Is fetal‑type posterior cerebral artery a risk factor for intracranial aneurysm as analyzed by multislice CT angiography?
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- Published online on: November 13, 2017 https://doi.org/10.3892/etm.2017.5504
- Pages: 838-846
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Copyright: © He et al. This is an open access article distributed under the terms of Creative Commons Attribution License.
Abstract
Introduction
Fetal-type posterior cerebral artery (FTP) is a common anatomic variation observed in the circle of Willis, and defined as a posterior cerebral artery that originates from the internal carotid artery (ICA) with or without a small connection with the basilar artery (BA) (1). In this condition, instead of the BA, the ICA supplies blood to the posterior cerebral artery (PCA) (2).
ICA is formed by extending from the paired dorsal aorta towards the cephalic side on embryonic days 28–30 (4–5.7 mm) (3). In the embryonic stage of 5–8 mm, pairs of longitudinal nerve arteries appear along the turbid brain and merge to form basilar arteries. The ICA and the caudal part of the anastomosed branch of the nerve artery form the PCA (4). In the stage of 40 mm (8 weeks), the PCA appears as an extension of the posterior communicating artery (PComA) (5). The vertebra-basilar system is then formed and contributes to the blood supply of the PCA through the P1 segment. During this period, the components of the circle of Willis have the same diameter (2). During development, the diameter of the PCA-P1 segment gradually increases, while that of the PComA gradually decreases. At the time of birth, the PCA-P1 segment diameter is greater than that of the PComA, which represents the most common type known as the adult PCA (5). Stagnation in the process of embryonic development results in the following two cases. If the P1 segment diameter is similar to that of the PComA, the result is intermediate-type PCA. In the present study, 2.75% of the subjects had intermediate-type PCA. However, absence of the P1 segment or the diameter of the P1 segment being less than that of the PComA is known as FTP. Absence of the P1 segment reflects full-type FTP. If the diameter of the P1 segment is less than that of the PComA, it is defined as partial-type FTP (6). In the present study, 117 strips of FTP in 91 patients were found, including 40 strips of full-type FTP and 77 strips of partial-type FTP.
Several studies have described FTP in great detail, including its incidence, the association between FTP and occipital lobe infarction, life-threatening headache and white matter degeneration (7,8). FTP has also been reported to be associated with the occurrence of PComA aneurysm (9). Thus, it is worthwhile to assess whether FTP is a risk factor for intracranial aneurysm. To the best of our knowledge, the characteristics of intracranial aneurysm, including its incidence, location, association with other variations of the circle of Willis, subarachnoid hemorrhage, aneurysmal morphology (the presence or absence of daughter sac) and size (diameter of the aneurysm neck and aneurysm height), have not been systematically analyzed. Computed tomography angiography (CTA) is a novel and effective examination method for the display of intracranial vessels with the use of three-dimensional reconstruction technology. Due to its great advantage for the detection and diagnosis of intracranial aneurysm over other techniques, it is a preferred choice for diagnosing this condition. In the present study, CTA data of the intracranial artery of 364 consecutive cases of patients who were suspected with cerebrovascular disease or intracranial aneurysm were reviewed, and a detailed analysis was performed to assess the association between FTP and intracranial aneurysm.
Patients and methods
Subjects
The CTA data of 364 consecutive Chinese cases of intracranial artery assessed at Tianjin Hospital (Tianjin, China) from January 2013 to January 2016 were reviewed. The cohort comprised 218 male and 146 female patients, with an average age of 61.73±13.33 years. All the patients were initially diagnosed with cerebral vascular disease or intracranial aneurysm by a neurologist, and the patients' symptoms included headache, vomiting, aphasia, difficulty swallowing, limb weakness, gait instability and hemiplegia. Patients were suggested to undergo CTA examinations of the intracranial artery. This study was approved by the Ethics Committee of Tianjin Hospital (Tianjin, China). Prior to CTA examination, all patients provided written informed consents, which approved the publication of the images and data in the present study.
Examination methods
A 16-slice spiral CT (GE Lightspeed; GE Healthcare, Little Chalfont, UK) and an LF 9000 high-pressure injector (Libel-Flarsheim Company, Cincinnati, OH, USA) were used in this study. The patients entered the scanner in the supine position with the head first. The scan level was from the third cervical vertebra to the calvaria. Scanning conditions were as follows: Scan thickness, 5 mm; pitch, 1.375:1; tube voltage, 140 kV; tube current, 200–250 mA. Iohexol (350 mgI/ml, 50 ml; Yangtze River Pharmaceutical Group, Taizhou, China) was administered through either the elbow or dorsal vein at a flow rate of 4 ml/sec. The scanning delay time was 20–25 sec. The dosage of the contrast agent was 50 ml in 2013–2014 and 1.0 ml/kg × body weight (kg) in 2014–2016.
