Anatomical features of the occipital artery on CTA and differences between patients with/without stenosis and occlusion of the internal carotid artery
- Authors:
- Published online on: December 29, 2021 https://doi.org/10.3892/mi.2021.28
- Article Number: 3
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Copyright: © Luan et al. This is an open access article distributed under the terms of Creative Commons Attribution License.
Abstract
Introduction
The occipital artery (OA) is a main artery that originates from the external carotid artery (ECA) and may be involved in a number of diseases, such as moyamoya disease, dural arteriovenous fistula (DAVF), aneurysm, etc. (1). Apart from its involvement in these diseases, the OA is a crucial artery due to its main anatomical variations and its usage in intra- and extracranial bypasses; therefore, the understanding of the anatomy of the OA is of utmost importance (2).
Previous studies on the OA have been mostly based on cadavers and catheter-based angiography procedures (3-5). Studies using computed tomography angiography (CTA) are rarely reported (6,7). Therefore, the present study was performed to examine the anatomy of the OA using CTA. Importantly, the data of the present study were derived from Han Chinese subjects, a population cohort that has rarely been reported in such cases.
In addition to providing OA parameters, most importantly, in the present study, the OA anatomy was compared between healthy subjects and patients with internal carotid artery (ICA) stenosis and occlusion to determine whether the OA parameters differed. This was performed as the OA can become dilated or thicker than normal in the presence of a DAVF (1). To date, no study has explored this issue, at least to the best of our knowledge.
Materials and methods
Study participants
An imaging study was performed on Han Chinese candidates, including healthy subjects and patients with ICA stenosis and occlusion, who underwent cervical CTA examinations between January, 2020 and September, 2021 at the First Hospital of Jilin University, Changchun, China. The healthy subjects underwent a cervical CTA examination during routine physical examinations.
Prior to the CTA, the following exclusion criteria were used: A history of severe or anaphylactic reaction to iodinated contrast, an inability to cooperate with scanning protocols, hemodynamic instability, diabetes, the administration of any anticoagulant medication, renal impairment.
The present study was approved by the Ethics Committee of the First Hospital of Jilin University (approval no. 2021-533). Written informed consent was obtained from all participants. The original CTA data were further processed on a GE Workstation (version 4.7) (GE Healthcare; Cytiva).
CTA imaging protocol
CTA examinations were performed on 256-multidetector CT scanners (GE Healthcare; Cytiva) at the First Hospital of Jilin University, utilizing THE test bolus injection technique to acquire the optimal trigger time (8). The scanned area ranged from the aortic arch to the frontal sinus. On the basis of the body weight of the patient, a dose of 2 ml/kg of nonionic iodinated contrast media (Omnipaque 350; GE Healthcare; Cytiva) and 20-30 ml saline chaser were injected via an antecubital vein through an 18-gauge peripheral intravenous line at a flow rate of 4 to 5 ml/sec using a MedRad Mark V power injector (MedRad, Inc.) (9).
Inclusion criteria and grouping
First, contrast-enhanced imaging upon CTA of the participants was clear, and the ECA and OA could be clearly seen and measured. For the healthy subjects, no ECA or ICA abnormities were observed. For patients with ICA stenosis and occlusion, the history of ICA stenosis and occlusion was chronic and progressive due to arteriosclerosis; in these patients, the ECA was normal and had no stenosis at the origin. No tumor or vascular diseases, such as scalp or intramuscular hemangioma (10,11), arteriovenous malformation (12), or DAVF (13), were associated with the OA.
Accordingly, the patients were divided into a mild and moderate ICA stenosis (ICA stenosis <70%) group, a severe ICA stenosis (ICA stenosis 70-99%) group and an ICA occlusion group (Fig. 1) (14).
