|History: Patient 1 - male, 18 years;Patient 2 - male, 13 years;Patient 3 - male, 61 years;Patient 4 - female, 37 years.|
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|Findings: Figure 1 (patient 1): Complete circle of Willis; Figure 2 (patient 2): Hypoplasia of the left posterior communicating artery(PCoA); Figure 3 (patient 3): Hypoplasia of the precommunicating(A1) segment of the left anterior cerbral artery(ACA); Figure 4 (patient 4): Fetal origin of posterior cerebral artery(PCA) from internal carotid artery(ICA) on left side.|
|Diagnosis: Normal variant of the circle of Willis: 1. complete circle of Willis; 2. hypopla of posterior communicating artery(PCoA); 3. hypoplasia of A1 segment of anterior cerebral artery (ACA); 4. fetal origin of posterior cerebral artery(PCA).|
|Discussion: The circle of Willis, located at the base of the brain, is an interconnecting arterial polygon that surrounds the ventral surface of the diencephalon adjacent to the optic nerves and tracts. It is a potential collateral pathway through which adequate distribution of cerebral blood flow can be maintained in case of impaired or decreased flow through one or more of its proximal feeding vessels. Its ability to reditribute blood flow depends on the presence and size of the component vessels. The distal internal carotid artery usually terminates by bifurcating into the anterior and middle cerebral arteries. The posterior cerebral arteries typically arise from the basilar artery. The following 10 vessels comprise the circle of Willis (fig 1): - two internal carotid artery (ICA); - two horizontal or precommunicating or A1 segments of both anterior cerebral arteries (ACA); - one anterior communicating artery (ACoA); - two posterior communicating artery PCoA); - two horizontal or precommunicating or P1 segments of both posterior cerebral arteries (PCA); - one basilar artery (BA). The ICA, ACA and ACoA form the anterior part of the circle. The BA, PCA and PCoA are termed the posterior circulation. Medial cerebral arteries (MCA), vertebral arteries (VA), A2 segments of ACA and P2 segments of PCA are not components of the circle of Willis. A complete circle of Willis in which no component is hypoplastic or absent is seen in only 20% to 25% of cases (fig 1). Common normal variants include hypoplasia of one or both posterior communicating arteries (PCoA), a hypoplastic or absent A1 anterior cerebral artery segment (ACA), and fetal origin of the posterior cerebral artery (PCA). Hypoplasia or absent of one or both posterior communicating arteries (PCoA) is the most common anatomic variant of the circle of Willis (fig 2), which is seen in about 25% to 33% of cases. It is found in 1/3 of anatomic dissections and in 1/4 of MR angiograms (MRA). Such anomalies can be very important when stenosis or obstruction of a major extracranial vessel occurs, since it normally connects the anterior and posterior circulaton, therefore provides an important collateral circulation. The precommunicating or A1 segment extends medially from the ACA origin to its junction with the anterior communicating artery (ACoA). Anatomic variations in the ACA-ACoA complex are very common. Hypoplastic A1 segment is seen in about 10% to 20% of anatomic dissections (fig 3). The true absent of A1 segment is not common, found only in 1% to 2% of cases. If A1 segment is hypoplastic on one side, the A2 segment on this side fill from the other side with the aid of the anterior communicating artery (ACoA). Fetal type posterior cerebral artery indicates those cases in which the major stem of the posterior cerebral artery arises from the ipsilateral internal carotid artery instead of from the basilar artery (fig 4). Usually there is hypoplasia or abscense of the P1 segment of the posterior cerebral artery at the same side. This variant is seen in 15% to 25% of cases. Large variations in the percentage of circle with a complete configuration and normal variations are found in the literature. Several factors can account for variations in these results: the selected study populations differ; the methods and techniques of examination are differents; investigators use different sets of criteria to define a "normal" or complete circle of Willis.|
|References: 1. Osborn AG. Diagnstic neuroradiology. 1st ed. St. Louis,MO: Mosby-Year Book, Inc. 1994. 2. Osborn AG. Angiografia Cerebral - Diagnóstica. 2nd ed. Rio de Janeiro,RJ. Livraria e Editora Revinter Ltda. 2002. 3. Monique J, Krabbe-Hartkamp, Grond JVD, Leeuw FE, Gront JC, Algra A, Hillen B, Breteler MMB, Mali WPTM. Circle of Willis: Morphologic variation on Three - Dimensional Time-of-Flight MR Angiogram. Radiology. 1998; 207: 103-111. 4. Tanaka H, Fujita N, Enokib b, Matsumoto, k, watanab Y, Murase K, Nakamura H. Relationship Between Variations in the Circle of Willis and Flow Rates in Internal Carotid and Basilar Arteries Determined by Means of Magnetic Ressonance Imaging with Semiautomated Lumen Segmentation: Reference Data from 125 Healthy Volunteers. American Journal of Neuroradiology. 2006; 27: 1770-1775.|
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Case Number: 30076837Last Updated: 08-07-2009 The reader is fully responsible for confirming the accuracy of this content.
The reader is fully responsible for confirming the accuracy of this content.