Image processing
Image reconstruction, including volume rendering and the multiplanar reconstructed image (MPR), were obtained on an AW4.5 workstation (GE Healthcare). The reconstruction thickness was 0.625 mm, the window level was 500 HU and the window width was 1,500 HU.
Interpretation of images
When the diameter of the PComA was greater than that of the P1 segment of PCA (PCA-P1), partial-type FTP was assumed, while full-type FTP was identified if the PCA-P1 segment was absent (Figs. 1 and 2).
Intracranial aneurysm was defined as abnormal expansion of the artery with congenital, infectious or traumatic causes. According to their shape, aneurysms were divided into saccular and fusiform types (10). Based on their location, saccular aneurysms were divided into bifurcation and lateral wall subtypes (11). ICA-PComA aneurysm is an aneurysm with the neck located in the ICA-PComA. ICA-PComA aneurysms were divided into 5 types according to the location of the aneurysm neck (12). In the present study, 3 types were considered, including the bifurcation (aneurysm neck occupying both ICA and PCA), ICA (aneurysm neck located mainly in the ICA) and PComA (aneurysm neck mainly located in the PComA) types.
Observation
CTA images of the intracranial artery were reviewed to further define FTP occurrence, location (side), type, potential combination with other variations of the circle of Willis (13) and potential combination with intracranial aneurysm.
Regarding aneurysms, the type (fusiform or saccular, bifurcation or lateral wall), location, presence or absence of daughter sac, presence or absence of subarachnoid hemorrhage and potential combination with FTP or other variations of the circle of Willis were assessed. ICA-PComA aneurysms were then classified. The neck of the aneurysm was displayed on an MPR image and the neck diameter and height of the aneurysm were measured.
Statistical analysis
All statistical analyses were performed with SPSS 21.0 software (IBM Corp., Armonk, NY, USA). The incidence of FTP, intracranial aneurysm and FTP with intracranial aneurysm was calculated. The chi-square test was used to assess the influence of FTP and gender on the above items, as well as on aneurysm daughter sac and subarachnoid hemorrhage. The chi-square correction test was performed with a total sample size of >40 and a minimum theoretical frequency between 1 and 5. Two independent sample Student's t-tests was used to compare the aneurysm neck diameter and aneurysm height between the FTP and non-FTP patients. Binary logistic regression analysis was performed to assess whether FTP and gender were risk factors for intracranial aneurysm and ICA-PComA aneurysm and the association was evaluated by the standards shown in Table I.
Results
Incidence of FTP
The total incidence of FTP and bilateral FTP (Fig. 3) was 25.00 and 7.14%, respectively (Table II). There was no statistical difference between the total incidence of FTP and bilateral FTP between males and females (χ2=2.577, P=0.108). A total of 117 strips of FTP were identified, including 77 strips of partial-type FTP and 40 strips of full-type FTP. In addition, 10 patients (2.75% in total) with intermediate-type PCA were identified.
 | Table II.Incidence of FTP, bilateral FTP, intracranial aneurysm and intracranial aneurysm associated with FTP/bilateral FTP and influence of gender. |
Other variations of the circle of Willis in FTP and non-FTP patients are presented in Table III. The percentage of other variations of the circle of Willis in patients with and without FTP was 49.45 and 7.69%, respectively (Table IV). There was a statistical difference on other variations of the circle of Willis between FTP and non-FTP patients (χ2=80.173, P<0.001).
| Table IV.Influence of FTP on other variations of circle of Willis, intracranial aneurysm, ICA-PComA aneurysm, other type of aneurysm in the anterior part of the circle of Willis, BA aneurysm, aneurysm in the posterior part of the circle of Willis, daughter sac of saccular aneurysm and subarachnoid hemorrhage analyzed by χ2 test. |
Incidence of intracranial aneurysms
Within the cohort (n=364), 50 patients (13.74%; 18 males and 32 females; mean age, 61.66±14.03 years) with 56 intracranial aneurysms were identified, including 11.54% (42/364; 48 aneurysms in 42 patients) saccular aneurysms and 2.20% (8/364; 8 aneurysms in 8 patients) fusiform aneurysms (Table II). Among them, multiple aneurysms accounted for 1.37% (5/364; 11 aneurysms in 5 patients). The dimensions of the intracranial saccular aneurysms were 2.93±1.6 mm (aneurysm neck diameter) ×3.52±2.47 mm (aneurysm height).
