European Journal of Cardiovascular Medicine

The Circle of Willis (CoW) is a crucial arterial anastomotic ring situated at the base of the brain, responsible for maintaining adequate cerebral perfusion, particularly during episodes of arterial occlusion or stenosis. This arterial configuration connects the anterior and posterior cerebral circulations through the internal carotid and vertebrobasilar systems, thus ensuring collateral flow and reducing the risk of cerebral ischemia during vascular compromise [1].

Anatomically, the Circle of Willis comprises the anterior cerebral arteries (ACAs), anterior communicating artery (ACoA), internal carotid arteries (ICAs), posterior communicating arteries (PCoAs), and posterior cerebral arteries (PCAs). In its classical or “complete” form, all these vessels are present and of normal caliber. However, numerous studies have shown that such a complete configuration is less common in the general population. In fact, anatomical variations in the CoW have been observed in 50–60% of individuals, with common anomalies including hypoplasia, duplication, or even absence of one or more arterial segments [2,3].

These anatomical variants significantly influence the capacity of the CoW to function as an effective collateral network, particularly in pathological conditions such as ischemic stroke, aneurysmal rupture, or arterial dissection. As such, a thorough understanding of these variations is vital for neurosurgeons, radiologists, and vascular neurologists, especially when planning interventions like carotid endarterectomy, aneurysm clipping, or revascularization procedures [4,5].

Despite the clinical importance of the CoW, there is a notable scarcity of population-specific anatomical data, particularly from the Indian subcontinent. Given the potential ethnic and geographic variability in vascular anatomy, studies in underrepresented populations are essential to inform both clinical practice and educational curricula [1,5]. Therefore, the present study was conducted to investigate the anatomical variations of the Circle of Willis in human cadavers from a tertiary institution in central India and to explore their potential clinical implications.

This study was undertaken to identify and classify anatomical variations in the Circle of Willis through cadaveric dissection, assess their prevalence, and explore their clinical relevance in cerebrovascular disease management and surgical planning.

Study Design and Setting:

This was a descriptive cross-sectional study conducted in the Department of Anatomy at Shri Vasantrao Naik Government Medical College (VNGMC), Yavatmal, Maharashtra, over a period of 16 months, from December 2021 to March 2023.

Study Sample:

A total of 50 formalin-fixed human cadaveric brains were obtained from routine undergraduate dissection sessions in the department. Cadavers with visible signs of traumatic brain injury, surgical intervention, or pathological lesions affecting the cerebral vasculature were excluded from the study. Brains were preserved in 10% formalin solution for at least two weeks prior to dissection.

Sampling Technique:

The sample was selected using a convenient sampling method, based on availability during the dissection schedule.

Dissection Procedure:

Standard neuroanatomical dissection techniques were employed. After removal of the calvarium and brain, the base of the brain was carefully examined. The arachnoid mater was gently removed to expose the Circle of Willis and its constituent arteries. Special care was taken to preserve arterial integrity during removal.

Data Collection:

Each Circle of Willis was inspected for: Completeness, Symmetry

Presence of anatomical variations (e.g., hypoplasia, absence, duplication, or fetal origin of arteries)

Measurements were taken using digital calipers, and photographic documentation was performed for each specimen. Arteries with diameters <1 mm were considered hypoplastic.

Classification of Variations:

Variations were categorized into four commonly observed types based on standard anatomical classifications:

• Type I: Complete classical configuration
• Type II: Hypoplastic or absent posterior communicating artery (PCoA)
• Type III: Hypoplastic anterior communicating artery (ACoA) or anterior cerebral artery (ACA)
• Type IV: Fetal origin of posterior cerebral artery (PCA)

Ethical Considerations:

Prior approval for the study was obtained from the Institutional Ethics Committee of VNGMC Yavatmal. All cadavers used were those legally donated or made available for academic teaching and research purposes.

Data Analysis:

Data were entered in Microsoft Excel and analyzed using descriptive statistics. Frequency and percentage were used to present the prevalence of complete/incomplete circles and the types of variations observed. Results were organized in tables and graphs for clarity.

A total of 50 formalin-fixed human cadaveric brains were examined during the one-year study period to assess the anatomical variations in the Circle of Willis (CoW). The mean age of the cadavers was 63.4 ± 8.2 years, with a range of 45 to 85 years. The sex distribution revealed a predominance of male cadavers (64%) compared to females (36%), as shown in Table 1.

Table 1: Demographic Distribution of Cadaveric Samples (N = 50)

Completeness of the Circle of Willis

Out of the 50 specimens, a complete Circle of Willis was observed in 30 cases (60%), while 20 cases (40%) exhibited incomplete configurations (Table 2). These findings suggest that a significant proportion of individuals may lack robust collateral circulation pathways.

