ADD-ONS / ANATOMY

Anatomical variants of cerebral arteries

David Goldemund M.D.
Updated on 07/09/2024, published on 24/11/2022

Overview of anatomical variants

Variants in the arrangement of blood vessels supplying the brain are quite common. While they are often of minor clinical significance, their knowledge and recognition are essential if surgical or endovascular treatment in the head or neck is considered.  Unfavorable vascular anatomy complicating endovascular treatment is discussed separately.

Anomalous origin
  • common origin
    • PCA+SCA
    • PICA+AICA
    • TBC+left CCA
    • left-sided brachiocephalic trunk
    • common origin of both CCAs
  • infundibulum (enlarged origin, e.g., AComA, PComA)   Posterior communicating artery infundibulum
  • anomalous origin due to persistent embryonic circulation
    • fetal type PCA (partial x complete, uni/bilateral)
    • cavernous origin of the ophthalmic artery
Anomalous number of vessels
  • increased number
    • duplicated MCA/ AComA / SCA
    • accessory MCA
    • > 3 MCA trunks
    • A2/M2 trifurcation
  • decreased number
    • agenesis or aplasia (ICA, AComA, A1, A2, PComA, Percheron artery – type 2)
    • azygos ACA
Anomalous vessel morphology
  • hypoplasia (ICA, CCA, A1, A2, PComA, VA)
  • fenestration (often BA)
  • early bifurcation (typically MCA)
  • tortuosity, kinking, coiling  Internal carotid artery anomalies
Persisting communicants and aberrant vessels
  • persistent primitive olfactory artery
  • aberrant lateral pharyngeal artery
  • ICA-BA anastomoses
    • persistent trigeminal artery (PTA)
    • persistent primitive hypoglossal artery (PPHA)
    • persistent otic artery
    • proatlantal intersegmental artery (PIA) (may as well arise from CEA or CCA)
  • ICA-ECA anastomosis
    • persistent stapedial artery

Clinical significance of anatomical variants

Distinguish anatomical variations from pathology, as it has implications for the supplied area and surrounding structures
  • distinguish infundibulum from an aneurysm, the ICA hypoplasia from dissection
  • abnormal findings on CTP 
    • relative hypoperfusion in the PICA territory due to AV hypoplasia
    • slow perfusion of the occipital lobes in the absence of PComA
  • trigeminal compression and neuralgia in the persistent trigeminal artery
  • tinnitus in the persistent stapedial artery
Hemodynamic consequences
  • improved or impaired collateral circulation
  • atypical stroke presentation
    • the PCA territory infarct due to the ICA stenosis (via fetal PCA)
    • bihemisferal ACA infarction with occlusion of dominant A1-A2 segment or azygos artery occlusion
    • bithalamic lesion caused by the Percheron artery occlusion
Association with other anomalies
  • aneurysms, AVMs, and other vascular malformations
  • developmental deformities  (e.g., migration disorders in patients with  azygos artery)
Planning a surgical or endovascular procedure
  • exclude carotid-basilar communication before the WADA test (risk of unconsciousness and respiratory arrest during the test )
  • ↓ risk of fatal bleeding
    • exclude persistent trigeminal artery before initiating procedures near the sella turcica
    • exclude stapedial artery before ENT procedures
    • exclude aberrant lateral pharyngeal arteries prior to surgery near the pharynx

Hypoplasia and absence of the carotid artery

Absence of ICA (agenesis and aplasia)

  • the term absence includes:
    • agenesis – absence of the artery + absence of the carotid canal
    • aplasia – diminutive carotid canal is present   (Given, 2001)
  • rare (approx. 0.01%), mostly unilateral anomaly; a few bilateral cases have been reported [Midkiff, 1995]  [Alexandre, 2016]
  • patients with ICA agenesis are usually asymptomatic due to collateral circulation
    • via AComA and/or PComA
    • connections from the ECA
  • ICA aplasia can be associated with the following conditions:
    • corpus callosum agenesis
    • neurofibromatosis
    • aortic coarctation
    • cardiac ventriculoseptal defects
    • cerebral hemiatrophy, ear malformations, facial nerve palsy
    • hemangiomas
    • aneurysms

