动脉瘤的手术策略中起重要作用。

患者和方法：

该研究涉及对54名女性和46名男性进行100次CTA扫描，范围为18-88岁（平均年龄52.49岁）。 使用多平面重建。 在冠状切面中分析BAB相对于矢状中线平面（MP）的位置和横向平面（TP）中的最低DS点（LDSP）。 在矢状切面测量BAB和DS之间的距离。

结果：

在研究的患者（n = 100）中，57％的病例BAB位于MP的右侧，41％的病例位于MP的左侧，2％的病例位于中线或中位。BAB在47％的病例中位于LDSP（TP）之上，在53％的病例中位于其之下。 没有BAB位于TP的情况。 BAB和MP之间的平均距离为0.35 mm; 标准尺1.91毫米; BAB与TP之间的平均距离为1.01 mm; 和SD 4.47毫米。 此外，BAB和DS之间的平均距离为9.34毫米; SD 2.61毫米。

结论：

该研究显示，根据性别，BAB位置无统计学差异。 然而，在45岁以上的人群中观察到BAB相对于TP（相对于DS）的高度显著差异和更高的位置。

关键词：

基底动脉分叉 - 基底动脉尖 - 基底动脉瘤 - 背鞍 - 计算机断层扫描

Introduction

Basilar artery bifurcation (BAB) is in most cases formed by two P1 segments of the posterior cerebral arteries at the level of the interpeduncular fos­sa, in close proximity to the dorsum sel­lae (DS). This type of division is cal­led bifurcation.

Below the bifurcation, the paired superior cerebel­lar arteries branch off of the basilar artery.

Together with two posterior cerebral arteries, they may co-form the BAB quad­rifurcation, which is observed in 38% of the cases [1,2].

Numerous pathologies may develop in this region, includ­­ing BAB aneurysms, which account for over 50% of vertebrobasilar aneurysms and 5– 8% of all intracranial aneurysms [3].

Due to the location of the BAB in proximity to vital anatomical structures, such as the brain stem, the internal carotid artery, the pituitary stalk and the oculomotor nerve, microsurgical treatment of BAB aneurysms still car­ries the risk of death at the level of almost 9– 10% [4– 8]; in the case of endovascular treatment, the risk is approximately 1.3– 2.7% [9,10]. Surgical conditions are additional­ly complicated by the fact that the arteries form­­ing the BAB give off numerous and fragile perforators responsible for blood supply to the diencephalon and mesencephalon. Intraoperative damage to these may lead to death or severe defects [11].

In 1932, Herbert Axel Olivecrona was the first to operate on a patient with an aneurysm of the posterior circle of Wil­lis. However, the pioneer of surgery on basilar artery aneurysms was Charles George Drake who, in the late 1950s, presented infratemporal access to the BAB by temporal lobe retraction [12]. This provides a lateral view at the BAB and the perforators. However, the risk of damag­­ing the temporal lobe or insuf­ficient inspection of the contralateral posterior cerebral artery, which are as­sociated with this approach, prompted researchers to search for other routes of acces­s [5]. In 1976, Gazi Yasargil proposed the anterolateral (pterional) approach through the Sylvian fis­sure, which makes the BAB acces­sible through a nar­row window formed between the internal carotid artery and the oculomotor nerve. This does not neces­sitate temporal lobe retraction but still al­lows as­ses­sment of the initial segments of the posterior cerebral arteries. However, this approach is as­sociated with a deep and nar­row surgical field with a limited view into the posterior surface of the BAB [2,3].

Despite the development of endovascular methods of aneurysm treatment, which are also employed in the treatment of BAB aneurysms, surgery is still neces­sary when these techniques are inef­fective or can­not be used [13,14].

Numerous authors believe that the position of an aneurysm in relation to the DS plays a decisive part in the surgery plan­n­­ing since, when located low, the DS may conceal the site of vascular clamping [15,16].

The aim of the study was to evaluate the position of the BAB in relation to the DS based on multiplanar reconstructions of CTA scans.

Patients and methods

The retrospective study involved 100 ano­nymised head scans from CTA of 54 women and 46 men in a range of 18– 88 years of age (mean age 52.49 years) dia­gnosed with headaches in 2012– 2016.

The scans were free of motion artefacts or foreign bodies, e. g. vascular clamps. The study excluded patients:

1. after craniocerebral trauma;
2. after a neurosurgical procedure (includ­­ing endovascular) within the head;
3. with a dia­gnosed intracranial tumour;
4. with pathology within the ves­sels of the CNS.

