(Stroke. 2000;31:1346.)
© 2000 American Heart Association, Inc.
Original Contributions |
Collateral Configuration of the Circle of Willis
Transcranial Color-Coded Duplex Ultrasonography and Comparison With Postmortem Anatomy
From the Departments of Neurology (A.W.J.H., L.C.), and Anesthesiology and Intensive Care (B.F.), University Medical School of Debrecen, Hungary, and the Department of Vascular Surgery, Academic Medical Center, Amsterdam, Netherlands (A.W.J.H., D.A.L.).
Correspondence to Dr D.A. Legemate, MD, Department of Vascular Surgery, G4-107, Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, Netherlands. E-mail d.a.legemate{at}amc.uva.nl
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Abstract |
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Background and Purpose—The anterior communicating artery (AcoA) and posterior communicating arteries (PcoA) of the circle of Willis provide the main route for collateral blood flow in cases of carotid artery obstruction. Transcranial color-coded duplex ultrasonography (TCCD) allows real-time measurement of the collateral function of the AcoA and PcoA. The primary objective of this study was to determine the collateral artery threshold diameters for supplying collateral flow.
Methods—In 12 acute stroke patients with a median age of 75 years (51 to 91 years), the collateral integrity of the circle of Willis as assessed by TCCD and carotid compression tests was compared with their postmortem anatomy. The lengths and diameters of the collateral arteries were measured.
Results—TCCD demonstrated absent anterior collateral flow in 3
patients. In 1 of these patients, absence of anterior cross-flow was
due to an occluded anterior cerebral artery, which was revealed at
autopsy. Absent posterior collateral flow was found in 14 hemispheres.
In 2 of these hemispheres, autopsy revealed a fetal configuration of
the posterior cerebral artery hampering posterior collateral flow. The
median (range) diameters as found at autopsy of the functional (n=19)
and nonfunctional (n=16) collateral arteries of the circle of Willis
were 1.1 (0.4 to 2.0) and 0.5 (0.3 to 0.7) mm, respectively
(P=0.003). PcoA diameters were found to correlate
negatively (
=-0.50, P=0.01) to the diameters of
their accessory P1 segments.
Conclusions—The threshold diameter allowing for cross-flow through the primary collateral arteries of the circle of Willis is between 0.4 and 0.6 mm.
Key Words: cerebral arteries • collateral circulation • hemodynamics • ultrasonography • Doppler • duplex • autopsy
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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The anterior communicating artery (AcoA) and posterior communicating arteries (PcoA) of the circle of Willis provide the main route for collateral blood flow in cases of severe internal carotid artery (ICA) stenosis or occlusion.1 Absence of collateral function due to hypoplasia (Figure 1
) or atherostenosis of
collateral arteries may lead to a higher risk of stroke in patients
with severe ICA occlusive disease.2 3 4 For this reason,
insight into the collateral function of the circle of Willis is of
clinical importance. Transcranial color-coded duplex
ultrasonography (TCCD) combined with common carotid artery (CCA)
compression tests allows real-time evaluation of the collateral ability
of the circle of Willis.5 6 Comparative studies between
transcranial ultrasound techniques and angiography have
demonstrated that transcranial ultrasound has a high level
of sensitivity and specificity in the assessment of the patency of the
AcoA and PcoA.5 7 8 This is the first study comparing
hemodynamic data on the collateral function of the
circle of Willis with anatomic findings obtained at autopsy. One goal
was to investigate whether the commonly used ultrasound criteria for
the definition of AcoA and PcoA collateral patency6 7 9 10
reflect true differences in arterial size, but our main
goal was to determine the collateral artery threshold diameters for
supplying collateral flow through the circle of Willis.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Patients
The study was performed at the Stroke Unit of the Department of Neurology and at the Department of Pathology of the Debrecen University Medical School in Hungary after being approved by the local ethics committee. This study was part of a larger study performed to assess the collateral function of the circle of Willis in acute stroke patients by means of TCCD and CCA compression tests. Because of the severity of stroke, a number of patients included in this larger study died. The circle of Willis of these patients was removed at autopsy, and the size of the collateral arteries was measured.
TCCD Examination
For all ultrasound examinations, a Hewlett Packard SONOS 2000
duplex scanner was used. Duplex scanning of the extracranial
brain-supplying arteries (4.5- to 5.5-MHz transducer) preceded
transcranial investigation. TCCD was performed with a 2.0-
to 2.5-MHz phased-array probe. Examination of the main trunk of the
middle cerebral artery (M1), the precommunicating part of the
anterior cerebral artery (A1), and the precommunicating part of the
posterior cerebral artery (P1) through the temporal bone window was
performed in a standard manner, details of which are reported
elsewhere.11 In cases of unilateral window failure,
examination of the A1 and P1 through the opposite temporal bone window
was attempted.
