Download Rare Neurovascular Variants Arising from the Internal Carotid Artery

EXTRACRANIAL VASCULAR
CME
ABBREVIATIONS KEY
Rare Neurovascular Variants Arising
from the Internal Carotid Artery
M. Gajjar, A.R. Honarmand, B.M. Patel, M.C. Hurley, A. Shaibani, and S.A. Ansari
AChA ⫽ anterior choroidal artery
CCA ⫽ common carotid artery
CT ⫽ computed tomography
DSA ⫽ digital subtraction
angiography
ECA ⫽ external carotid artery
ICA ⫽ internal carotid artery
MRA ⫽ magnetic resonance
angiography
PCA ⫽ posterior cerebral artery
PCoA ⫽ posterior communicating
artery
Received October 13, 2013;
accepted April 15, 2015.
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ABSTRACT
We reviewed rare cervical and intracranial ICA anatomic variants, aberrant ECA origins;
aberrant or aplastic ICAs; CCA absence; persistent proatlantal, stapedial, hypoglossal, and
trigeminal arteries; primitive maxillary and mandibular arteries; and duplicated posterior
communicating arteries, including a persistent primitive fetal branch or hyperplastic anterior choroidal artery, infraoptic anterior cerebral artery, and dorsal-ventral ophthalmic arteries. Variants of the ICA are rare, typically asymptomatic, and incidental findings. However, these variants can occasionally influence surgical or endovascular strategies,
temporary occlusion times, inadvertent embolization risk, and collateral pathways, with a
potential impact on procedural complications and patient outcomes.
From the Northwestern University
Feinberg School of Medicine,
Chicago, Illinois.
Presentation at American Society
of Neuroradiology Annual Meeting,
San Diego, CA, May 18-23, 2013.
Please address correspondence to
Sameer A. Ansari, MD, PhD, Departments of Radiology, Neurology,
and Neurological Surgery, Northwestern University Feinberg School
of Medicine, 676 N. St. Clair Street,
Suite 800, Chicago, IL 60611-2927;
e-mail: [email protected]
http://dx.doi.org/10.3174/ng.4160164
Learning Objective: Review anatomic variations of the internal carotid artery (ICA), including imaging appearances, embryologic development, prevalence, and associations of clinically relevant variants with neurovascular pathology.
INTRODUCTION
The development of the primitive carotid
artery appears by the third or fourth week
of gestation, from which all major intracranial branches arise at 7.5 weeks of gestation. The internal carotid artery (ICA)
can be identified at Padget stage 1 of brain
vascular development (28 –29 day) in
which both the hindbrain and forebrain
are supplied by primitive carotid arteries.
The primitive ICA connects to the contralateral ICA posterior to the Rathke
pouch after dividing into anterior (olfactory,
rostral) and posterior (mesencephalic, caudal) branches. The early anterior cerebral
artery will develop from an olfactory
branch, and the middle cerebral artery
(MCA) will arise as a lateral branch of the
anterior cerebral artery. Two branches
from the proximal ICA and 2 branches
from the primitive aorta will supply the
hindbrain by connecting to the longitudinal neural arteries, and are named according to the accompanying nerves: trigeminal
and otic branches from the proximal ICA,
and hypoglossal and proatlantal branches
from the primitive aorta.1
At the time of the developing diencephalon and mesencephalon, the posterior
communicating artery (PCoA) extends as
the primary caudal division (caudal ramus)
of the distal primitive ICA (which terminates as the primitive mesencephalic
branches). It connects the carotid and vertebrobasilar systems and is the major supply for the deep white matter (diencephalic
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and some mesencephalic territories) via multiple thalamotuberal penetrating arteries. The PCoA connects to the P1
segment of the posterior cerebral artery (PCA), which is the
terminal branch of the basilar artery (formed by midline
fusion of longitudinal neural arteries) and supplies the occipital and posteromedial temporal lobes. The anterior choroidal artery (AChA) arises from the rostral division of
primitive ICA that supplies the posteromedial hemispheric
territory.2 As the gestational age increases, a posterior choroidal artery develops from the caudal division and takes
over the supply to this region. At age 35 days of gestation, 2
primitive ophthalmic arteries are recognizable: a ventral
ophthalmic artery, which arises from the anterior cerebral
artery, and the dorsal ophthalmic artery, which originates
from the ICA carotid siphon with subsequent intraorbital
and intradural anastomoses, respectively. The ophthalmic
artery forms after regression of the proximal parts of the
dorsal and ventral ophthalmic arteries.3
The purpose of this review article was to explore anatomic variations along the ICA and highlight unusual configurations that might be encountered when performing a
conventional cervical-cerebral angiogram or interpreting
CTA-MRA imaging. The angiographic appearances, embryologic development, prevalence, and associations of
these clinically relevant variants with neurovascular pathology are discussed.