The distance between the BAB and the sagittal midline plane (MP) as well as the transverse plane (TP) selected at the lowest DS point was determined us­­ing the bone window (W = 1500, L = 300) for CTA with multiplanar reconstruction. The distance between the BAB and the DS was measured in the sagittal section. The statistical analysis was conducted for the BAB position depend­­ing on gender and age (the analysed age groups were: group 1 –  up to 45 years of age; group 2 –  over 45 years of age).

Determination of the BAB

The BAB was determined in the coronal section of CTA at the site where both P1 segments of the posterior cerebral arteries originate. This point was marked on the screen us­­ing a marker (cros­s) (Fig. 1).

Position of the BAB in relation to the DS

The coronal CTA scans were used to visualise the DS and to determine the MP and the lowest DS point (LDSP) within the TP. Subsequently, the BAB position was saved on the workstation screen with a marker (cros­s) and, us­­ing multiplanar reconstructions, its position was mapped in relation to the analysed planes (in relation to the DS).

• A. In order to evaluate the position of the BAB in relation to the MP, the sagittal plane BABsag, paral­lel to the MP and encompas­s­­ing the BAB, was selected. In the case of the BAB position on the right of the MP, negative values were as­sumed, whilst positive values were used when it was located on the left of the plane. In the case of the midline BAB location in the MP, the 0 value was as­sumed (Fig. 2).
• B. In order to evaluate the position of the BAB in relation to the LDSP, the BABtp plane, paral­lel to the TP includ­­ing the LDSP and encompas­s­­ing the BAB, was selected. In the case of the BAB position above the LDSP (TP), positive values were as­sumed, whilst negative values were used when it was located below the plane. In the case of BAB location on the level of the LDSP (TP), the 0 value was as­sumed (Fig. 3).
• C. In order to evaluate the distance between the BAB and the DS in the sagittal section, the fol­low­­ing two planes were selected: the oblique coronal plane DSp in the Wackenheim clivus baseline and the oblique coronal plane DSbab paral­lel to the DS plane and encompas­s­­ing the BAB (Fig. 4).

Statistical analysis

Statistical calculations were performed in PQStat version 1.6.2.901 (PQStat Software, Poznań/ Plewiska, Poland).

The BAB sym­metry in relation to the MP and TP in men and women was analysed us­­ing the chi2 test and Fisher’s exact test.

The position of the BAB depend­­ing on gender was compared using the Student’s t-test.

The position of the BAB depend­­ing on age was analysed with the Student’s t-test for independent samples.

The tested probability was deemed significant at p < 0.05, and highly significant at p < 0.01.

Results

BAB sym­metry in relation to the MP and TP

In the studied patients (n = 100), the BAB was located on the right of the MP in 57% of the cases, on the left of the MP in 41% of the cases and medial­ly in 2% of the cases.

The BAB was located above the TP in 47% of the cases and below it in 53% of the cases. There were no cases with the BAB located in the TP (Tab. 1).

There were no significant dif­ferences (p > 0.05) between BAB sym­metry and the investigated planes (MP and TP) depend­­ing on gender.

Position of the BAB in relation to the DS

In the studied patients (n = 100), the mean distance between the BAB and MP was – 0.35 mm (the BAB was located on the right of the MP), and the mean distance between the BAB and TP was – 1.01 mm (the BAB was located below the TP). The mean distance between the BAB and DS was 9.34 mm (Tab. 2).

There were no significant dif­ferences (p >0.05) between BAB position depend­­ing on gender.

Position of the BAB depend­­ing on age

The position of the BAB in relation to the DS was also analysed in two groups of patients: group 1 –  above 45 years of age and group 2 –  up to 45 years of age. As for the BAB location in relation to the TP (in relation to the DS), the study revealed a statistical­ly significant dif­ference (p < 0.01) and higher BAB position in persons above 45 years of age (Tab. 3).

Position of the BAB in relation to the TP (height ranges)

The position of the BAB in relation to the TP was evaluated in seven 5-mil­limetre height ranges. In 41% of the cases, the BAB was found between 0 mm and 5 mm above the TP; this was the most com­mon range. The location of the BAB in extreme ranges (from – 20 mm to – 15 mm and from 10 mm to 15 m­m) was the rarest and noted in only 1% of the cases (Tab. 4).

