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POROČILO O PRIMERU / CASE REPORT POROČILO O PRIMERU / CASE REPORT
Članek prispel / Received
23. 10. 2023 
Članek sprejet / Accepted
15. 11. 2023
Abstract
Purpose: The cerebral arterial cir-
cuit provides collateral circulation 
to the brain and protects the brain 
from ischemia, by sustaining tissue 
perfusion in case of reduced or impa-
ired blood flow through its branches. 
Although many random anatomi-
cal variations in the circle of Willis 
(CoW) are of minor importance, 
some increase the risk of ischemic 
events, aneurysm, and atheroscle-
rosis, and play an important role in 
endovascular and surgical treatment 
planning. 
Case presentation: We report a 
rare arterial variation of the CoW 
diagnosed by computed tomography 
angiography in a patient with ische-
mic stroke. We found aplasia of the 
A1 segment of the left anterior cereb-
ral artery and ipsilateral hypoplasia 
of the P1 segment of the left posterior 
cerebral artery. 
Izvleček
Namen: Cerebralni arterijski krog za-
gotavlja kolateralni obtok v možganih 
in ščiti možgane pred ishemijo, saj v 
primeru zmanjšanega ali oslabljenega 
pretoka krvi vzdržuje perfuzijo tkiva 
po njegovih vejah. Čeprav so številne 
naključne anatomske variacije v Wil-
lisovem krogu (CoW) manj pomem-
bne, pa nekatere med njimi poveču-
jejo tveganje za ishemične dogodke, 
anevrizme in aterosklerozo ter imajo 
pomembno vlogo pri načrtovanju 
tako endovaskularnega kot kirurške-
ga zdravljenja.
Predstavitev primera: Poročamo 
o primeru redke arterijske varia-
cije CoW, ki je bila diagnosticirana 
z računalniško tomografsko an-
giografijo pri bolniku z ishemično 
možgansko kapjo, ki je v literatu-
ri redko opisana. Ugotovili smo 
aplazijo segmenta A1 leve sprednje 
možganske arterije  in ipsilateralno 
Ključne besede:  
arterijske variacije, možganske 
arterije, Willisov krog, možganska 
cirkulacija, možganska kap     
Key words: 
arterial variations, cerebral arter-
ies, circle of Willis, brain circulation, 
stroke
Redka anatomska variacija Willisovega kroga 
pri bolniku z ishemično možgansko kapjo
A rare anatomical variation of the circle of 
Willis in a patient with ischemic stroke
Avtor / Author Lidija Kocbek Šaherl
1
,
 
Klemen Lah
2
Ustanova / Institute 
1 
Univerza v Mariboru, Medicinska fakulteta, Inštitut za anatomijo, histologijo in 
embriologijo, Maribor, Slovenija; 
2 
Splošna bolnišnica Celje, Urgentni center Celje, Celje, 
Slovenija;
1 
University of Maribor , Faculty of Medicine, Institute of Anatomy, Histology and Embryology, 
Maribor, Slovenia; 
2 
General Hospital Celje, Emergency Department Celje, Celje, Slovenia;
Naslov za dopisovanje /  
Correspondence
Lidija Kocbek Šaherl
University of Maribor, Faculty of 
Medicine, Institute of Anatomy, 
Histology and Embryology
Taborska ulica 8, SI-2000 Maribor, 
Slovenia
+386 2 234 58 61
lidija.kocbek-saherl@um.si
https:/ /doi.org/10.18690/actabiomed.257, CC BY 4.0
© 2023 Avtor(ji) / The Author(s)
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INTRODUCTION
Reduced blood flow to the brain activates compensatory 
mechanisms via the collateral supply of the circle of 
Willis (CoW), which protects the brain from ischemia 
and infarction (1). Variations in CoW lead to changes 
in cerebral hemodynamics and are associated with 
increased risk of stroke, aneurysm rupture, white matter 
disease, migraine, atherosclerosis, and vasospasm after 
subarachnoid hemorrhage. CoW variations determine 
the type and method of treatment, may affect the success 
of the procedure due to potential complications, and 
may influence the safety and outcome of treatment (2).
Here, we present a case of a patient with ischemic stroke 
with a rare variation of the CoW and address the role 
of CoW variations as important risk factors for stroke. 
CASE PRESENT A TION
A 71-year-old right-handed man was brought to the 
emergency department with suspicion of ischemic stroke, 
as he suddenly developed right-sided hemiplegia and 
central facial palsy, dysarthria, and diplopia when looking 
to the right, homonymous hemianopia, and the condition 
progressed. Glasgow Coma Scale (GCS) was 15/15, and 
he was conscious and oriented. Tendon reflexes on the 
right side were less responsive and right-sided extensor 
plantar reflex was present. The electrocardiogram 
recorded a normal sinus rhythm. His past medical history 
included hypertension, hyperlipidemia, benign prostatic 
hyperplasia, symptomatic epileptic seizures, alcohol 
dependence syndrome, and toxic liver damage. He had 
been a smoker for 30 years. The patient had no history of 
ischemic stroke or transient ischemic attack, head injury, 
or trauma. His family history of stroke was negative. 
Initial computed tomography and computed 
tomography angiography revealed left parieto-
occipital subacute ischemia, with reduced perfusion 
time, volume, and flow. A 90% occlusion of the left 
internal carotid artery (ICA) was found. The left 
vertebral artery (VA) was subtotally narrowed in 
the V4 segment. The A1 segment of the left anterior 
cerebral artery (ACA) was absent (Figures 1 and 
2). The P1 segment of the left posterior cerebral 
artery (PCA) was hypoplastic (Figures 1 and 2). The 
PCA was filled distally via a strong left posterior 
communicating artery (PCoA).