DEVELOPMENTAL AND PRIMITIVE EMBRYOLOGIC
ICA ANOMALIES
ICA Agenesis and Aplastic ICA
Congenital absence of the ICA has a prevalence of 0.01%
and is secondary to the failure in development of the primordial ICA from the third aortic arch and dorsal aorta
before 24 days of embryogenesis.4 A CT of the skull base
and CTA in such cases demonstrate an absence or hypoplasia of the carotid canal and the ICA.5 The anomaly may be
unilateral or, more rarely, bilateral, and has been described
in association with PHACES (Posterior fossa malformations– hemangiomas–arterial anomalies– cardiac defectseye abnormalities–sternal cleft and supraumbilical raphe
syndrome) and congenital Horner syndrome.4,6
Fig 1. CCA absence. Three-dimensional CTA reconstruction depicts the
absence of the right CCA, with separate ICA and ECA origins arising directly from the brachiocephalic trunk (white arrows).
the brachiocephalic artery proximal to the ECA. In the absence of the left CCA, the aortic arch gives rise to both the
ECA and the ICA.9
The ductus caroticus (a segment of the embryonic dorsal
aorta connecting the third and fourth aortic arches that
normally regress) provid the origin to the primitive CCA
during developmental stage IV.1 Either a persistent ductus
caroticus with concurrent regression of the third aortic arch
or the persistent third aortic arch with concurrent regression of the fourth aortic arches have been proposed as the
embryologic abnormalities that lead to the absence of the
CCA.10,11 Persistence of the ductus caroticus also has been
proposed as a congenital mechanism for low or early carotid bifurcation. Intrathoracic carotid bifurcations have
been described in association with the Klippel-Feil anomaly.12 In addition, we previously described a very unusual
anomaly of the left ICA, which arises directly from the
pulmonary artery. The embryologic mechanism was postulated to be secondary to impaired regression of the primitive
aortic arch (Fig 2).13
Aberrant ICA
Congenital Common Carotid Artery Absence and Early
ICA Bifurcation or Aberrant Origin
The exact incidence of common carotid artery (CCA) agenesis is not well described; however, ⬍35 cases of the absence
of CCA have been reported in the literature to date (Fig 1).7
Unilateral and bilateral agenesis of CCA has been described.7,8 They are usually asymptomatic and incidentally
detected during the evaluation for other conditions or at
autopsies. In the absence of the right CCA, in most cases,
the external carotid artery (ECA) originates from the brachiocephalic artery proximal to the ICA, which arises from
the distal subclavian artery. However, in an unusual case
described by Maybody et al,9 the right ICA originated from
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Recognized aberrant ICAs include intratympanic and retropharyngeal variants. The normal ICA anatomy of the carotid canal is characterized by entry into the petrous bone
medial to the styloid process. The initial vertical petrous
segment of the ICA courses anterior to the tympanic cavity,
adjacent to the middle ear cavity, and is covered by a thin
bony plate, which is absorbed in adulthood (Fig 3). An
unusual course of the ICA directly toward the middle ear is
exceedingly rare and is termed an “aberrant ICA.” This
anomaly may be secondary to disturbed differentiation of
the third branchial artery, which results in an enlarged inferior tympanic artery (branch of the ascending pharyngeal
artery) that enters the tympanic cavity via the inferior tym-
inferolateral trunk and as a branch of the meningohypophyseal trunk.1 The posterior inferior hypophyseal artery gives
rise to the medial clival artery and its branches, which then
go on to anastomose with the contralateral posterior inferior hypophyseal artery. These transclival anastomoses provide a collateral or developmental route to the contralateral
ICA in a setting of ICA occlusion. Alternatively, persistent
primitive maxillary artery supply to the ICA may become a
dominant pathway as an intercarotid anastomosis in cases
of embryologic ICA agenesis at or below the petrous
segment.