Software and tools used for post-processing
The raw CTA data were post-processed using the GE Workstation (version 4.7; GE Healthcare; Cytiva). The raw CTA data were primarily reconstructed using volume rendering. Structures that affected the measurements were removed using the cutting tool. The diameter of the vessel and the distances between structures were obtained using the ‘measure distance’ tool. The curved length of a vessel was measured using the two-click AVA tool, and the curved three-dimensional length could be measured accurately. All the parameters were measured three times, and the average value was used for analysis.
Measured parameters Vessel diameter
The vessel diameters included the ECA diameter at the OA origin, the OA diameter at its origin, the OA diameter at the superior nuchal line, the OA diameter 2 cm above the superior nuchal line and the OA diameter at the midline of the foramen magnum. When measuring the OA diameter at the superior nuchal line and OA diameter 2 cm above the superior nuchal line, if the OA was branched, the thickest portion of the main trunk was measured.
Curve length of the vessel. The curve lengths included the lengths of the OA from its origin to the superior nuchal line, between the superior nuchal line and 2 cm above the superior nuchal line, and from the midline level of the foramen magnum to the superior nuchal line. When measuring these parameters, if the OA was branched, the thickest portion of the main trunk was measured.
Spatial distance. The spatial distances included the distance from the edge of the foramen magnum to the OA and the distance from the midline to the OA at the level of the superior nuchal line. For the distance from the midline to the OA at the level of the superior nuchal line, if the OA was branched, the thickest portion of the main trunk was measured. All measured parameters are illustrated in Fig. 2.
OA variations
A number of variations were recorded, including the common origin with other ECA branches arising from the ICA and OA-vertebral artery anastomosis (Fig. 3).
Statistical analysis
Statistical assessments were performed using GraphPad Prism (version 8.02) software (GraphPad Software, Inc.). Continuous variables are expressed as the mean ± standard deviation. A paired t-test was used for the comparison of two continuous variables. Ordinary one-way ANOVA followed by Tukey's multiple comparisons test was used for the comparison of multiple continuous variables. The Chi-squared test was used to compare count data among multiple groups. A P-value <0.05 was considered to indicate a statistically significant difference.
Results
General information
A total of 205 Han Chinese participants who met the inclusion criteria were selected for further investigation. A total of 50 healthy subjects (100 sides, left and/or right) were selected as the control group. In total, 155 patients (180 sides) were selected as the stenosis and occlusion groups, including the mild and moderate ICA stenosis group (50 sides, left and/or right), severe ICA stenosis group (80 sides, left and/or right) and ICA occlusion group (50 sides, left and/or right).
In the control group (50 subjects), the average age was 60.14±11.56 years (range, 33-88 years), and the ratio of males to females was 1.94:1 (33/17). In the mild and moderate ICA stenosis group (38 patients), the average age was 64.26±9.38 years (range, 37-80), and the ratio of males to females was 2.8:1 (28/10). In the severe ICA stenosis group (71 patients), the average age was 64.73±8.31 years (range, 33-79 years), and the ratio of males to females was 1.54:1 (43/28). In the ICA occlusion group (46 patients), the average age was 62.04±9.361 years (range, 41-80 years), and the ratio of males to females was 1.54:1 (30/16). The age and sex data, and their comparisons are summarized in Table I, Table II and Table III.
It should be noted that for the participants, their weight, height and body mass index should be included. However, as some data were from the out-patient department, these data could not be provided.