For reliable assessment of AcoA and PcoA functional patency in patients with no ICA occlusive disease, CCA compressions are needed.7 Collateral supply through the AcoA to the M1 was demonstrated by reversal of blood flow in the A1 ipsilateral to the compressed CCA, combined with an enhanced blood flow velocity in the contralateral A1. Both A1 segments were routinely investigated with ipsilateral and contralateral CCA compression. Functional patency of the PcoA was defined by a peak systolic velocity (PSV) increase of >20% in the P1 ipsilateral to the compressed CCA, this value being twice as much as expected from normal variation and measurement error.7 9 The PSV increase was always measured over the highest peaks on the Doppler spectrum. If the PSV increase in the P1 was <20%, the PcoA was defined as hypofunctional. Velocity measurements were taken proximally in the A1 and P1 with the sample volume set as narrow as possible. Measurements in the P1 were taken as close as possible to the top of the basilar artery.
Carotid Occlusion
In patients with unilateral ICA occlusion, AcoA function was
proven if the A1 ipsilateral to the occluded ICA demonstrated
spontaneous reversed flow. PcoA function on the side of the ICA
occlusion was demonstrated if the mean blood flow velocity in the
ipsilateral P1 was more than the mean blood flow velocity+2
SD10 from an age- and sex-matched group of atherosclerotic
patients with no ICA occlusive disease or cerebral
symptoms.12 PcoA function contralateral to the occluded
ICA was tested by compression of the nonoccluded CCA. The presence of
leptomeningeal collaterals could not be assessed by TCCD.
Compression Tests
To avoid a systemic cardiovascular reaction,
compressions of the CCA were applied low in the neck just proximal to
the sternal head of the clavicle for a maximum of 4 cardiac cycles. To
minimize the risk of embolus, compressions were performed only in those
patients with no atherosclerotic plaques in the proximal CCA as judged
by the B-mode image of the duplex scan. To ensure the efficacy of the
compression, a pulse oximeter (Eagle 3000, Marquette), which generated
pulse tracings on a separate monitor, was attached to the earlobe on
the same side as the compressed artery. Flattening of this pulse wave
indicated cessation of blood flow through the CCA and thus an adequate
compression.
Autopsy
The circle of Willis was dissected out after removal of the
brain from the cranial cavity. All minute branches arising from the
main vessels were carefully cut away. Pathology of the cerebral vessels
was judged by a neuropathologist familiar with the medical history of
the patient. A rough drawing of the anatomy was made to prevent
mixing up the right and left sides of the circle of Willis during
further procedures. Blood was carefully washed out from the circle of
Willis with isotonic saline. Photographs were taken before and after
dissection from the brain and after the blood was washed out.
Measurements
To measure the lengths and diameters of the arteries, the circle
of Willis was put on a glass plate. After the arteries had been
straightened out, the lengths of the PcoAs and A1 segments were
measured with a ruler with a millimeter scale. These measurements were
rounded off to whole millimeters. For the measurement of the length of
the AcoA and P1 segments, a transparent 10x10-cm sheet with a
millimeter scale was placed on top of the circle. Under a microscope
with a x10 magnification, AcoA and P1 lengths were measured with an
approximation of 0.1 mm. For the measurement of the
arterial diameters, a second glass plate was clamped on top
of the first, with the circle of Willis and the transparent sheet in
between. Sufficient force was applied to induce complete obliteration
of the vessel lumina but not to flatten the arterial
walls.13 14 If atherostenosis was found to be
present in the middle cerebral artery or the basilar artery, this
was cut away first. This was done to allow application of equal
pressure on all 4 corners of the glass plates for obliteration of the
vessel lumina. The set of glass plates was put under the microscope
again, and in this way, readings representing half of the
arterial circumference were obtained. Measurements were
taken at proximal, middle, and distal sites of the AcoA, A1, PcoA, and
P1 and approximated to the nearest 0.1 mm. The narrowest part of
each artery was used for further analysis, because we
considered that the width at this part determined the collateral
ability. Assuming the arteries to be circular, their external diameter
could be calculated by the formula diameter=circumference/
. All data
in this study are reported as medians with ranges.
Nonparametric tests were used to analyze the data.
Significance was assumed at the 5% level.
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Results |
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The study comprises TCCD and autopsy data from 12 patients, 8 men and 4 women, with a median age of 75 years (51 to 91 years). The median time between TCCD and death was 53 hours (4 hours to 36 days), and the median time between death and autopsy was 22 hours (5 hours to 3.5 days). Six patients had a unilateral ICA occlusion without significant contralateral ICA stenosis (Table 1
). In Table 2, the TCCD results are listed together
with the arterial sizes obtained at autopsy. With the
exception of patient 3 (thrombus in the left anterior cerebral artery),
no patients were found to have severely stenosed or occluded collateral
arteries. Patient 10 showed a large thrombus occluding the left middle
cerebral artery.