Primitive Mandibular Artery
Fig 2. Aberrant ICA origin via the pulmonary artery. Aberrant aortic arch
anatomy demonstrates the origin of the enlarged left ECA (LECA) that
overlaps the right CCA (RCCA) origin (white arrowhead), pigtail catheter
(black arrow) in the ascending aorta, aberrant right subclavian artery
(asterisk) that supplies the right vertebral artery (RVERT), and the normal left subclavian artery (LSUB). Delayed imaging demonstrates retrograde filling of the left ICA (LICA) from the carotid bifurcation down to an
unusual left pulmonary artery origin (black arrowhead). Contrast opacification and washout into the left pulmonary artery (double arrowheads)
due to its low-pressure venous system, which results in a functional arteriovenous shunt (with permission from Hurley et al, J Neurosurg Pediatr 200813).
panic canaliculus (Jacobsen canal) and develops a persistent
anastomosis to the distal horizontal petrous segment of the
ICA via the caroticotympanic branch (hyostapedial remnant). Preoperative recognition of an aberrant intratympanic ICA is essential to avoid potentially serious complications during otologic surgery.14 Accidental injury may
result in uncontrollable intratympanic hemorrhage and require surgical occlusion and risk for anterior circulation
ischemia. This intratympanic variant, as with a persistent
stapedial artery, may mimic a hypervascular tumor, such as
a glomus tympanicum.
The rare retropharyngeal variant of an aberrant ICA is
an anomalous course of the distal cervical segment that
extends to or near the midline of the posterior pharyngeal
wall, which lies beneath the mucosa.15 Its presence places a
patient at higher surgical risk for carotid injury during traumatic intubation, oropharyngeal tumor resection, tonsillectomy, adenoidectomy, or palatopharyngoplasty. The etiology has been assumed to be secondary to the disturbance
in the descent process of the heart and large vessels, including the dorsal aortic root (which is the origin of the ICA),
into the mediastinum and results in the persistence of the
embryologic angulations of the cervical ICA.16
During early development, the ventral aorta and dorsal
aorta are connected via arterial anastomoses known as the
primitive aortic arches.1 As the fetus reaches the 4-mm
stage, the first and second primitive aortic arches involute
and their remnants become the primitive mandibular artery
and hyoid artery, respectively, incorporated as branches of
the dorsal aorta. At this stage, the primitive mandibular
artery and the hyoid artery anastomose with a complex
vascular plexus in the region that will later become the
future face and pharynx, with its territory later annexed by
the external carotid circulation.
As the third primitive aortic arch develops and gives rise
to the cervical ICA in the 5– 6 –mm stage, the primitive
mandibular artery starts to regress, and by 7–12 mm, it is no
longer present. The remnants of its plexus consolidate to
become the mandibulovidian artery that arises from the
foramen lacerum segment of the ICA, rarely noted in normal angiographic studies. The mandibular branch courses
anteroinferiorly to supply the soft tissues of the pterygoid
muscles, posterior pharyngeal wall, and possibly, the mandibular condyle. The vidian branch enters its attenuated
osseous canal, along with the greater superficial petrosal
nerve, and anastomoses with the vidian branches of the
internal maxillary artery in the region of the pterygopalatine fossa, with potential persistent supply to the posterior
nasopharynx, nasal septum, and turbinates in response to
the internal maxillary artery developmental hypoplasia or
injury and/or occlusion. The clinical significance of this
mandibulovidian remnant is most often evident in recruitment of the ICA–vidian artery supply to hypervascular tumors such as juvenile nasopharyngeal angiofibromas (Fig
4). In addition, ECA collateral networks to the ICA-mandibulovidian artery (via both the internal maxillary artery–
vidian branch and ascending pharyngeal artery–superior
pharyngeal branch) require caution when performing head
and neck embolizations in this region.17
Primitive Maxillary Artery
PERSISTENT PRIMITIVE CAROTID–
VERTEBROBASILAR ANASTOMOSES
The primitive maxillary artery is derived from the proximal
remnant of the first pharyngeal arch and is represented in
the adult pattern as the posterior inferior hypophyseal artery, which arises from the cavernous ICA, posterior to the
Primitive carotid anastomoses to the longitudinal neural
arteries provide a transient arterial supply to the hindbrain
in the very early 4-mm embryo, thus persistent ICA-vertebrobasilar anastomoses represent the oldest brain vascular
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Fig 3. Aberrant ICA. Coronal MRA (A), anteroposterior DSA (B), and axial CT (C) images demonstrate an aberrant intratympanic course of the right ICA,
exhibiting a relatively hypoplastic proximal caliber, lateral deviation into the middle ear, acute angulation (A, B) (white and black arrows) into the
horizontal petrous segment, which indicates the level of inferior tympanic to caroticotympanic artery anastomosis, and a thin osseous carotid plate (C)
(white arrow).