Of the intracranial aneurysms, 62% (31/50) were associated with variations of the circle of Willis, particularly the FTP variation (Table V). Among the 48 saccular aneurysms, 12 were accompanied with a daughter sac and 9 with subarachnoid hemorrhage. In addition, 6 saccular aneurysms had an aneurysm daughter sac combined with subarachnoid hemorrhage. The probability of subarachnoid hemorrhage in intracranial saccular aneurysm with daughter sac was significantly higher than that in saccular aneurysm without daughter sac, as analyzed by Continuity Correction χ2 test (χ2=7.704, P=0.006; Table VI). In addition, the presence of an aneurysm daughter sac was closely associated with subarachnoid hemorrhage as analyzed by binary logistic regression, with a strong association (OR=11.000; Table VII).
| Table V.Variations in circle of Willis associated with intracranial aneurysm in a total of 50 patients with intracranial aneurysms. |
Intracranial aneurysms in FTP
The incidence of intracranial aneurysm with FTP in all patients was 4.40% (16/364) (Table II). The rates of intracranial aneurysm combined with unilateral or bilateral FTP in female patients (37.5% for unilateral and 12.5% for bilateral FTP) were higher compared with those in males (22.22% for unilateral and 5.56% for bilateral) (Table II). The incidence of FTP in intracranial aneurysm patients was 32% (16/50), including 10% (5/50) of bilateral FTP cases. The incidence rate of intracranial aneurysm in FTP patients was 17.58% (16/91), which was slightly higher than that in non-FTP patients with 12.45% (34/273), but the difference was not statistically significant (χ2=1.285, P=0.257; Table IV). Table VIII presents the location of intracranial aneurysm in FTP and non-FTP patients.
Incidence of FTP and ICA-PComA aneurysm
In the present study, 29 ICA-PComA aneurysms (24 patients) were identified. Of the patients with FTP, 12.09% (11/91) presented with ICA-PComA aneurysm and 2.20% (2/91) of cases occurred in the initial part of PComA (Table IX), and were predominantly identified in females. Table IX displayed the ICA-PComA aneurysm type in FTP and non-FTP patients. The ICA type was predominant type in FTP or non-FTP patients. With the exception of ICA-PComA aneurysm, there was no statistical difference regarding intracranial aneurysms at other positions between FTP and no-FTP patients (Table IV).
Influences of FTP and gender on intracranial aneurysm and ICA-PComA aneurysm
Table II indicates the incidence of FTP, bilateral FTP, intracranial aneurysm, intracranial aneurysm with FTP and intracranial aneurysm with bilateral FTP between females and males by the χ2 test. A statistically significant difference in the incidence of intracranial aneurysm between females and males was identified (χ2=16.524, P<0.001). More females than males had intracranial aneurysm with FTP and bilateral FTP; however, the difference was not significant. No statistically significant differences in the incidence of any of the other conditions mentioned above were noted between females and males.
Table IV displays the influence of FTP on other variations of the circle of Willis, intracranial aneurysm, ICA-PComA aneurysm, other aneurysms in the anterior part of the circle of Willis, BA aneurysm, aneurysm in the posterior part of the circle of Willis, daughter sac of saccular aneurysm and subarachnoid hemorrhage as analyzed by the χ2 test. Statistically significant differences in the incidence of other variations of the circle of Willis (χ2=80.173, P<0.001) and ICA-PComA aneurysm (χ2=4.437, P=0.035) were identified between FTP and non-FTP patients (Table IV). No statistically significant differences in the incidence of any of the other conditions mentioned above were noted between FTP and non-FTP patients.
Table X presents the association of FTP and gender with intracranial aneurysm. A weak association was identified between FTP and intracranial aneurysm (OR=1.365), while there was a stronger association between gender and intracranial aneurysm (OR=0.328).
| Table X.Influence of FTP and gender on intracranial aneurysm by binary logistic regression analysis. |
Table XI displays the association of FTP and gender with ICA-PComA aneurysm. A moderate association was identified between FTP and ICA-PComA aneurysm (OR=2.762). In addition, a moderate association was present between gender and ICA-PComA aneurysm (OR=0.357).
