The value of phase-contrast magnetic resonance angiography of the circle of Willis in predicting cerebral ischemia-hypoxia (shunt need) during carotid endarterectomy
Background: Cerebral perfusion during carotid cross-clamping depends on collateral function of the circle of Willis. The aim of this study was to determine the value of 3D Phase-Contrast (3D PC) MR angiography in predicting pre-operatively the need of shunting. Methods: 3D PC MR angiography were performed in 121 patients before carotid endarterectomy under locoregional anaesthesia. Based on the MR analysis, the risk of cerebral ischemia–hypoxia during clamping was classified as high, moderate and low. The analysis was then correlated with intraoperative neurological examination. Results: In patients with high risk of cerebral ischemia (n=9), immediate cerebral ischemia developed in all patients within one min of clamping (P<0.001). All nine underwent shunt placement. In six of the patients with moderate risk (n=46), cerebral ischemia occurred between 20 and 25 min after clamping; All occurred during intraoperative hypotension. Conclusion: 3D PC MR angiography can significantly determine the need of shunting in patients with important risk of immediate intraoperative cerebral ischemia. It also focuses on the intraoperative blood pressure stability in patients with moderate risk of ischemia.
KeywordsCerebral ischemia Magnetic Resonance angiography Carotid surgery Carotid artery Circle of Willis Anatomy
The clinical tolerance of acute carotid artery cross-clamping during carotid endarterectomy (CE) under locoregional anaesthesia depends on the functionality of the intracranial cerebral collateral circulation and on the blood pressure stability . The collateral flow to the cerebral hemisphere is mainly through the circle of Willis. Lazorthes and Gouaze  have described 22 different configurations of the circle of Willis. The typical normal polygon configuration is present only in 50% of the general population [1, 3, 9, 12]. Moreover, the collateral circulation is not functionnal and require placement of a shunt in about 10% of patients undergoing CE . The use of an intraluminal shunt can be performed systematically or selectively. It appears that shunt placement complications such as dissection or thromboembolism might be reduced by selective-shunting only for severe persisting ischemia during carotid clamping . The traditional methods used to assess pre-operatively the need for shunt placement are conventional angiography and transcranial Doppler ultrasonography [2, 6, 7, 19, 20, 21]. The purpose of this study was to determine the value of 3D phase contrast magnetic resonance angiography (3D PC MR) in predicting pre-operatively the risk of severe cerebral ischemia and therefore the need of shunting.
Material and methods
From October 1998 to December 2001, 121 patients benefited from pre-operative 3D PC intracranial MR angiograms. There were 95 male and 26 female patients with a mean age of 69 years (range 49–89 years). The criteria of the North American Symptomatic Carotid Endarterectomy Trial were used for the determination of percent stenosis. There were 45 symptomatic patients with >70% carotid stenosis on duplex scanning and 76 asymptomatic patients with >80% carotid stenosis on duplex scanning. Seven patients had a contralateral internal carotid occlusion.
The risk of cerebral ischemia was considered as high when flow was absent or weak in a major segment of both anterior (contralateral M1, A1 controlateral, Com Ant) and posterior (Com Post, P1) collateral pathway of cerebral blood flow. The risk was considered as moderate when flow was absent or weak in a segment of anterior or posterior collateral pathway and it was considered as low when flow was normal in the collateral circulation.
Anaesthetic and surgical techniques
Baseline blood pressure was recorded during anaesthetic examination 1 week before operation. Patients underwent a superficial and deep cervical plexus block especially during carotid clamping. Neurologic evaluation and blood pressure control were repeated during carotid clamping. Intraluminal shunts were placed whenever neurologic deterioration occurred.
The risk of cerebral ischemia was correlated with the history of severe ischemia requiring shunt placement during carotid clamping and with the blood pressure level. Statistical analysis were performed using computerized software (Statwiew, brain power Inc, Calabass, CA, USA). Statistical significance was evaluated with the χ2 test. P values less than 0.05 were considered significant.
Comparison between anomalies observed on 3D PC MRI and reported on anatomical studies (Polygons configurations according to Lazorthes classification )
Comparison between predictive risk of shunt need, blood pressure level, frequency and timing of cerebral ischemia
Number of cerebral ischemia: n patients
Timing of cerebral ischemia after clamping (min)
Blood pressure level (mmHg)a
Predictive risk of cerebral ischemia: n patients
Low: 66 Moderate: 40
0.6 [0. 2–1. 5]
In patients with carotid stenosis, pre-operative evaluation of cerebrovascular structure depends more and more on non-invasive vascular imaging modalities such as MR or CT angiography to reduce the morbidity related to conventional angiography, which averages 2% rate [11, 22]. Three dimensional phase contrast MR angiography is known to have a high sensitivity for the flow quantification in intracranial cerebral arteries [15, 17]. Our observations are in line with previous anatomical studies which also confirm the sensitivity for depicting the anatomy of the circle of Willis (Table 2). The present study relates circle of Willis hemodynamic and morphology on pre-operative 3D PC angiograms to immediate transient ischemia during carotid clamping in patients undergoing endarterectomy under locoregional anaesthesia. Statistically significant relation was found between the pre-operative risk and the presence of intraoperative ischemia requiring shunt placement. Concerning the high-risk patients on pre-operative evaluation, our results confirm previous reports in patients with controlateral carotid occlusion [10, 14, 18, 19]. The absence of collateral flow on MRA from the posterior circulation is significantly associated with immediate ischemia.
In the patients who had unilateral stenosis or bilateral stenosis, all high-risk patients (n=5) on pre-operative evaluation (absence of collateral flow in major arterial segment in both anterior and posterior parts of the circle of Willis) had an immediate and severe ischemia independent on blood pressure level. These findings are at the opposite to Lee et al.’s study . They observed ischemia in only one of eight patients with incompleteness in both parts of the circle of Willis. However, the major difference in this study is that the features of anterior and posterior parts of the circle of Willis were categorized according to the vessel size seen on 3D time-of-flight MR angiograms. Contrary to our study in which we evaluated the risk on intravascular flow.
Moreover, the findings observed in the six patients who experienced transient ischemia related to systemic hypotension focuses on the hemodynamic influence during carotid clamping. Consequently, pre-operative evaluation can improve intraoperative management of cerebral perfusion. Significant correlation between intracranial collateral circulation and immediate cerebral ischemia may help prevent brain damage. High-risk patient may benefit from techniques that reduce brain metabolism, such as general anaesthesia with immediate shunt placement to avoid flow-related ischemia . In patients with moderate risk, ischemic vulnerability is dependent on blood pressure stability . We attempt, as recommended by Lawrence et al. to maintain systolic blood pressure at least 20 mmHg above the pre-operative pressure .
In conclusion, 3D PC MR angiography is a noninvasive method which provides useful information about blood flow in the circle of Willis. Evaluation of cross-filling from the contralateral carotid artery can significantly predict cerebral ischemia which requires an intraluminal shunt procedure.
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