anomaly that can be clinically identified.18 The 4 primitive
carotid-vertebrobasilar anastomoses are mainly categorized into presegmental (trigeminal, otic, and hypoglossal)
and proatlantal intersegmental arteries. These anastomoses
provide carotid supply to 2 longitudinal neural arteries that
distally fuse to form the basilar artery. Later in the 5– 6 –mm
embryo, a PcoA develops from the anastomosis of the distal
ICA and the corresponding longitudinal neural artery respectively, followed by obliteration of primitive otic, hypoglossal,
trigeminal, and proatlantal intersegmental arteries.19 Failure
in regression of these primitive arteries leads to formation of
persistent carotid-vertebrobasilar anastomoses.
Persistent Primitive Trigeminal Artery
The persistent primitive trigeminal artery is the most common and most cephalic of the persistent carotid-vertebrobasilar anastomoses (Fig 5A, -B). Its reported prevalence is
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0.1%– 0.6%.20 This artery originates from the ICA immediately after its exit from the petrous carotid canal and
anastomoses with the midbasilar artery. After arising from
the cavernous portion of the ICA, the persistent trigeminal
artery may enter the sella and penetrate the dura connecting
to the basilar artery between the anterior inferior cerebellar
and the superior cerebellar artery origins (trans-sellar). It
may also course laterally (parasellar), adjacent to the sensory root of trigeminal ganglion connecting to the basilar
artery.21 Due to its higher incidence, it is not uncommon to
encounter a persistent primitive trigeminal artery during
diagnostic cerebral angiography or neuroendovascular procedures. The presence of carotid-vertebrobasilar anastomoses
warrants caution in transsphenoidal surgeries and Wada testing, which require superselective catheterization distal to these
variants to avoid the risk of inadvertent brain stem anesthesia
from primitive anterior to posterior circulation shunts.
Saltzman20 classified the angiographic appearance of a
persistent primitive trigeminal artery into 3 types. In type 1,
the PcoA is hypoplastic or absent and the persistent primitive trigeminal artery joins the basilar artery between the
anterior inferior cerebellar arteries and superior cerebellar
arteries, which provides the main supply for the distal basilar, bilateral superior cerebellar arteries, and PCA. In type
2, the persistent primitive trigeminal artery joins the basilar
artery, as in type 1, by supplying the bilateral superior cerebellar arteries, but the ICA may supply the PCAs via patent
PcoAs. The prevalence of these 2 types is relatively equal,
and the basilar artery is usually hypoplastic caudal to the
anastomosis of the persistent primitive trigeminal artery. In
a type 3 variation, usually considered a combination of the
previous types, the persistent primitive trigeminal artery
joins the ipsilateral superior cerebellar arteries (Fig 5C) directly with variable supply to the PCAs.20
Persistent Primitive Otic Artery
The persistent primitive otic artery is the rarest variant of
the primitive carotid–vertebrobasilar anastomoses due to
its earliest obliteration between the presegmental and
proatlantal intersegmental arteries in the fourth week of
embryogenesis. It arises from the petrous ICA within the
carotid canal, coursing in the internal acoustic meatus, and
joins the caudal basilar artery between the anterior inferior
cerebellar and superior cerebellar arteries.18 Approximately
8 cases of this variant have been reported; and it has even
been associated with an intracranial aneurysm.5
Persistent Primitive Hypoglossal Artery
The persistent primitive hypoglossal artery is the second
most common carotid-vertebrobasilar anastomosis, with a
prevalence of 0.02%– 0.10%.22 It originates from the ICA
at the C1 through C3 levels, courses through the hypoglossal canal (not the foramen magnum), and joins the proximal
basilar artery. In 79% of cases, the PcoAs are hypoplastic,
and, in 78% of cases, the proximal vertebral artery is hypoplastic.5 A definitive diagnosis is based on the recognition
of an anomalous artery centered within an enlarged hypoglossal canal. Clinically, a persistent hypoglossal artery is the
most likely to cause neurovascular compression symptoms
and may present with glossopharyngeal neuralgia and hypoglossal nerve paralysis.