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Anterior Part of Circle of Willis
In 2 patients (patients 2 and 3) with ICA occlusion, spontaneous
collateral flow through the anterior part of the circle of Willis
toward the M1 was absent. In patient 3, the ipsilateral A1 could not be
visualized by TCCD, and the velocity in the contralateral A1 was not
enhanced. In patient 12, who had no significant ICA stenosis,
cross-flow through the AcoA could not be provoked by CCA compressions.
At autopsy, a single AcoA was found in all cases; no duplications or
triplications14 15 were found. Three AcoAs had a typical
hourglass form, with a smaller diameter in the midsection of the artery
than at the site of junction with the A1 segments. In 1 case, a third
anterior cerebral artery originated from the AcoA. All A1 segments had
a diameter of 
1.0 mm (Table 2
). Patients 2 and 12 showed
very small AcoA diameters, 0.3 and 0.5 mm, respectively, which had
impeded interhemispheric cross-flow. In patient 3, a large thrombus in
the left anterior cerebral artery was found that blocked outflow from
the AcoA, explaining the absence of anterior cross-flow on TCCD. The
median AcoA diameter in patients with a nonfunctional anterior
collateral pathway (n=2, patient 3 excluded) was 0.4 mm, and the
median AcoA diameter of patients with a functional anterior collateral
pathway (n=9) was 1.1 mm (Table 2).
Posterior Part of Circle of Willis
In 14 hemispheres, spontaneous collateral flow through the
posterior part of the circle of Willis either was absent or could not
be provoked by CCA compressions. In 2 hemispheres (right hemispheres of
patients 6 and 8), we were able to measure the blood flow velocity in
the PcoA directly. Instead of reversal of flow, ipsilateral CCA
compression caused a PSV decrease of 89% and 61%, respectively.
Autopsy revealed a fetal configuration of the posterior cerebral artery
(Figure 2), consisting of a wide PcoA
(diameter of 2.1 mm in both) combined with a narrow ipsilateral P1
(0.5 and 0.6 mm, respectively; Table 2). PcoAs classified
as functional by TCCD had significantly larger diameters than
nonfunctional PcoAs, 0.9 versus 0.6 mm (P=0.008). PcoA
diameters were found to correlate negatively (=-0.50,
P=0.01, Spearman’s rank-order correlation) to the diameters
of their ipsilateral P1 segments (Figure 3).
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The diameters of all functional collateral arteries of the circle of
Willis were significantly larger than the diameters of all
nonfunctional collateral arteries (including the diameters of the P1
segments of the 2 fetal posterior cerebral arteries), 1.1 versus
0.5 mm (P=0.003, Figure 4).
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Despite the limited number of patients and the relatively simple method of measuring the external arterial diameters, we believe our results to be unique, because this is the first time this type of information has been obtained from a study population as opposed to a model. The TCCD criteria for determining the collateral function of the circle of Willis used in the literature6 7 9 10 truly correspond with significant differences in arterial diameter, despite the slight overlap in diameters of functional and nonfunctional collateral arteries (Figure 4
). Overall, the arterial diameters
found in this study are very similar to the diameters reported by
Baptista14 and by Murray16 but slightly
smaller than the diameters reported by other
authors.15 17 18 19 Studies should be compared only with
caution because of the difference in study populations and methods of
measurement. How far postmortem arterial diameters are
equal to in vivo diameters is unknown. The absence of perfusion
pressure and possible shrinking of the arterial wall due to
muscle cell decay might result in the measurement of smaller diameters.
Whether this process had a significant influence on our results remains
speculative. Because of the nature of our study population, we assume
that we found smaller diameters than some other
authors.15 17 18 19 It has been shown that stroke patients
have less well-developed circles of Willis because of a higher
prevalence of thread-like vessels.20 21 22 Another reason is
that we used the narrowest parts of the arteries in the
analysis, because these parts probably determine collateral
ability. The AcoA in particular can have a widely differing diameter
along its course because of an hourglass (3 cases in this study) or
triangular shape.15 23 The reported negative correlation
between PcoA and P1 diameters (Figure 3) refutes the findings of
Puchades-Orts et al24 and Orlandini et al25 ;
however, Kamath18 found the same negative correlation, and
in the detailed analysis by Hillen,13 comparable
correlation coefficients were found.