Table 2: Completeness of the Circle of Willis (N = 50)

Figure 1.Completeness of the Circle of Willis

Component-wise Arterial Variations

The anterior communicating artery (ACoA) was found to be normal in 90% of cases, while hypoplasia or absence was observed in 10%. The posterior communicating arteries (PCoA) were the most variable component, with only 56% showing normal bilateral configuration. Hypoplasia or absence was noted in 44% of the cases. Similar trends were observed in the anterior cerebral artery (ACA) and posterior cerebral artery (PCA), where 92% and 84% of arteries were normal, respectively (Table 3).

Table 3: Variations in Components of the Circle of Willis (N = 50)

Figure 2: Variations in Components of the Circle of Willis

Classification of Anatomical Variants

The variations observed were classified into four common types based on existing literature. The classic complete configuration (Type I) was seen in 60% of cases. Type II variation, characterized by hypoplastic or absent PCoA, accounted for 20% of cases. Type III, involving hypoplastic ACoA or ACA, and Type IV, involving fetal origin of PCA, were each observed in 10% of cases (Table 4).

Table 4: Types of Anatomical Variations in the Circle of Willis (N = 50)

Laterality of Variations

Assessment of the laterality of these variations revealed unilateral involvement in 28% of cases and bilateral variations in 12% of cases (Table 5).

Table 5: Laterality of Variations in Circle of Willis Components (N = 50)

Unilateral hypoplasia was more frequently observed on the right side, although specific side dominance was not statistically assessed in this study.

The present study aimed to evaluate the morphological variations of the Circle of Willis (CoW) in 50 formalin-fixed human cadaveric brains at VNGMC Yavatmal, Maharashtra. A complete configuration of the CoW was observed in 60% of specimens, while the remaining 40% demonstrated incomplete or variant forms. This finding is consistent with previous studies, which have reported the incidence of a complete CoW to range from 36% to 60%, depending on the population studied and the diagnostic modality used (e.g., dissection, MR angiography, CT angiography) [5,6,7].

Among the individual arterial components, the posterior communicating arteries (PCoAs) were the most frequently variable, being hypoplastic or absent in 44% of cases. Similar trends were noted in other cadaveric and radiological studies, including those by Kapoor et al. and Eftekhar et al., where PCoA anomalies were the most common [9,10]. The anterior communicating artery (ACoA) was absent or hypoplastic in 10% of specimens, potentially compromising anterior collateral flow during ischemic episodes. These observations align with those of Fattahian et al. [6], who also reported a high incidence of ACoA anomalies in their autopsy series.

Variations were classified into four types: Type I (classic complete configuration) was the most common (60%), followed by Type II (hypoplastic/absent PCoA) in 20% of cases. Type III (hypoplastic ACoA/ACA) and Type IV (fetal origin of PCA) were each seen in 10% of specimens. These patterns resemble the anatomical variants described in populations from South Trinidad, Malawi, and the Middle East [5,7,8].

The laterality of variations further revealed that unilateral anomalies were more prevalent (28%) than bilateral variations (12%). This asymmetry may hold clinical relevance; unilateral defects may still permit partial compensation via the intact side, whereas bilateral deficiencies severely limit the collateral capability of the CoW. Similar findings were documented by Nyasa et al. in a Malawian cadaveric study [7].

From a clinical perspective, these anatomical variations have significant implications. Incomplete or variant configurations may reduce the CoW’s ability to provide adequate collateral flow during cerebral ischemia, increasing the risk of stroke, aneurysm rupture, or poor outcomes following vascular surgeries. Therefore, preoperative evaluation using MR angiography or CT angiography is strongly recommended for patients undergoing neurovascular interventions [6,10].

Beyond clinical practice, the study's findings offer important pedagogical value. Given the high prevalence of non-classical configurations in the population, it is essential for medical students, anatomists, neurosurgeons, and radiologists to understand these patterns for effective neurovascular planning and interpretation. This aligns with observations from Alharbi and Al Saffar, who emphasized the role of anatomical variability in both educational and surgical settings [8].

Limitations

This study was limited by its sample size and the use of cadaveric specimens, which may not fully replicate in vivo cerebral circulation. Additionally, age-related or pathological changes in vessel integrity could not be excluded, despite screening for gross abnormalities.

Future Directions

Further studies using larger sample sizes and radiological correlation across diverse populations are needed to better understand the clinical impact of these anatomical variations. Comparative data from patients with cerebrovascular accidents could elucidate the protective or predisposing roles of specific CoW configurations.

This cadaveric study highlights the high prevalence of anatomical variations in the Circle of Willis, with only 60% of specimens showing a complete configuration. The most commonly affected component was the posterior communicating artery, often hypoplastic or absent. These structural deviations may compromise cerebral collateral circulation and influence susceptibility to ischemic events. Knowledge of such variations is crucial for neurosurgical planning, stroke management, and radiological interpretation. The findings underscore the importance of routine preoperative cerebrovascular imaging to identify at-risk individuals. Further studies with larger sample sizes and radiological correlation are recommended to better understand the clinical significance of these anatomical patterns.

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