Fig 4. Primitive mandibular artery. Lateral oblique DSA image identifies
a mandibolovidian artery–vidian branch (black arrow) arising from the
foramen lacerum segment of the ICA due to recruitment from a hypervascular juvenile nasopharyngeal angiofibroma. Note the prominent hypertrophied arteries arising superiorly from the inferolateral trunk (foramen rotundum and ovale branches [black arrowhead]) to also supply the
hypervascular tumor (arrowhead).
Proatlantal Intersegmental Artery
The proatlantal intersegmental artery (Fig 6) originates
from the CCA bifurcation, ECA, or ICA at the C2 through
C4 levels, joins the horizontal V3 segment of the vertebral
artery in the suboccipital region, and traverses the foramen
Fig 5. Persistent primitive trigeminal artery. Three-dimensional TOF MRA (A) and oblique DSA (B) images show a prominent Saltzman type 1 persistent
primitive trigeminal artery (white and black arrows) arising from the cavernous right ICA with direct anastomosis to the basilar artery. Note the
hypoplasia of the vertebral-basilar artery proximal to the anastomosis. An oblique DSA (C) image demonstrates a rare Saltzman type 3 persistent
primitive trigeminal artery variant (black arrow) directly supplying the ipsilateral superior cerebellar artery.
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The AChA arises from the rostral division of the primitive
ICA and supplies the posteromedial hemispheric territory in
the early embryologic stage. As the gestational age increases, a posterior choroidal artery develops from the caudal division and replaces the supply to this region. If there is
a failure of the posterior choroidal artery to augment this
distribution, it can lead to the persistence of the AChA as
the main supply for the temporal-occipital lobes. Other
AChA anomalies include agenesis, an anomalous origin
from the middle cerebral artery and PCoA, and AChA duplication. A hyperplastic AChA may mimic a duplicated
PCoA–fetal PCA in the setting of a separate prominent
PCoA or fetal PCA. A duplicated branch that arises from
the ICA distal to the origin of the native PCoA–fetal PCA,
and that courses as an isolated temporal branch has been
classified as a persistent primitive PCA, although it can
easily be mistaken for a hyperplastic AChA (Fig 7).26
PERSISTENT PRIMITIVE ICA-ECA ANASTOMOSES
Fig 6. Proatlantal intersegmental artery. Oblique lateral DSA image demonstrates a rare proatlantal intersegmental artery (white arrow) arising
from the CCA, consistent with a persistent primitive carotid–vertebral
anastomosis.
magnum. Three variants have been described: type 1 originates from the dorsal aspect of the ICA and accounts for
38% of cases, type 2 arises from the ECA and comprises
57% of cases, and type 3 rarely arises from the CCA or
bifurcation (5%).19 Associated cerebrovascular abnormalities, including intracranial aneurysms, have been described
in patients with carotid-vertebrobasilar anastomoses, including the proatlantal intersegmental artery.19,23,24Also,
aplasia or hypoplasia of one or both vertebral arteries proximal to the anastomosis may be identified in 50% of cases.24
PcoA and Fetal PCA
PCoA variants have been extensively described with multiple types of anatomic anomalies and morphologies related
to its early embryologic development as described: hypoplasia and/or aplasia (agenesis), different lengths and courses,
fetal configuration of the PCA branching directly from the
ICA, vessels that originate directly from the PCoA (eg,
AChA), PCoA infundibulum, fenestration, and partial or
complete duplications (with or without a patent basilar
P1 segment).2 Interestingly, the variations of the right
PCoA have been described to be more common than the
left.
Hyperplastic AChA
A hyperplastic AChA is defined by the posterior temporal
branch of the PCA that arises from the AChA and can be
identified in 1.1% to 2.3% of angiographic examinations.25
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The primitive ICA arises from the third aortic arch. Although there are controversial comments regarding the origins of the CCA and ECA in the literature, it seems that the
CCA evolves from the ductus caroticus and the ECA develops from the ventral pharyngeal artery that arises either
directly from the third aortic arch or from the aortic sac that
migrates to the third aortic arch.12,27 Anastomoses between
the ICA and ECA branches become angiographically apparent and are commonly identified secondary to high-flow
recruitment from neurovascular pathologies, such as hypervascular tumors, dural arteriovenous fistulas (AVF), arteriovenous malformations (AVM), or proximal ICA occlusions. Multiple primitive ICA-ECA anastomoses persist
into adulthood, and several notable collateral pathways include intracavernous meningohypophyseal trunk branches
of the ICA with the ascending pharyngeal artery; the inferolateral trunk of the ICA via the foramen rotundum and
foramen ovale branches of the internal maxillary artery and
middle meningeal arteries, the mandibulovidian trunk of
the intrapetrous ICA via branches of the internal maxillary
and the ascending pharyngeal artery; and ophthalmic artery
via branches of the facial, internal maxillary artery, and the
middle meningeal artery.
