The posterior cerebral artery (PCA) shares in the formation of the posterior part of the circle of Willis (CoW). PCA develops in embryonic life from the internal carotid system then through anastomosis between the carotid and basilar systems, the basilar system substitutes the carotid system to provide the main supply to the distal segment of PCA to become a classical type of PCA while the proximal segment of the embryonic PCA regresses to become the posterior communicating artery (PComA). If the internal carotid system fails to develop a mature anastomosis with the basilar system, or the development of the vertebrobasilar system is lagging, PCA doesn’t modify its source of blood supply and the fetal PCA (FPCA) will persist where PCA gets its supply from the internal carotid system [1]. A diagrammatic illustration of the development of PCA is shown in Fig. 1.

Figure 1. Diagrammatic Illustration for the development of posterior cerebral artery (PCA). (A) Fetal PCA (FPCA) arises from internal carotid. (B) FPCA makes weak anastomotic network with basilar artery (BA) terminals (Future P1 segment of PCA). (C) The anastomotic network progresses into a larger anastomotic vessel. (D) The proximal part of the FPCA starts to regress to become posterior communicating artery (PComA). 1. Vertebral arteries, 2. BA, 3. BA terminals (future P1 segment of PCA), 4. superior cerebellar arteries, 5. internal carotid artery (ICA), 6. FPCA, 7. anastomosis of ICA with BA terminals, 8. P2 segment of PCA, 9. PComA.

PCA duplication is extremely rare and may involve its different segments [2-4]. Superior cerebellar arteries (SCAs) usually arise from the basilar artery (BA). Duplication of the SCA is a common variant [5]. Anomalous origin of the SCA from the PCA is a rare variant [6, 7]. To the best of our knowledge, the partial duplication of the distal part of the P1 segment (P1b) of the FPCA associated with vertebral artery hypoplasia and bilateral duplication of the SCA with its unilateral anomalous origin from FPCA was not reported.

A rare cerebral vascular variation was found during a cadaveric dissection (Fig. 2). The right PComA was large in diameter (2.23 mm) and had a distal attachment to the junction between a small P1 and a large P2 segments of the PCA demonstrating the partial FPCA. The distal part of the P1 segment (P1b) of the right PCA had a double attachment to PComA. Both arms of this duplication were almost of similar diameter (0.83 and 0.87 mm), and both had smaller diameters compared to the proximal part of the P1 segment (P1a) of the right PCA (1.34 mm in diameter). The P1a of the right PCA was hypoplastic compared to the left P1 segment (2.34 mm in diameter). The left PComA was of a classical type and had a small diameter (0.86 mm). Bilateral duplication of SCAs was found. The two left SCAs as well as the lower right SCA originated from BA, while the upper right SCA originated from the P1a segment of the right PCA. The upper right SCA had a smaller diameter of 0.63 mm while the lower right SCA had a larger diameter of 1.82 mm. The 2 left SCAs were similar in diameter (1.24 and 1.13 mm). The right vertebral artery was hypoplastic (1.56 mm in diameter) while the left one was of large diameter (4.43 mm) and had almost the same diameter as the BA (4.51 mm). No additional anatomical variations were found in the other ipsilateral or contralateral cerebral arteries.

Figure 2. (A) Dissection of the base of the brain and circle of Willis. (B) Illustrative drawing for the brain vessels of the present case. 1. Right internal carotid artery (ICA), 2. left ICA, 3. right posterior communicating artery (PComA) (fetal variant posterior cerebral artery [PCA]), 4. left PComA, 5. right vertebral artery, 6. left vertebral artery, 7. basilar artery, 8. P1a (proximal part of P1 segment) of right PCA, 9. anterior limb of the duplicated P1b (distal part of P1 segment) of right PCA, 10. posterior limb of the duplicated P1b (distal part of P1 segment) of right PCA, 11. P1 segment of left PCA, 12. P2 segment of right PCA (fetal variant), 13. P2 segment of left PCA, 14. upper right superior cerebellar artery (SCA), 15. lower right SCA, 16. upper left SCA, 17. lower left SCA.

This work was done on cadaveric specimens obtained from the dissection room of the Department of Anatomy and Neurobiology, College of Medicine and Health Sciences, National University of Science and Technology. The research protocol was approved by the Ethics Committee of the College of Medicine and Health Sciences, National University of Science and Technology (IRB No: NU/COMH/EBC0031/2022).