Discussion
FTP is a posterior cyclic variation of the circle of Willis. Blood supply of the PCA on the FTP side is exclusively from the ipsilateral ICA, or from both the ipsilateral ICA and the BA, but predominantly from the ICA. Under normal circumstances, intracranial blood supply on both sides simultaneously relies on the cervical and vertebral basilar system, and the cerebral blood flow pressure remains similar between both sides. In the case of FTP, the blood flow of the ICA and vertebral basilar system is unbalanced, leading to a series of hemodynamic changes in circle of Willis components (14). First, blood flow is increased in the ICA-PComA and the blood pressure is enhanced, leading to increased impact on the vessel wall (15). Furthermore, the membrane lacks the muscle layer in the blood vessel wall of the arterial bifurcation and the blood vessel wall appears to be thinning (16).
In addition, the present study identified some other variations of the circle of Willis in FTP and non-FTP patients. The percentage of other variations of the circle of Willis in patients with and without FTP was 49.45 and 7.69%, respectively. The former was identified to be significantly higher compared with the latter. In theory, the hemodynamic changes of the circle of Willis would be more complex if FTP was combined with other variations (17). Previous studies have reported that anatomical variations of the circle of Willis, including persistent trigeminal artery, arterial window and anterior cerebral artery (ACA) -A1 dysplasia or absence, are associated with the occurrence of intracranial aneurysm (18,19). In fact, the present study also indicated that the incidence of intracranial aneurysm in FTP with other variations of the circle of Willis was higher than that in non-FTP patients; however, there was no significant difference between them. In addition, some variations in the circle of Willis were demonstrated to be associated with intracranial aneurysm in the present study. Among them, FTP was the most common variation associated with intracranial aneurysm. The incidence of FTP in intracranial aneurysm patients was 30%, including 10% for bilateral FTP cases, which was in line with the results of a previous study (20).
In 50 patients with intracranial aneurysm, there were 18 males and 32 females. There was significant difference between females and males who had intracranial aneurysms. Of the 48 intracranial saccular aneurysms identified in the present study, 12 had a daughter sac, 9 occurred with subarachnoid hemorrhage and 6 simultaneously occurred with both aneurysm daughter sac and subarachnoid hemorrhage. Analysis by Continuity Correction χ2 test revealed that saccular aneurysms with daughter sacs demonstrated a higher chance of subarachnoid hemorrhage, which was consistent with previous study (21). Furthermore, 3 saccular aneurysms were present with the daughter sac and FTP, and 1 saccular aneurysm was indicated with subarachnoid hemorrhage and FTP. However, no association between FTP and subarachnoid hemorrhage was identified in the present study. Results demonstrated the location of intracranial aneurysm, including the ACA, AComA, ICA-PComA, MCA, PCA and BA between FTP and non-FTP patients. Regardless of patients with FTP and patients without FTP, ICA-PComA aneurysm accounted for the largest proportion.
As mentioned above, there was no statistically significant difference between FTP and non-FTP patients regarding the incidence of intracranial aneurysm. However, a statistical difference was identified between FTP and non-FTP with ICA-PComA aneurysms. No significant differences were determined between FTP and non-FTP in intracranial aneurysms located elsewhere. In the present study, the ICA-PComA aneurysms were divided into 3 types, including the bifurcation type (aneurysm neck occupying both ICA and PCA), the ICA type and the PComA type. The ICA type was the predominant type in FTP and non-FTP patients. These results corroborated with the findings of Zada et al (22). Of note, ICA-PcomA aneurysm require distinguishing from the PComA funnel due to differences in treatment (12); the PComA funnel is a variation which does not require surgical treatment.
Binary logistic regression analysis revealed that gender was a risk factor for intracranial aneurysm and ICA-PComA aneurysm. A strong association was identified between gender and intracranial aneurysm (OR=0.328), and a moderate association between gender and ICA-PComA aneurysm (OR=0.357). Among patients with unilateral and bilateral FTP, more female than male patients with intracranial aneurysm were identified. This result was in accordance with that of a previous study, which proved that the prevalence of unruptured intracranial aneurysms in women was higher than that in men (23). The significant difference in the prevalence between males and females may be due to estrogen levels (24), which are also easily influenced by age, and the interplay among these factors deserves further research.
In conclusion, the present study indicated that female is an independent risk factor for intracranial aneurysm, and FTP and female are independent risk factors for ICA-PcomA aneurysm. It is known that age, gender, smoking, alcohol consumption, hypertension, coronary heart disease and diabetes are risk factors for intracranial aneurysm (25,26). Therefore, clinicians should pay sufficient attention to female patients with FTP, and a comprehensive follow-up program combined with risk factors of other aneurysms should be designed for the early prevention and treatment of intracranial aneurysm.
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