Introduction 

   The anterior communicating artery (ACoA) has shown many types of anomalous structures : plexiform (33%), dimple (33%), fenestration (21%), duplication (18%), string (18%), fusion (12%), median artery of the corpus callosum (6%), and azygous anterior cerebral artery (ACA) (0.3~3%).^4)12)15) Azygous ACA is a single A2 which originates from both A1 without ACoA and feeds both hemispheres.⁵⁾ Azygous ACA is related to cerebral aneurysm in 8.6% cases.¹⁰⁾ 
   Aneurysm associated with azygous ACA is rare.⁹⁾ Clinically, its unexpected structure can cause confusion during surgical approach to its associated cerebral aneurysm. We report our experience of cerebral aneurysm associated with the azygous ACA. 

Case presentation 

   35-year-old female patient developed sudden severe headache. CT scans revealed subarachnoid clots around the basal cisterns (Fig. 1). Conventional angiography was performed (Integris Allura^® V500, Philips, Netherlands) and two saccular aneurysms were found at the proximal A2 and left internal carotid artery (ICA) bifurcation. A2 was thicker than both A1, but before surgery it was considered to overlap both A2 (Fig. 2). 
   The patient underwent clipping surgery through the pterional approach. First, the aneurysm at the left ICA bifurcation was exposed and its neck was clipped (Sugita^® #84, 4mm, vent type). Then around ACoA complex was dissected. We could not find the opposite side A2. Both A1 had fused and formed a single A2. A 5 6mm size aneurysm was located on the proximal single A2 with a blood clot around it. After careful dissection, the neck was clipped (Sugita^® #81, 5mm, straight type). After the operation, the patient recovered without any neurologic deficit. 
   Cerebral angiography demonstrated that both sides of the A1 segment were fused without a usual ACoA anterior communicating artery. A saccular aneurysm was found at the proximal portion of the single midline A2. A 5×6mm size aneurysm was directed inferiorly. The diameter of the A2 segment was larger than the left A1 segment. On the A2 bifurcation, the ACA separated into the pericallosal and callosomarginal arteries (Fig. 2, 3). On the left ICA bifurcation, an unruptured aneurysm was located in a posterolateral direction and measured 3×4mm. 

Discussion 

   In the circle of Willis, anomalous ACoA is estimated in about 8~20% of causes.^4)12)15) However, Serizawa et al.¹²⁾ reported it to be as high as 60%. Among the research, the incidence of azygous ACA was reported about 0.8~3%.¹²⁾¹⁵⁾ In 8.6% of cases, azygous ACA is associated with cerebral aneusrysm.¹⁰⁾ These reports can be integralized into a total incidence of azygous ACA aneurysm about 0.0056 %~0.15 %. However, each group in many reports had a very different mean age. The group which was analyzed by Serizawa et al.¹²⁾ had a mean age at 75.3 years. Kayembe et al.⁸⁾ dealt with about a 56-year-old group. Furthermore, their studies consisted almost of with cadaver research only, so it is very difficult to know the real incidence of anomalous ACA or azygous ACA and its associated aneurysm in past research or in future works. 
   However, all reports indicated that azygous ACA with aneurysm was rare and one needed to consider its clinical implication. 
   Many cases of congenital cerebrovascular anomaly are related with abnormal circulatory flow. Abnormal flow causes higher hemodynamic stress on the vascular wall and causes vascular dilatation, aneurysm or intracranial hemorrhage.^2)3)13)14) Some authors found a definite correlation between anomalous ACA and aneurysms in ACoA by analyzing their data groups.^8)13)14) Hashimoto et al.⁷⁾ proved this hypothesis by their experimentally induced aneurysms. However, not all aneurysms develop at the increased impingement site and a few authors have mentioned the unclear role of increased hemodynamic stress.^6)8)11) Not only does hemodynamic structure affect the development of an aneurysm, but pathological vascular wall structure might also cause the development of the aneurysm since the embryo. 
   The anomalies of cerebral circulation make false positive dynamic imaging due to its complexities. Also, its confused structure can cause a misreading of the imaging study and difficulty in the surgical approach.¹²⁾ Precise identification should be done before surgical intervention, but the limitation of reserving diagnostic tools and lack of experience can cause confusion or misread. In angiographic images, vascular anomaly looks like overlapping, weak flow, incompetent or clouded image and winding.¹⁾ We found its anomalous vasculature in the operation field, even though we did conventional angiography before the operation, because we thought the azygous ACA was overlapping both A2. Large azygous ACA is easily mistaken as overlapping both ACAs. 

Conclusion 

   Azygous ACA is a rare type of ACA anomaly which is sometimes associated with cerebral aneurysm. In conventional angiography, suspicion of its anomalous structure is needed to avoid misreading or confusion in the operation field. 

REFERENCES

1. Alekr GJ Jr, Abdel-Dayem HM, OH YS, Leslie EV, Donoghue G. False positive dynamic imaging of the cerebral circulation due to a congenital anomaly. Clin Nucl Med 6(11):532-6, 1981 

2. Alpers BJ, Berry RG, Paddison RM. Anatomical studies of the circle of Willis in normal brain. The anatomical structure Arch Neurol Psychiatry 81:409-18, 1959 

3. Alpers BJ, Berry RJ. Circle of Willis in cerebral vascular disorders. Arch Neurol 8:398-402, 1963 

4. Aydin IH, Takçi E, Kadioğlu HH, Tüzün Y, Kayaoğlu CR, Barlas E. Vascular variations associated with anterior communicating artery aneurysms-an intraoperative study. Minim Invasive Neurosurg 40(1):17-21, 1997 

5. Baptista AG. Studies on the arteries of the brain. II. The anterior cerebral artery: Some anatomic features and their clinical implications. Neurology 13:825-35, 1963 

6. Ferguson GG. Physical factors in the initiation, growth, and rupture of human intracranial saccular aneurysms. J Neurosurg 37:666-77, 1972 

7. Hashimoto N, Handa H, Nagata I, Hazama F. Experimentally induced cerebral aneurysms in rats. Part V : Relation of hemodynamics in the circle of Willis to formation of aneurysms. Surg Neurol 13:41-5, 1980 

8. Kayembe KN, Sasahara M, Hazama F. Cerebral aneurysms and variations in the circle of Willis. Stroke 15:846-50, 1984 

9. Eom KS, Kang SD. A2 Anomaly associated with anterior cerebral artery aneurysm, Kor J Cerebrovascular Disease 3:159-62, 2001 

10. Ohno K, Monma S, Suzuki R, Masaoka H, Matsushima Y, Hirakawa K. Saccular aneurysms of the distal anterior cerebral artery. Neurosurgery 27:907-13, 1990 

11. Sekhar LN, Heros RC. Origin, growth and rupture of saccular aneurysms: A review. Neurosurgery 8:248-60, 1981 

12. Serizawa T, Saeki N, Yamaura A. Microsurgical Anatomy and clinical significance of the anterior communicating artery and its perforating branches. Neurosurgery 40:1211-18, 1997 

13. Stehbens WE. Aneurysms and anatomical variation of the cerebral arteries. Arch Pathol 75:45-64, 1963 

14. Wilson G, Riggs HE, Rupp C. The pathologic anatomy of ruptured cerebral aneurysms. J Neurosurg 11:28-34, 1954 

15. Windle BC. The arteries forming the circle of Willis. J Anat Physiol 22:289-93, 1888