Measured parameters
Among the healthy subjects, the ECA diameter at the OA origin was 4.37±0.93 mm (range, 2.5-8.7 mm). The OA diameter at its origin was 1.94±0.33 mm (range, 1.3-2.8 mm). The length of the OA from its origin to the superior nuchal line was 142.2±19.39 mm (range, 104.9-185.9 mm). The OA diameter at the superior nuchal line was 1.68±0.35 mm (range, 0.8-3.0 mm). The OA diameter 2 cm above the superior nuchal line was 1.14±0.39 mm (range, 0.4-2.3 mm). The length of the OA between the superior nuchal line and 2 cm above the superior nuchal line was 39.34±9.1 mm (range, 22.6-71.2 mm). The OA diameter at the midline of the foramen magnum was 2.07±0.39 mm (range, 1.5-3.1 mm). The distance from the edge of the foramen magnum to the OA was 29.88±3.47 mm (range, 24.1-42.1 mm). The length of the OA from the midline level of the foramen magnum to the superior nuchal line was 89.82±14.89 mm (range, 56.9-129.5 mm). The distance from the midline to the OA at the level of the superior nuchal line was 36.09±4.42 mm (range, 25.6-48.7 mm). The overall data and their comparisons are summarized in detail in Tables IV and V.
OA variations
A common origin of the OA and the ascending pharyngeal artery was found on 15.4% of sides (43/280). A common origin of the OA and the posterior auricular artery was observed on 3.6% of sides (10/280). The OA-vertebral artery was found in 2.1% of sides (6/280). The OA arising from the ICA was not observed.
Results of the statistical analysis
There were no significant differences in age, sex, or anatomical parameters of the OA among the control, mild and moderate ICA stenosis group, severe ICA stenosis group and ICA occlusion groups (P>0.05) (Table I, Table II, Table III, Table IV and Table V). In the control group, no significant difference in anatomical parameters between the left and right sides was observed (Table VI).
Discussion
The OA is a crucial structure (1), and is often be used as a donor graft for cerebral revascularization (15). This vessel may also be involved in high-flow arteriovenous shunts, functioning as an accomplice, such as in cases of DAVF where the OA is dilated and hyperplastic (Fig. 4) (1). In cases of stenosis and occlusion of the ICA, the blood flow through the ECA is entirely from the common carotid artery (16). In these cases, it is unclear whether the OA becomes thicker or longer. If the OA becomes thicker or longer, performing a bypass between the suboccipital segment of the OA and intracranial artery may help treat brain hypoperfusion.
In the present study, CTA data were collected from a control group with a normal ICA and patients with stenosis and occlusion of the ICA. The analysis of the OA diameter did not reveal any differences among the different locations and OA lengths among all groups, indicating that the redistribution of blood flow due to severe stenosis or occlusion of the ICA was insufficient in changing the anatomic characteristics of the OA. In addition, these OA data are valuable, as there are no previous studies that have provided CTA data on the OA in Han Chinese population, at least to the best of our knowledge.
The OA is a large artery that has been reported to provide a mean blood flow of 15 to 80 ml/min when used for posterior fossa bypass (17). In a previous microanatomical study by Alvernia et al (18), the outer diameter of the OA at the origin varied from 2.2-2.9 mm.
In the present study, in healthy subjects, the OA diameter at its origin was 1.94 mm, which is smaller than that in the study by Alvernia et al (18); this may be due to the fact that the OA diameter in the present study was the inner diameter. Additionally, in the present study, in healthy subjects, the ECA diameter at the OA origin was measured to be 4.37 mm, which is much larger than that of the OA. An inner diameter of 1.94 mm is sufficient for endovascular treatment through the OA, easily allowing double 0.017' microcatheters to pass through (19).
The OA branches from the ECA; in rare cases, the OA can arise from the ICA (20). In the present study, the latter was not found, perhaps due to the small sample size. However, a common trunk of the OA with other branches of the ECA or OA anastomosis with the vertebral artery were found; a common origin with the ascending pharyngeal artery was observed in 15.4% of sides, and a common origin with the posterior auricular artery in 3.6% of sides. In the study by Hayashi et al (21), the incidence of the origin of the ascending pharyngeal artery from the OA was 19%, which was similar to that presented herein. These variations are important; for instance, during endovascular treatment via the OA, care must be taken to avoid damaging the ascending pharyngeal artery and posterior auricular artery. The OA-vertebral artery anastomosis was in 2.1% of sides in the present study. Anastomosis is important, and when the ICA or vertebral artery is occluded, the OA can serve as an important path of collateral circulation.
