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V./1.2.: Anterior cerebral circulation
V./1.2.1.: Internal carotid artery (ICA)
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The anterior cerebral circulation is supplied by the right and left internal carotid artery (ICA). Based on its course extra- and intracranial segments are distinguished. The intracranial segment is divided into extra-, inter- and intradural parts. The intracranial extradural segment of the ICA enters the base of the skull through the carotid canal of the petrous bone. First ascends nearly vertically a short distance, then it runs along in line with the long axis of the petrous bone and exits the bony canal through the foramen lacerum. The interdural part of the artery then again curves upward to run along the lateral surface of the body of the sphenoid bone in the cavernous sinus. This intracavernous segment shows great variability in its course from the steeply ascending form to the curved „S”-like form, which was called the carotid siphon. Afterwards the artery exits the cavernous sinus at the lateral surface of the anterior clinoid process of the sphenoid bone, where the dura mater, forming the roof of the sinus, is divided into two layers attaching to the superior and inferior surface of the process. Based on its position lateral to the process this short -also interdural- segment is recently called the paraclinoid carotid. The last segment of the internal carotid artery is located intradurally, after a short distance it bifurcates to its terminal branches forming the anterior (ACA) and the middle (ACM) cerebral artery.
V./1.2.2.: The segments of internal carotid artery used in clinical practice
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In clinical practice the beforementioned segments are also called C1-C7, where C1 is referring to the intradural, C2-C5 to the interdural (paraclinoid and intracavernous) and C6 to the extradural (intrapetrous) segments. According to this nomenclature C7 segment stands for the cervical (extracranial) part of the ACI.
V./1.2.3.: Interdural branches of the internal and external carotid artery
Despite the long course of the internal carotid artery, it has only a few and variably present braches.
Generally ICA gives no extrcranial branches, therefore these arteries rarely arise from it. However in those rare cases the corresponding blood supply of the meninges similarly to the cerebrum is derived from the ICA.
V./1.2.3.2.: Caroticotympanic arteries and artery of the pterygoid canal (Vidian artery)
The caroticotympanic arteries enter the tympanic cavity on its anterior (carotid) wall through several minute foramina and anastomose with its other supplying branches.
The artery of the pterygoid canal passes along the corresponding bony canal and accompanies the pterygoid canal nerve at the bottom of the cavity of the sphenoid bone (sphenoid sinus) in the pterygopalatine fossa.
V./1.2.3.3.: Branches of the intracavernous segment
In most cases two main trunks can be identified, however the intracavernous branches also show variability based on its origin and number. The proximal trunk originating a few millimeters above from the beginning of the intracavernous segment called the meningohypophyseal trunk. Generally it has three clinically significant branches: the dorsal meningeal artery dividing into the medial and lateral clival branch supplies the tissues adjacent the vertex of the petrous bone, the clivus and the dorsum sellae.
The tentorial artery or the artery of Bernasconi-Cassinari is a thin branch, which runs posterior and enters the anterior part of the cerebellar tentorium. The inferior hypophyseal artery supplies the neurohypophysis. The inferolateral trunk is also clinically relevant. With several smaller branches it supplies the gasserian ganglion and the meninges and cranial nerves of the cavernous. Its origin shows a great variabilitiy. Its occasionally absent capsular artery of McConnell supplies the meninges of the sella turcica and the pituitary gland.
V./1.2.4.: Intradural branches of the internal carotid artery
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Due to limitation of scope of the present chapter our main objective is to give a general idea of the clinically relevant arteries.
V./1.2.4.1.: Ophthalmic artery (OA)
The ophthalmic artery is considered as the first branch of the intradural (supraclinoid) segment of the internal carotid artery, however it may also originate from the intracavernous (infraclinoid) segment of the ICA. Intradural origin is only seen in 10% of all cases. The most commonly it originates from the paraclinoid segment, seen in 75% of all cases. OA passes through the optic canal with and under the optic nerve. In the majority of cases (70%) it runs along inferomedially, but rarely it may course medially or laterally to the optic nerve. Branches of the ophthalmic artery supply not only the contents of the orbit, but also the supraorbital part of the forehead, the inner canthi of the eyes, root of the nose, the lateral wall of the nasal cavity and the upper part of the nose hedge. In the beforementioned regions the branches of the ophthalmic artery forms anastomoses with branches of the external carotid artery.