ICA hypoplasia

  • diffuse narrowing of the ICA (usually accompanied by CCA hypoplasia) along its entire course, with continuous but markedly decreased peak systolic velocities on ultrasound  Hypoplastic internal carotid artery on CT angiography
  • hypoplasia is usually associated with a narrowed carotid canal (best visualized on CTA)
Hypoplastic right ICA on MRA and ultrasound. CTA shows a hypoplastic carotid canal.
Hypoplastic CCA and ICA on MRA. Doppler ultrasound shows presteal phenomenon in CCA
The left ICA hypoplasia

Hypoplasia of the right CCA and ICA (CCA 3.4 mm)
Hypoplastic CCA with aplasia of ICA. The ipsilateral MCA is supplied via a massive PComA
Internal carotid artery hypoplasia (B mode)

Hypoplasia/aplasia of the vertebral artery

Aplasia

  • rare anomaly (~1%)
  • contralateral VA is compensatorily enlarged  (diameter >4 mm)
  • only the vertebral vein can be detected on the affected side

Hypoplasia

  • relatively common anatomical variety (~1-6%; more often on the right side)  Hypoplastic left verterbal artery on ultrasound Hypoplastic left vertebral artery (2.4 mm)
  • lumen diameter < 2-3 mm (different cut-offs reported)
  • ultrasound findings: PSV < 40cm/s, increased peripheral resistance (↑ PI, RI) but non-zero diastolic flow (adjust scale properly)
  • compensatory enlargement of the contralateral VA accompanies hypoplasia/aplasia
  • hypoplastic vessel may terminate in PICA  (~ 0.2%) Hypoplastic vertebral artery (VA) terminating as PICA. Hypoplastic A1 segment and fetal PCA.

Fenestrations and duplications

  • duplication – two separate arteries instead of one
  • fenestration – the partial splitting of an artery (each part has its own media and intima, adventitia may be shared)
Proximal basilar artery fenestration
The fenestration of the right vertebral artery. Repeated CTA ruled out short dissection
Basilar artery fenestration on CTA
Fenestration in the proximal segment of the basilar artery on MRA

Variants of the circle of Willis

  • a complete circle of Willis is present in < 30% of patients; individual anatomical variants are common
    • different diameters of the vessels on the right and left side
    • predominance of carotid or vertebral blood flow
    • absence/hypoplasia of one or more arteries
The circle of Willis variants

Azygos anterior cerebral artery

  • the persistent embryonic middle artery supplying the corpus callosum is a rare vascular anomaly with a prevalence of 0.2-4% [Dimmick, 2009]
  • both ACA territories are supplied by a single A2 trunk ⇒ its occlusion causes a bihemispheric ischemia
The anterior azygos artery on CTA
The anterior azygos artery on MRA

Bihemisferic ACA

  • hypoplasia of one A2 segment; contralateral ACA supplies both hemispheres
  • prevalence 2-7%
  • hypoplastic A2 segment distinguishes this variety from the anterior azygos artery; the clinical significance is similar
Bihemisferic ACA with the left A2 segment hypoplasia (green arrow)

ACA trifurcation

  • presence of three A2 segments arising from AComA
  • prevalence 2-13%
Trifurcation of the anterior cerebral artery (ACA)
Triple A2 segements

A1 segment hypoplasia/aplasia

  • asymmetry of A1 segments is common
  • hypoplasia (width < 1.5 mm) occurs in up to 10% of cases, complete absence (aplasia) in 1-2%
  • AComA is usually prominent
  • occlusion of the contralateral (normal) A1 segment can result in bihemispheric infarction
Hypoplastic A1 segment of the anterior cerebral artery
Aplasia of the right A1 segment
Left A1 segment aplasia (CTA)
Aplasia of the right A1 segment of the anterior cerebral artery (ACA)

Anterior communicating artery (AComA) hypoplasia/aplasia

  • AComA may not be apparent on angiography; however, this does not necessarily indicate its absence
  • the prevalence of true aplasia is about 5% based on surgical findings

Accessory and duplicated middle cerebral artery

  • an accessory MCA arises from the A1 segment of the ACA and runs parallel to the M1 segment of the MCA; prevalence is 2-3%
  • a duplicated MCA arises from the distal ICA
  • both the recurrent Heubner artery and the accessory MCA represent primitive anastomoses with the piriform cortex. The accessory MCA disappears during normal development.
  • clinical significance
    • aneurysms may form in the proximal segment of the accessory ACM
    • accessory MCA may serve as a collateral circulation during occlusion of the main MCA
Accessory and duplicated MCA