An important finding in this study is that AcoAs and PcoAs with a
diameter considerably less than 1 mm can still supply collateral
flow, which can be detected by TCCD. Our results indicate that the
threshold diameter for collateral function lies between 0.4 and
0.6 mm (Table 2, Figure 4). Because of its greater
length, it seems valid that the PcoA threshold diameter for collateral
function is slightly higher than the AcoA threshold diameter, because
the resistance to blood flow is higher in longer vessels as a result of
the larger area of endoluminal vessel wall. Our data (Table 2)
endorse this hypothesis, but numbers are too small to draw a definite
conclusion. A threshold diameter for supplying collateral flow of
between 0.4 and 0.6 mm is probably also applicable to the P1. In
the hemispheres with a fetal posterior cerebral artery, the ipsilateral
P1 diameters were 0.5 and 0.6 mm, respectively. CCA compression
could not provoke a flow reversal in their wide accessory PcoAs
(diameter of 2.1 mm in both).
Up to now, a threshold of 1 mm to define hypoplasia or inadequacy of collateral vessels has been widely used in anatomic studies.17 19 22 26 27 28 29 30 In clinical studies, an increased risk of ischemic cerebral infarction after ICA occlusion2 and an increased risk of brain stem ischemia after basilar artery occlusion31 have been associated with PcoA diameters <1 mm. Only in a minority of studies was a threshold of 0.5 mm used to define hypoplasia of collateral arteries.18 20 The varying definitions of hypoplasia of circle of Willis collaterals and the different populations studied have resulted in a large variability of anomalous or "incomplete" circles of Willis throughout the literature.14 18 20 22 26 27 32 The prevalence of the "normal" textbook polygon ranges from 21% to 76%. However, the arbitrary diameters of 0.5 or 1 mm have never been discussed in terms of functional significance. Fluid-dynamic mathematical models have been developed to study the effect of collateral artery diameter on cerebral blood flow after ICA stenosis or occlusion.33 34 35 Cassot et al33 and Dickey et al34 independently showed that the smallest luminal diameter allowing for cross-flow through the AcoA was 0.4 mm. Moreover, Dickey et al found that in patients with ICA occlusion and a well-functioning AcoA, the collateral supply from the PcoA to the deprived hemisphere fell to zero when its diameter was set at levels <0.5 to 0.6 mm.34 The results of our study confirm those of Cassot et al and Dickey et al. Therefore, we suggest that the term hypoplasia be reserved for those vessels that cannot supply collateral flow. Our results indicate that in practice, communicating arteries with a diameter <0.5 mm should be labeled hypoplastic. This may result in greater uniformity in radiological studies on the collateral integrity of the circle of Willis. This is important, because there is increasing evidence that a well-functioning, complete circle of Willis plays a protective role against cerebral ischemia in patients with carotid artery occlusive disease.2 3 4 9 Furthermore, it is quite possible that in the future, particularly in patients with an asymptomatic severe carotid stenosis, the decision whether to operate or not will be influenced in part by the collateral ability of the circle of Willis.36 Therefore, uniformity in the definition of a complete circle of Willis is a first requirement.
Although TCCD can be used to assess the presence of cross-flow through the AcoA and PcoA, it should be kept in mind that volumetric blood flow cannot be measured by this technique. A TCCD diagnosis that collateral flow to a deprived hemisphere is present or can be provoked does not guarantee that this is sufficient to protect the hemisphere against ischemia. Moreover, the presence of leptomeningeal collaterals, which might be vital for hemispherical perfusion in some cases, cannot be assessed by TCCD. One method of assessing the amount of collateral flow via the AcoA and/or PcoA to a deprived hemisphere is measurement of the proportional velocity decrease in the middle cerebral artery after carotid compression6 or carotid cross-clamping during carotid endarterectomy.37 38 The degree of velocity decrease is correlated with the frequency of electroencephalographic changes and the development of cerebral ischemia during carotid clamping.37 38
In summary, the ultrasound criteria used in the literature to discern functional from nonfunctional collateral arteries reflect significant differences in arterial size. The arterial threshold diameter allowing for collateral flow through the circle of Willis lies between 0.4 and 0.6 mm. This threshold diameter might be used in prospective studies evaluating the influence of the collateral ability of the circle of Willis on the development of ischemic strokes in patients with carotid artery occlusive disease.
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Acknowledgments |
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The authors wish to thank Csáti Géza and Síró Péter for their practical assistance during the duplex examinations. Drs Bereczki Dániel, Fekete István, Csepány Tünde, and Valikovics Attila are thanked for enlisting patients into the study. We thank Professors Hegedüs Katalin, Nemes Zoltán, and Gomba Szabolcs for their technical advice at autopsy.
Received January 28, 2000; revision received March 23, 2000; accepted March 23, 2000.
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