V./1.2.4.2.: Superior hypophyseal artery
It is a branch of the supraclinoid segment of the internal carotid artery. The left and the right arteries anastomose along the midline and supply the optic chiasm and nerves, the tuber cinereum and the infundibulum and the anterior lobe of the pituitary gland through several perforating branches.
V./1.2.4.3.: Posterior communicating artery (Pcomm, PCOM)
The posterior communicating artery extends few millimeters proximally to the intradural course of the internal carotid artery. It passes posteriorly and then joins the posterior cerebral artery between it’s P1 and P2 segments connecting the anterior and posterior cerebral circulatory system. In 3-11% of the cases one side is missing and often hypoplastic. Interesting observation, that in 25% of the cases the PCOM is thicker than the posterior cerebral artery (fetal type/origin).
It courses along the medial surface of the temporal lobe and shortly to the optic tract, first in the carotid cistern than in the interpeduncular cistern. The posterior communicating artery supplies the optic chiasm, the optic tract, the tuber cinereum, than the mamillary body and the posterior lobe of the pituatory gland. It gives off perforating branches to the thalamus and subthalamus through the thalamoperforating arteries (thalamotuberal and premamillar arteries) and the thalamogeniculate artery.
V./1.2.4.4.: Anterior choroidal artery
It originates from the internal carotid artery, few millimeters distal to the beforementioned artery, though it also rarely arises directly from the bifurcation of the ICA or from the opening segment of the middle cerebral artery. Most of the descriptions are only dealing with it’s ventricular supply, but it also gives important branches for example to the olfactory cortex, the basal ganglia, the diencephalon or to the different structures of the visual pathway.
It runs shortly in the carotid cistern, than following the optic tract it runs toward to the medial surface of the temporal lobe in the crural cistern, where it usually with a smaller trunk (uncal artery) gives off branches to the amygdala, the anterior part of the hippocampus (pes hippocampi), the posterior part of the caudate nucleus and the piriform area.
Afterwards it courses along in the ambient cistern and with several smaller branches supplies the optic chiasm and tract and gives branches to the globus pallidus, the substancia nigra, the red nucleus and to the knee of the internal capsule. Also supplies the anterior nuclei of the thalamus (ventral anterior and lateral nucleus). Pushing forward the epitheIial lamina of the lateral ventricle and the pia mater the anterior choroidal artery reaches the choroidal fissure and enters the lateral ventricle to supply the choroid plexus of the inferior horn . From this segment it also gives branches to the lateral geniculate body (LGB)and to the optic radiation.
V./1.2.4.5.: Anterior cerebral artery (ACA)
The anterior cerebral artery together with the other terminal branch of the ACI, the medial cerebral artery originates from the bifurcation of the internal carotid artery, in three quarters of the cases with a smaller trunk. It courses anteriorly in the cistern of lamina terminalis, than in the corpus callosal cistern. The anterior cerebral arteries of the two sides form a short, but highly variable anastomotic connection through the anterior communicating artery. The segment of ACA from ACI bifurcation to the origin of the beforementioned artery is its pre-communicating segment, also called segment A1 or ACA proximal. After the antastomosis ACA forms the postcommunicating segment, A2 segment, ACA distal or pericallosal artery.
V./1.2.4.5.1.: Pre-communicating or A1 segment (proximal ACA)
It shows higher variability (7-46%) between the two sides, than the postcommunicating segment. Absence artery of one side only occurs in 1-2% of all cases. It supplies the surrounding structures (lamina terminalis, anterior commissure, colums of fornix, optic chiasm, superior part of the optic nerves, pellucid septum). Besides the ventral part of the hypothalamus, it also plays a great role in supplying the anterior limb of the internal capsule. Rarely through the anterior perforated substance it even irrigates the medial part of the striatum.
V./1.2.4.5.2.: Post-communicating or A2-szegment (distal ACA)
It usually supplies the frontobasal (medial frontonasal artery) and frontopolar (frontopolar artery) regions of the skull base, the corpus callosum till the splenium, the frontal and parietal parts of the cortex at the the medial surface of the cerebral hemispheres, and the cingulate gyrus.