Variations of MCA branching

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Fetal PCA

  • during early development, the PCA arises from the ICA; persistence of this configuration is termed a fetal-type PCA
  • PComA typically has a larger caliber compared  to the P1 segment of the PCA
  • classification of fetal PCA
    • complete – P1 segment is absent
    • partial – P1 segment is hypoplastic and has a smaller diameter than the PComA
  • prevalence approx. 10-26%
  • bilateral fetal PCA may be associated with hypoplasia/aplasia of the terminal segment of the basilar artery   Hypoplasia of the terminal segment of the basilar artery (from the SCA origin). Bilateral fetal type of PCA is present
Fetal posterior cerebral artery (PCA) on CTA
Bilateral fetal PCA (CTA)
Fetal PCA

Percheron artery

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Common origin of PCA and SCA

  • common PCA-SCA trunk is present in 2-22%
  • clinically insignificant finding
Common origin of the PCA and the SCA

Persistent communication between the ICA and the BA

  • a craniocaudal series of primitive vessels connects the anterior to the posterior circulations during embryogenesis
  • the most superior of these connections persists as the PComA
    • in approximately 20% of patients, the embryonic, fetal condition (with the PCA being derived primarily from the ipsilateral ICA via a prominent PComA and a small ipsilateral P1 segment) persists (fetal origin of the PCA)
  • other primitive vessels disappear or transform into others
  • rarely, primitive vessels persist after birth (prevalence ~ 0.1-1%)
    • Persistent Primitive Trigeminal Artery (PPTA)
    • Persistent otic artery
    • Persistent hypoglossal artery
    • Proatlantal Intersegmental Artery (PIA)
  • persistent carotid-vertebrobasilar anastomoses are important to recognize during angiography for endovascular and surgical planning
Persisting ICA-BA communicants

Persistent trigeminal artery (PTA)

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Persistent otic artery

  • rarely visualized angiographically
  • passes from the petrous segment of the ICA to the basilar artery

Persistent primitive hypoglossal artery (PPHA)

  • PPHA is the second most common variant, with a prevalence of 0.02-0.10%
  • arises from the ICA at the level of C1-3 and passes intracranially via the hypoglossal canal (not through the foramen magnum)  Hypoglossal canal with persistent hypoglossal artery
  • it eventually anastomoses with the basilar artery
  • often asymptomatic
Persistent primitive hypoglossal artery (PPHA) on CTA

Proatlantal intersegmental artery (PIA)

  • type I: (~55%)
    • arises from the ICA
    • corresponds to the first segmental artery
    • does not pass through intervertebral foramina
  • type II: (~40%)
    • arises from the ECA
    • corresponds to the second segmental artery
  • it passes through the foramen magnum (as opposed to the hypoglossal artery, which enters intracranially via the hypoglossal canal)
Proatlantal intersegmental artery on CTA

Persistent communication between ICA and ECA

Persistent stapedial artery (PSA)

  • temporary embryonic communication between the ECA and ICA
  • persists in ~ 0.5% of the population
  • clinical significance:
    • may cause pulsatile tinnitus
    • may be misdiagnosed as a glomus tumor on CTA
    • may complicate otolaryngological (ENT) surgery and cause severe bleeding
  • management varies from observation to surgical intervention, depending on the symptomatic burden and associated pathology

Other anomalies

  • left-sided brachiocephalic trunk (BCT)
  • bilateral absence of ECA (often associated with PIA)
  • persistent first cervical intersegmental artery
    • rare; can cause compressive symptoms or be a surgical consideration in neck operations
  • ACE arising from the aortic arch in combination with ipsilateral PIA
    • may complicate surgical or interventional procedures
  • common origin of the BCT and the left CCA from the aortic arch  Common origin of brachiocephalic trunk and the left common carotid artery (CTA)  Common origin of brachiocephalic trunk and the left common carotid artery (CTA)

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Anatomical variants of cerebral arteries
link: https://www.stroke-manual.com/anatomical-variants-of-cerebral-arteries/