DISCUSSION
The CoW is complete in less than half of the population 
due to developmental anomalies. Twice as many 
complete circles are in the anterior half, whereas only 
one-third are complete in the posterior half of the 
circle (3). Aplasia of the A1 segment is found in 1% of 
people (1). Thus, both A2 segments are supplied via 
the available A1 segment, which covers the entire ACA 
territory (4). Variation is crucial between intravascular 
hipoplazijo segmenta P1 leve zadnje možganske arterije.
Zaključek: Za nevrokirurge, nevrologe in nevrora-
diologe je podrobno poznavanje številnih anatomskih 
variacij v arterijski oskrbi možganov ključnega pome-
na za postavitev pravilne diagnoze in izbiro ustreznega 
zdravljenja. Tip CoW prav tako vpliva na resnost in izid 
možganske kapi, pri čemer je nepopoln Willisov krog 
povezan s težjimi stanji (višji rezultati po National In-
stitutes of Health Stroke Scale, hujši nevrološki izpadi in 
slabši nevrološki izidi) in slabšo prognozo.
Conclusion: For neurosurgeons, neurologists, and neuro-
radiologists, detailed knowledge of the numerous anatomi-
cal variations in the arterial supply to the brain is crucial 
to make a correct diagnosis and choose the appropriate 
treatment. The type of CoW variation influences stroke 
severity and outcome, with an incomplete CoW related to 
more severe conditions (higher National Institutes of He-
alth Stroke Scale scores, more severe neurological deficits, 
and worse neurological outcomes) and poor prognosis.
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interventions in the anterior communicating artery 
territory or ischemic events, as it increases the risk and 
extent of brain ischemia in the frontal lobe (5). 
Two-thirds of cases have more common anatomical 
variants of CoW in the posterior region (4). When the 
P1 segment is hypoplastic, the main blood supply to 
the occipital lobe comes from the ICA, and the PCA 
is filled distally via a stronger PCoA, as in our patient. 
The variation is found unilaterally in 11%–29% and 
bilaterally in 1%–9% (6).
Studies on CoW variations are numerous and vary 
widely depending on age, sex, health status, side and 
number of vessels affected, location within the CoW, and 
method for data collection (4). Despite the high degree of 
CoW variability described and classified into 22 different 
types by Riggs, Lazorthes, and Krabbe–Hartkamp (2), to 
our knowledge, there is no research on the incidence of 
simultaneous A1 aplasia and ipsilateral P1 hypoplasia in 
the literature. He et al. (7) found a similar case with left 
A1 aplasia but contralateral hypoplasia of the P1 segment 
ipsilateral to ICA occlusion. When the P1 segment is 
hypoplastic, the main blood supply to the occipital lobe 
comes from the ICA. In P1 hypoplasia, flow through 
the dilated PCoA is preserved in 5–20% of cases, as in 
our patient (9). When large artery stenosis or occlusion 
is present in patients with variations in the posterior 
portion of the CoW, the brain tissue is vulnerable to 
hypoxicity because of inadequate compensation by 
collateral blood flow. Variations, incompleteness, and 
hypoplasia of any part of the CoW are 1.4 times more 
likely to cause ischemic stroke compared with patients 
with normal anatomy. Poor CoW integrity is associated 
with frequent stroke recurrence in patients with severe 
intracranial atherosclerotic stenosis and poor prognosis 
in acute ischemic stroke. Recent studies suggest that 
CoW variants affect the development and progression 
of intracranial atherosclerosis (10).
The CoW is strongly associated with cerebral 
infarction in patients with severe stenosis of the ICA 
or vertebrobasilar artery (10), as in our case, as the 
Figure 1. Scheme of anatomical variation of the Willis polygon with 
absent A1 segment of the left ACA and hypoplastic P1 segment of 
the ipsilateral PCA. ACA – anterior cerebral artery; PCA – posterior 
cerebral artery.
in a patient with hypertension, who 
complained of long-term dizziness, 
fatigue, depression, insomnia, 
and memory loss. Sonobe et al. (8) 
reported a case of A1 aplasia and 
bilateral P1 hypoplasia in an infant 
who suffered a massive infarction 
during extracorporeal life support 
because of a rare anatomic variation. 
An incomplete anterior part of the 
CoW combined with an incomplete 
posterior part of the CoW results in 
anterior circulation stroke (4).
The effectiveness of compensation of 
the CoW depends on the presence of 
its vessels and their size. A complete 
CoW has an effective intracranial 
collateral circulation and a greater 
compensatory capacity in cerebral 
ischemia, but variations in the CoW 
cause changes in hemodynamics 
(4). In the case of ICA occlusion 
and A1 hypoplasia or aplasia, 
collateral supply to the ACA and 
middle cerebral artery occurs 
through reverse flow in the PCoA 
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Figure 2. 3D computed tomography angiography reconstruction of the cerebral arteries – anterolateral view. 
Absence of the proximal segment (A1) of the left ACA (arrow) and hypoplastic proximal segment (P1) of the 
left PCA (asterisk). ACA – anterior cerebral artery; PCA – posterior cerebral artery.
patient had 90% occlusion of the left ICA and subtotal 
stenosis of the V4 segment of the left VA. Henderson and 
Eliasziw (11) found that the rate of hemispheric cerebral 
infarction or transient cerebral ischemic attack was 
significantly lower in patients with >70% ICA stenosis 
in the perioperative or long-term treatment period when 
the CoW developed normally.
CONCLUSIONS
Understanding the anatomical variations in the arterial 
supply to the brain is important for planning surgical 
or interventional treatment of ischemic attacks and 
strokes, and has a significant impact on disease outcome 
and patient recovery. 
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