Conversely, persistent primitive ICA-ECA anastomoses are very rare. The very early anastomosis between
ICA and ECA forms at 4 –5 weeks of fetal life via the
hyostapedial trunk when the inferior division stapedial
artery (arising from the hyoid artery near the proximal
ICA as a dorsal remnant of the second aortic arch) transiently connects to the ventral primitive pharyngeal arteries,
which will develop the final ECA. The stapedial artery regresses after this anastomosis, but the hyoid artery persists to
form the adult caroticotympanic branch of the petrous portion
of the ICA. Abnormal or interrupted stapedial artery regression allows for the congenital manifestation of ICA-ECA
anastomoses.1,28
Fig 8. Persistent primitive ECA-ICA anastomoses. Lateral DSA images
depict direct origins of the occipital artery (A) and ascending pharyngeal
artery (B) arising directly from the ICA (black arrowhead). Note hypertrophied ascending pharyngeal artery–neuromeningeal trunk supplies a
contralateral dural AVF of the left anterior condylar vein via multiple
ascending clival branches.
Occipital Artery and Ascending Pharyngeal Artery
Arising from the ICA
Usually no branches originate from the cervical segment of
the ICA in the neck. Occasionally, as described above, the
proatlantal intersegmental and hypoglossal arteries persist
from the cervical segment of the ICA into adult life. The
occipital artery (Fig 8A) seems to be the most common ECA
branch to arise directly from the cervical ICA, followed by
the ascending pharyngeal artery (Fig 8B). The occipital artery and ascending pharyngeal artery develop a pharyngealoccipital system at the craniocervical junction, which supplies cervical somites C1, C2, C3, and the third branchial
arch.3,29 In addition, several case reports describe the unusual origination of lingual, middle meningeal, facial, and
laryngeal arteries from the CCA and ICA.30-33
Persistent Stapedial Artery
Fig 7. Duplicated fetal PCA with hyperplastic AChA. A, Three-dimensional rotational DSA axial multiplanar reconstructions demonstrates
separate origins of both proximal PCoA–fetal PCA (1) and a distal fetal
PCA (2), initially suspected to be a persistent primitive PCA that supplies
an isolated posterior temporal branch territory. B, However, a small
AChA branch is identified arising from the distal fetal PCA and extends
through the choroidal fissure (arrow), consistent with a hyperplastic
AChA. C, 3-dimensional reconstructions further delineate the distal hyperplastic AChA supplying the posterior temporal branch (curved arrow), and separate proximal PCoA–fetal PCA (arrow).
The stapedial artery is a normally transient embryonic anastomosis between the branches of the future external carotid
artery and ICA, as described above. A persistent stapedial
artery has a reported prevalence of 0.48%.34 The stapedial
artery originates from the vertical segment of the petrous
ICA, passes through the obturator foramen of the stapes,
and terminates as the middle meningeal artery. CT findings
along the course of a persistent stapedial artery may include
a small canaliculus at the distal end of the carotid canal; a
linear structure that crosses the promontory of the tympanic cavity; an enlarged facial nerve canal or a separate
canal that parallels the facial nerve canal; and the absence of
the foramen spinosum, which normally contains the middle
meningeal artery.5 The stapedial artery may be mistaken for
a vascular neoplasm of the middle ear (glomus tympanicum
tumor) and may present with pulsatile tinnitus. It is important to identify a persistent stapedial artery before surgery
because its presence may complicate tympanotomy, stapedectomy, and cholesteatoma resection as well as hinder cochlear implantation.
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PARACLINOID AND SUPRACLINOID ICA ANOMALIES
According to the classification by Lasjaunias and SantoyoVazquez,35 the first embryologic segment of the ICA is derived from the third aortic arch, whereas the remaining 6
embryologic distal segments come from the dorsal aorta.