After leaving the ECA, the OA then runs in the occipital groove of the temporal bone, medial to the digastric groove (Fig. 5) (22). The digastric groove is an important landmark; a number of studies have measured the diameter of the OA at the level of the digastric groove. For instance, in a previous microanatomical study by Matsuo et al (3), the diameter of the OA at the level of the digastric groove was 2.1 mm. In another microanatomical study by Kawashima et al (23), the diameter of the OA was 2.05 mm at its exit from the digastric groove. The present study measured the OA diameter at the midline of the foramen magnum, which corresponds to the digastric groove; the diameter in healthy subjects was 2.07 mm; thus the results of the present study were similar to those of the previous aforementioned studies. Furthermore, the distance from the edge of the foramen magnum to the OA was measured, which was 29.88 mm, providing the spatial position of the OA at the level.
For a bypass between the OA and intracranial artery, the OA should be dissected from its distal end, and the dissection should extend to the level of the mastoid process after the digastric groove (4). In a previous microsurgical study by Kawashima et al (23), the mean length of the OA from the exit of the digastric groove to the level of the superior nuchal line was 81.9 mm. In the present study, the length in the healthy subjects was 89.82 mm. This difference may be derived from the fact that the study by Kawashima et al (23) was a microsurgical study, and the dissection could not reach the digastric groove; however, the present study used CTA, avoiding muscle disturbances. Nossek et al (24) recommend that a total length of 10-12 cm be dissected to reach the anastomosis site to avoid any tension in the region of the anastomotic sutures. In the present study, the length of the OA between the superior nuchal line and 2 cm above the superior nuchal line in healthy subjects was 39.34 mm, and the total length from the digastric groove was 12.92 cm (89.82 plus 39.34 mm), which was a sufficient length.
The OA crossed upwards over the superior nuchal line; however, its diameter did not decrease significantly at this point. In the study by Kawashima et al (23), the mean diameter of the OA was 2.01 mm at the level of the superior nuchal line. In the present study, this value in healthy subjects was 1.68 mm, which is smaller than that in the aforementioned study. The reason for this difference may be that the parameter measured for the peripheral OA on CTA was the inner diameter, which is expected to be smaller than the values obtained in microanatomical studies. In addition, due to the poor filling of peripheral vessels, the measured parameters were not accurate (25).
For surgical purposes, it is important to consider that the diameter of the main trunk of the OA remains >1.0 mm. In the study by Alvernia et al (18), the diameter of the OA was still >1.0 mm at 50 mm above the superior nuchal line. In the present study, ot was found that this measurement became inaccurate 20 mm above the superior nuchal line o CTA; thus, the OA diameter at this point was measured only in healthy subjects and a value of 1.14 mm was obtained. For an OA bypass, an accurate understanding of the orientation is crucial, and previous studies have demonstrated that the OA travels 3.0-4.5 cm on the superior nuchal line lateral to the inion (3,4). In the present study, this parameter in healthy subjects was measured to be 36.09 mm, in agreement with the aforementioned studies (3,4).
In conclusion, the presents study reported data on the OA diameter in Han Chinese patients. In addition, it was demonstrated that the stenosis and occlusion of the ICA did not significantly alter the anatomy of the OA.
Acknowledgements
Not applicable.
Funding
Funding: No funding was received.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Authors' contributions
JY designed the study and drafted the manuscript. TL collected the data. TL and JY confirm the authenticity of all the raw data. JY and TL revised the manuscript. Both authors have read and approved the final manuscript.
Ethics approval and consent to participate
The present study was approved by the Ethics Committee of the First Hospital of Jilin University (Approval no. 2021-533). Written informed consent was obtained from the participants.
Patient consent for publication
The participants or their parents/guardians provided consent and agreed to have their data (shown in the figures) published.
Competing interests
The authors declare that they have no competing interests.
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