Here we present an exceedingly rare case of partial duplication of the distal part of the right P1 segment (P1b) of a partial FPCA associated with hypoplastic right vertebral artery and bilateral duplication of SCAs, of which, the upper right SCA originated from the right PCA. The PComA can be classified according to its diameter into small (<1 mm), moderate (1–2 mm), or large (>2 mm) [8]. In the present case, the right PComA was large (2.23 mm) as compared to the right PCA P1 segment (1.34 mm), confirming the partial FPCA while the left PComA was small (0.86 mm) confirming classical type left PCA. The mean diameter of P1 of PCA can range from 1.60–3 mm [9]. In the case presented here, the P1 segment of the left PCA had a normal diameter of 2.34 mm while the P1a of the right PCA had a small diameter of 1.34 mm. Both limbs of the duplicated P1b were smaller in diameter (0.83 and 0.87 mm). The P2 segment of the right PCA had almost the same diameter (2.26 mm) as the right PComA while the P2 segment of the left PCA had the same diameter as the P1 segment of the left PCA. The average diameter for BA ranges from 3–7 mm [10]. Its diameter in the present case was 4.51 mm. The normal diameter of the vertebral artery ranges from 2.9 to 3.4 mm [10]. The left vertebral artery had almost the same diameter (4.43 mm) as the BA while the right vertebral artery was 1.56 mm in diameter indicating hypoplasia. The diameter of the SCA ranges from 1.28 mm to 2 mm [11]. The upper right SCA in the present case was hypoplastic (0.63 mm in diameter) while the diameters of the other three were within the normal range.

In a recent study on Andhra population of India (multiethnic), the incidence of FPCA was 5.6% out of two hundred and 31 magnetic resonance angiography images of CoW of patients with different neurological symptoms [12]. The FPCA may be a full variant when the precommunicating P1 segment of the PCA is absent, a partial variant when the P1 segment is smaller than the PComA as in the present case, an intermediate variant when the P1 segment and PComA have the same diameter, or a true variant when two independent PCAs are demonstrated and this is extremely rare [1].

In another study done in India (multiethnic) including three hundred forty PCAs dissected from human cadaveric brains, PCA duplication was found to be extremely rare (2.3%) [2]. PCA duplication may be a partial duplication in the P1 segment, in the P2 segment [3], or rarely between the P1 and P2 segments [4]. However, to the best of our knowledge, the present case is the first to report a partial duplication of the P1b segment of the FPCA. In addition, duplication of the SCA is a common variant. Its frequency varies from 3% to 30% [5]. Anomalous origin of the SCA from the PCA is a rare variant (1.9%) [6]. However, it is rare to encounter this variation bilaterally [7]. To the best of our knowledge, the present case is the first to demonstrate the association of bilateral duplication of the SCA with its unilateral anomalous origin from FPCA.

During embryonic development, multiple events occur that may lead to the formation of anatomical variations [13]. In the early stages of development, the posterior circulation is supplied mainly by the internal carotid artery (ICA). Some PCA precursors could be seen in 4 to 6-mm embryos arising from ICA. As brain growth progresses, the brainstem and cerebellum enlarge; blood supply from the ICA becomes scanty, bringing about the development of the posterior circulation from the basilar system. In 12 to 14 mm embryos (35 days of age), PCA makes a network of vessels that anastomose with the termination of BA. This network fuses later on. At later stages, the proximal portion of embryonic PCA thins out and forms the adult PComA. The two terminal branches of the BA namely P1 segments of the right and left PCAs take over and become the main source of blood supply for the posterior circulation. We hypothesize that the poor development of the right half of the vertebrobasilar system, as demonstrated by the hypoplastic right vertebral artery, caused the FPCA to persist, acquiring the main supply from the ICA without substituting it with the BA as in classical type PCA. In addition, the remnants of the weak anastomosis between the embryonic PCA and the basilar system persisted as a partial duplication of P1b of the right PCA [14]. Furthermore, the hypoplasia of the right half of the vertebrobasilar system caused the right upper SCA to originate from the PCA instead of the BA.

Awareness about the rare variations in CoW and their hemodynamics is important as they are associated with cerebral aneurysm and stroke. In addition, knowledge about these variations may help in surgical planning for the management of the associated aneurysm and stroke [13, 15].

Conceptualization: EME, HRHE. Data acquisition: all authors. Data analysis or interpretation: EME, HRHE. Drafting of the manuscript: all authors. Critical revision of the manuscript: EME, HRHE. Approval of the final version of the manuscript: all authors.

No potential conflict of interest relevant to this article was reported.

None.