The diagrams (Fig. 5 A, B) show the position of the terminal BAB (red spot) in women (n = 54) and men (n = 46) in a two-dimensional Cartesian coordinate system, where:

• point 0, cal­led the origin of the coordinate system, was set at a point where the perpendicular axes x and y meet;
• x, the abscis­sa, represents the TP;
• y, the ordinate, represents the MP.

The fol­low­­ing values were as­sumed to determine the BAB:

• negative values: position of the BAB on the right of the MP (y axis) and below the TP (x axis),
• positive values: position of the BAB on the left of the MP (y axis) and above the TP (x axis).

Discus­sion

Surgery for BAB aneurysms is es­sential when the method of choice, i.e. endovascular treatment, can­not be applied [14]. BAB aneurysm clipp­­ing is performed in a nar­row and deep surgical field. Surgical dif­ficulties result from the location of the BAB in the neighbourhood of vital anatomical structures, such as the brain stem, internal carotid artery, pituitary gland, oculomotor nerve or perforators that supply the diencephalon and mesencephalon [11].

Accord­­ing to Yamaura et al., the position of a BAB aneurysm in relation to the DS plays a significant role in surgical plan­ning [16]. Tjahjadi et al., however, disagree. They be­lieve that the height in relation to the posterior clinoid process should be the point of reference [3]. Moreover, Majchrzak et al. also indicate that the position of the BAB in relation to the line between the posterior clinoid proces­ses is significant in the selection of a surgical approach to aneurysms in this region [17]. Caruso et al., in turn, claim that a surgical approach should depend on the distance of the BAB from the DS [18]. 

Smoker et al. have analysed the BAB position in relation to the DS in CT scans of the head (5-mm cuts) in 126 patients. The BAB was found at the level of the suprasel­lar cistern (one cut above the DS) in 71 patients (56%), whereas 45 patients (36%) had the BAB located at the level of the DS or below it. The least numerous group were 10 patients (7.9%) with the BAB found at the level of the floor of the third ventricle (one cut above the suprasel­lar cistern) [19].

In the present study, the analysis of 100 CTA scans (54 women and 46 men) showed that the mean distance of the BAB in relation to the TP was 1.01 mm below the TP, with most cases (41%) found in the range from 0 to 5 mm above the TP.

In the study by Żurada et al., the mean distance between the BAB and DS, evaluated in CTA head scans of 98 patients, was 9.55 m­m [20]. In our study, the mean distance between the BAB and DS was 9.34 m­m. Smoker et al., on the other hand (n = 126), found the BAB in the midline in 69 patients (55%), in the paramedian position (medial to the lateral margins of the DS) in 22 patients (17.4%) and in the medial right position in 33 patients (26%) [19].

The right location of the BAB in relation to the MP was found in our study in 57% of the cases with the mean distance from this plane at the level of 0.35 m­m.

As did Żurada et al. [20], we did not find any significant dif­ferences in the BAB position in relation to the DS depend­­ing on gender. However, the statistical analysis of the results depend­­ing on age showed a highly significant dif­ference (p < 0.01) and a higher BAB position above the TP (mean 0.06 m­m) in patients above 45 years of age. This may reflect the tendency toward basilar artery elongation with age. Such a position limits the pos­sibilities of Drake’s and pterional approaches. A dif­ferent angle of view is of­fered by fronto-orbito-zygomatic approaches, where the trajectory runs upwards from below without significant retraction of the temporal lobe.

Gonzalez et al. have divided aneurysms found in this region depend­­ing on the position in relation to the DS. They dis­tinguished: high-ly­­ing lesions for aneurysms located 5 mm above the DS and low-ly­­ing lesions for aneurysms located 5 mm below the DS [21]. In our study, the position of the BAB in relation to the DS was evaluated in 5-mm ranges. In most cases (41%), the BAB was found between 0 mm and 5 mm above the DS.

Conclusions

This study proposed methodology to fa­­cilitate accurate and reproducible deter­mination of the BAB distance from the DS. The authors believe that it may help plan safe surgical approaches to basilar tip aneurysms, particularly in patients with the BAB located near the DS.

The authors declare they have no potential conflicts of interest concerning drugs, products, or services used in the study.

The Editorial Board declares that the manu­script met the ICMJE “uniform requirements” for biomedical papers.

Wojciech Ilków

Department of Neurosurgery

University Teaching Hospital in Opole

al. Witosa 26

45-401 Opole

Poland

e-mail: wojciechilkow@gmail.com

Accepted for review: 1. 2. 2018

Accepted for print: 18. 7. 2018