The embryonic branches of the ICA, including the hyostapedial trunk, primitive mandibular artery, primitive maxillary artery, trigeminal artery, dorsal ophthalmic artery, and
primitive ophthalmic artery separate the ICA segments consecutively as the cervical ICA (segment 1), ascending petrous (segment 2), horizontal petrous (segment 3), ascending cavernous (segment 4), horizontal cavernous (segment
5), clinoid (segment 6), and supraclinoid-terminal (segment
7) ICAs. The origin of the dorsal ophthalmic artery (defined
by Lasjaunias and Santoyo-Vazquez35) is between segments
5 and 6, which form the adult inferolateral trunk, whereas
the origin of the primitive ophthalmic artery (defined by
Padget1) is between segments 6 and 7, and forms the adult
ophthalmic artery.
Infraoptic Anterior Cerebral Artery
An infraoptic course of the precommunicating anterior cerebral artery is a rare anomaly. In 2008, Wong et al36 summarized a total of only 42 examples reported in the literature, and they reported 2 additional cases. Overall, 59% of
these cases were associated with cerebral aneurysms. An
infraoptic A1 segment (Fig 9) is characterized by 3 features:
(1) the anomalous artery branches off the ICA at the level
of the ophthalmic artery, (2) it travels beneath the optic
nerve, and (3) it supplies the vascular territory of a normal anterior cerebral artery. The configurations of the
proximal anterior cerebral arteries in the presence of an
infraoptic A1 may be classified into 4 types according to
the presence or absence of ipsilateral supraoptic A1 and
contralateral A1 segments.36
The embryogenesis of this anomaly is still controversial.
Proposed explanations in the literature are as follows: (1)
the persistence of the embryonic anastomosis between the
primitive maxillary artery and the anterior cerebral artery,
(2) the persistence and enlargement of an embryologic anastomotic loop between the primitive dorsal and ventral ophthalmic arteries, and (3) the enlargement of the prechiasmal
arterial anastomosis.
Dorsal and Ventral Ophthalmic Arteries
During early embryonic development, 2 primitive ophthalmic arteries are present: the ventral and dorsal ophthalmic
arteries. The ventral ophthalmic artery, which is thought to
normally persist by migrating toward the normal ophthalmic artery origin, arises from the anterior cerebral artery
passing through the optic canal. Subsequently, the medial
posterior ciliary artery arises from the ventral ophthalmic
artery and supplies the medial side of the optic vesicle. The
proximal segment of the ventral ophthalmic artery regresses.37 The dorsal ophthalmic artery eventually regresses
with its vestiges represented by the anteromedial branch of
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Fig 9. Infraoptic anterior cerebral artery. Oblique DSA (A) and 3-dimensional TOF MRA axial source images (B, C) demonstrate an anomalous,
infraoptic course of the left A1 segment (white arrows) arising at the level
of the ophthalmic artery (arrowhead), coursing under the optic nerve
and supplying the A2 segments of bilateral anterior cerebral arteries.
the inferolateral trunk or the recurrent tentorial branch of
the ophthalmic artery. In some instances, the dorsal ophthalmic artery may persist as the dominant supply to the
orbit, which arises from the carotid siphon between the fifth
(horizontal cavernous) and sixth (clinoid) ICA segments
(Fig 10). On CT, this anomalous artery is seen to arise from
the dorsal aspect of the extracranial paraclinoid segment of
the ICA as a branch of the inferolateral trunk and enters the
7.
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11.
12.
Fig 10. Persistent dorsal ophthalmic artery. A lateral DSA image demonstrates a prominent dorsal ophthalmic artery (black arrow) with an extracranial origin from the expected location of the inferolateral trunk and
supplies the orbit via the superior orbital fissure.
orbit through the superior orbital fissure instead of the optic
canal.
13.
14.
15.
CONCLUSION
Variants of the cervical and intracranial ICA are rare, typically asymptomatic, and often incidental findings. However, these variants can occasionally influence surgical strategy, balloon test occlusion studies, temporary occlusion
time, Wada testing, and embolization risk in the setting of
hypervascular tumors or dural AVFs, with potential impact
on procedural complications and patient outcomes. In addition, they inherently provide redundant but valuable embryologic collaterals to maintain intracranial flow in settings of ICA compromise or occlusion.
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