Notable branches arising at the beginning of this segment are the callosal marginal arteryand the recurrent artery of Heubner .The recurrent artery of Heubner curves back sharply on itself, paralleling the A1 segment to the ACI bifurcation and supplies the putamen, the anterior portion of the caudate nucleus, the lateral part of the pallidum and anterior limb of the internal capsule. Symmetric appearence of the two sides is tipical of A2 (~90%) and mainly it supplies the ipsilateral structures (unihemispheric), however rarely branches coursing towards the other side above the corpus callosum are seen (bihemisphericus). In rare cases the artery is unpaired (azygos variant) or triplicated („median callosal artery”).
V./1.2.4.6.: Anterior communicating artery (AcommA, ACOM, ACoA)
Among the arteries of the skull base it is the most variable. For example Busse in 1921 reported 227 (!) variations. This short, according to most documentation 0,1-3 mm long artery interconnects the two anterior cerebral arteries in the cistern of lamina terminalis. In rare cases it is absent, which is mainly explained with the early fusion of the A2 segments. Contrarily fenestrated or reticular forms are frequently seen, where the A1 and A2 segments are interconnected with multiple arteries, which can even form complex anastomoses. It often gives off important perforating branches to the preoptic area of the hypothalamus, the optic nerves and chiasm and the subcallosal area.
V./1.2.4.7.: Middle cerebral artery (MCA)
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The middle cerebral artery is the other, usually largest terminal branch of ACI. Along it’s course it leaves the carotid cistern to enter the depths of the cistern of lateral cerebral fossa (Sylvian cistern). The artery supplies with numerous branches to the majority of the telencephalon, the insula and the basal ganglia. On the basis of its course the middle cerebral artery is usually classified into sphenoidal, insular, opercular and terminal or M1-M4 segments.
V./1.2.4.7.1. Sphenoidal or M1 segment
runs parallel to the lesser wing of the sphenoid bone from the bifurcation of the internal carotid artery toward the limen of insula. It’s principal branches are the temporal, orbitofrontal and lateral lenticulostriate arteries, the first two irrigate the lateral frontobase and the anterior portion of the temporal lobe, while the vascular territory of the lenticulostriate branches is the dorsolateral portion of head of the caudate nucleus, part of the anterior limb of the internal capsule, the putamen, the lateral portion of the globus pallidus as well as the external capsule and claustrum.
V./1.2.4.7.2.: Insular or M2 segment
The insular segment originates in the level of the limen of insula, where the M1 segment may bifurcate or sometimes trifurcate („upper, medial and lower trunk”), but other variations may occur. In practice the most important variation is the pseudobifurcation, where the orbitofrontal and temporopolar arteries originate from the M1 segment with a common, M1-likely large trunk.
The main branches of the M2 segment supply the frontal, parietal and temporal lobe, including several clinically relevant cortical areas (sensory, motor and association cortex), out of the frontal branches for example the precentral (pre-Rolandic) artery (supplying the medial and inferior frontal gyrus as well as the inferior part of the precentral gyrus), the central (Rolandic) artery (supplying the posterior part of the precentral and inferior part of the postcentral gyrus). Among the parietal branches the angular artery should be highlighted, which not only irrigates the angular, but also the superior temporal gyrus as well as the auditory cortex (Heschl's gyri or Heschl's convolutions). The temporal branches mainly supply the parietal lobe, however perfusion of the insula is considerably provided by the posterior temporal artery.
V./1.2.4.7.3.: Opercular or M3 segment
It originates from the branches of the beforementioned M2 segment and extends from the circular sulcus of insula towards the lateral (Sylvian) fissure. It bends back sharply to travel along the surface of the operculum and directly supplies it with smaller perforating branches.
V./1.2.4.7.4.: Terminal, cortical or M4 segment
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Terminal segment is the summarized name for the branches emerging from the sylvian fissure onto the surface of the hemisphere and terminating towards the longitudinal cerebral fissure, where it forms several anastomoses with the pericallosal artery originating from the anterior cerebral artery.
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