Aneurysms of the Anterior Communicating Artery and Gross Anomalies of the Circle of Willis* HOMER D. KIRGIS, M.D., WILLIAML. FISHER, !VI.D., RAEBURN C. LLEWELLYN,M.D., AND EDWARD1~r I~EEBLES, ProD. Section of Neurosurgery, Ochsner Clinic, and Department of Surgery and Anatomy, Tulane University School of Medicine, New Orleans, Louisiana hE various theories of development of aneurysms of the intracranial vessels T are well known. Although early in- vestigators thought embolic and inflamma- tory reactions were important factors in their formation, later workers stressed the prob- able importance of developmental defects of the media at sites of branching, supple- mented by degenerative changes of the in- ternal elastic membrane. Turnbull ~3 sug- gested that cerebral aneurysms formed as a result of degeneration of the media at sites of inherent weakness of the media as at points of branching. Forbus 7 concluded, however, that although a small diverticulum may oc- FIG. 1. Normal circle of Willis. It is complete, symmet- rical and has thread-like communicating arteries. cur at the site of a medial defect associated with the bifurcation of an artery, an "ana- tomical" aneurysm will not develop while Willis might be a cause of psychiatric illness. the internal elastic membrane is still intact. Actually, if by normal we mean a completely He concluded further that miliary aneu- symmetrical circle with thread-like com- rysms as such are not congenital malforma- municating arteries illustrated in current tions but are acquired lesions arising from a texts (Fig. 1), then the incidence of anoma- combination of focal weakness in the vessel lous circles is probably even greater than 50 wall, secondary to a congenital defect of the per cent. In the treatment of intracranial media and degeneration of the internal elas- aneurysms with subarachnoid hemorrhage, tic membrane, due to continued overstretch- gross anatomical anomalies of the circle have ing of this membrane. While some 3,4,8 have, been considered more as a factor in circula- in general, supported this theory of the de- tory intolerance of carotid occlusion than as velopment of aneurysms, others have dis- a cause of the aneurysm itself, u Chase ~ men- sented. Bassett and Lemmen 1 remarked that tioned that gross congenital anomalies of the most aneurysms of the cerebral arteries rep- circle of Willis are very common and sug- resent vestiges of the primitive circulatory gested that these anomalies may be expected system. to be accompanied by structural defects in It is now well known that the incidence of the walls of the vessels. Wilson et al., 14 how- gross anatomical anomalies of the circle of ever, reported that in 85 per cent of 40 aneu- Willis is high. Approximately 60 years ago, rysms found on the anterior communicating however, Blackburn, 2 finding at autopsy artery were associated with hypoplasia of the that 5~ per cent of the circles of Willis in a first portion of one anterior cerebral artery. group of psychiatric patients were anomalous, Stehbens ~2 stated that this was the only ana- concluded that anomalies of the circle of tomical variation that he was able to corre- late with the location of cerebral aneurysm * Supported in part by Public Health Service Re- in a study of ~51 brains with 333 aneurysms. search Grant NBO 49~6--03, National Institute of Asymmetry of the circle of Willis seems to Neurological Diseases and Blindness. Presented at meeting of the Harvey Cushing Society, be primarily the result of a difference in size, Los Angeles, California, April ~1, 1964. and, in turn, a difference in the pattern of 73 74 Kirgis, Fisher, Llewellyn and McC. Peebles cation and normal adult bifurcation has been described as transitional trifurcation, s In addition to the anterior trifurcation pattern of terminal branching, the internal carotid may have a transitional posterior communi- cating branch (Fig. 5). When combined with a contralateral internal carotid that fails to bifurcate (Fig. 5), this is the most dominant form of internal carotid. Of interest also in Fig. 5 is the congenitally small vertebral artery and the relatively large contralateral vertebral artery. A great discrepancy in the caliber of the vertebral arteries is not un- common but we have noted no consistent rela- tion between the asymmetry of these arteries Fro. 2. Anomalous circle of Willis. It is complete but and the asymmetry of the circle of Willis. asymmetrical due to fetal trifurcation of the left internal carotid artery and normal bifurcation of the right inter- Material and Methods nal carotid. The present study is an analysis of 1,000 branching of the internal carotid arteries, s,9 circles of Willis obtained at autopsy at Ochsner There are ~ relatively common, variations Foundation Hospital. The circles, with the subarachnoid portion of each vertebral artery, from the normal pattern of branching of this the basilar artery, and each major cerebral artery. One is the persistence of the fetal pat- artery to the point of its first major division tern of trifurcation in which all 3 of the ipsi- distal to the circle, were fixed in 10 per cent lateral major cerebral arteries arise from the formalin. internal carotid while the posterior cerebral The incidence of aneurysms in this series may segment of the circle continues, as in the be higher than expected for 2 reasons. Our neuro- early fetus, to be a small, thread-like, poste- surgical service represents a relatively large pro- rior communicating type of vessel (Fig. ~). In portion of the hospital population and thus a the other common variation the artery di- comparatively large number of patients with sub- vides into the middle cerebral artery and an arachnoid hemorrhage are admitted. Moreover, the series represents a special professional interest unusually large anterior cerebral branch. The in the dissections and probably few aneurysms latter in turn, divides at the anterior pole of were overlooked. the circle into both anterior cerebral arteries. This pattern of terminal branching of the internal carotid artery has been termed anterior trifurcation (Fig. 3). The proximal part of the branch to the opposite side is ori- ented in the manner of the usual anterior communicating artery but is several times larger. It usually is connected to the opposite internal carotid by a hair-like, communicat- ing type of vessel. The opposite, anterior cerebral segment of the circle, however, may be missing, in which case the contralateral internal carotid simply continues as the middle cerebral artery (Fig. 4) or may bifur- cate into the posterior cerebral artery and the middle cerebral artery (Fig. 3). As would be expected, there are many gradations between the normal bifurcation FIG. 3. Anomalous circle of Willis. It is complete but of the internal carotid and each of these asymmetrical due to anterior trifurcation of the left forms of trifurcation. A form representing a internal carotid artery and reverse bifurcation of the transition stage between early fetal trifur- right internal carotid. .