The Circle of Willis is an anatomic, arterial, and anastomotic (i.e., relating to a connection between various blood vessels) structure located within the brain which connects the anterior and posterior arterial circulations of the brain.1 It was originally described by Thomas Willis nearly 400 years ago. The Circle of Willis was initially thought of as a series of collateral, arterial networks which maintain adequate circulation to the brain in the setting of low perfusion or occlusion from one or more arteries supplying the brain, though more recently additional theories have been proposed.2 One of the more recent theories is that it serves to protect the blood–brain barrier against hemodynamic stress, permitting flow from areas of high pressure to low pressure via the collateral circulations;2 however, the theory of collateral circulation in the setting of hypoperfusion or occlusion still predominates.
Given the current conception of the Circle of Willis as a perfusion failsafe in the setting of unilateral ischemia, it merits consideration for anesthesia and surgery in certain cases. A 2023 prospective case-series investigated the role of Willisian collaterals during carotid endarterectomies (CEA).3 CEA is a procedure indicated for patients with certain degrees and combinations of carotid arterial atherosclerotic burden and symptomaticity. During this procedure, the internal carotid artery from which the atheroma (i.e., collection of atherosclerosis) is removed is clamped, effectively inhibiting cerebral blood flow from that artery.4 Thus, the concern is that there may be insufficient cerebral blood flow when the atherosclerosed artery is clamped intraoperatively in patients with blockages or limited blood flow in the other, perfusing internal carotid artery. The 2023 prospective case-series yielded data suggestive of this concern, noting that a missing or non-functional bilateral posterior communicating artery (an artery within the Circle of Willis) was more frequently seen in the group of patients who developed neurologic symptoms, a sign of hypoperfusion, during the cross-clamping portion of the CEA than in those who did not develop symptoms during cross-clamping (Chi-squared 24.4; p = 10-7).3 This was further substantiated by a 2024 single-center retrospective study, which studied the effect of anatomical variations in the Circle of Willis on neurologic outcomes immediately following CEA where no shunting (i.e., human-made intentional moving of blood from one circuit to the next) in patients with occlusion of the non-operative carotid artery.5 This study yielded data demonstrating that the absence of both posterior communicating arteries—arteries involved in the collateral circulation—was greatly associated with immediate neurologic outcomes (odds ratio: 11.10; 95% confidence interval 1.04-118.60).5
Currently, there are no guideline-specific directions regarding intraoperative blood pressure management for the anesthesia team during a CEA. However, it is generally recommended to maintain mean arterial pressure between the patient’s baseline and a 20% elevation from baseline,6 to mitigate the risk of hypoperfusion or ischemic stroke, given substantial data suggestive of risk for regional ischemia intraoperatively.
In conclusion, the Circle of Willis represents a series of anastomotic arterial connections within the brain which provide collateral circulation and function as a failsafe in the setting of low perfusion or inhibition of blood flow from one side of the brain. During CEA, a common procedure to remove atheroma from the internal carotid arteries, there is increased risk of cerebral hypoperfusion. While there are no guideline-specific recommendations, general practice maintains the mean arterial pressure between baseline and a 20% increase from baseline to prevent symptomatic, cerebral hypoperfusion. However, given significant anatomic variations in the Circle of Willis, as well as differential atheroma burden, ultimately, it is up to the discretion of the anesthesiologist, and blood pressure management may be best informed by a multidisciplinary discussion with the anesthesiologist and vascular surgeon as well as hemodynamic monitoring throughout the case.
References
1. Jones JD, Castanho P, Bazira P, Sanders K. Anatomical variations of the circle of Willis and their prevalence, with a focus on the posterior communicating artery: A literature review and meta-analysis. Clin Anat N Y N. 2021;34(7):978-990. doi:10.1002/ca.23662
2. Vrselja Z, Brkic H, Mrdenovic S, Radic R, Curic G. Function of circle of Willis. J Cereb Blood Flow Metab Off J Int Soc Cereb Blood Flow Metab. 2014;34(4):578-584. doi:10.1038/jcbfm.2014.7
3. Gyöngyösi Z, Belán I, Nagy E, et al. Incomplete circle of Willis as a risk factor for intraoperative ischemic events during carotid endarterectomies performed under regional anesthesia - A prospective case-series. Transl Neurosci. 2023;14(1):20220293. doi:10.1515/tnsci-2022-0293
4. Howell SJ. Carotid endarterectomy. Br J Anaesth. 2007;99(1):119-131. doi:10.1093/bja/aem137
5. Lengyel B, Vecsey-Nagy M, Peter C, et al. The Circle of Willis Status Influences Neurological Complications of Carotid Endarterectomy with Contralateral Carotid Occlusion. Ann Vasc Surg. 2024;108:410-418. doi:10.1016/j.avsg.2024.05.021
6. Stoneham MD, Thompson JP. Arterial pressure management and carotid endarterectomy. Br J Anaesth. 2009;102(4):442-452. doi:10.1093/bja/aep012