INTRODUCTION
It is our belief that a successful vascular neurosurgery procedure is edified on five solid foundations: (i) anatomical knowledge; (ii) capability to understand the radiological information and correlate it with anatomy; (iii) capability of presurgical planning based on anatomical and radiological correlation; (iv) capability of bringing the anatomy, radiology and presurgical planning into the surgery; (v) fine surgical skill through extensive training.
In this chapter, the authors will review the basic arterial anatomy of the brain and its angiographic correlation, which will cover the first two of those five foundations.
The brain is supplied by two main arterial systems: (i) the carotid system and (ii) the vertebrobasilar system; the carotid system supplies most of the cerebrum and the vertebrobasilar system supplies the brainstem, the cerebellum and part of the cerebrum. Both carotid systems are communicated by the anterior communicating artery (A Com) complex. The carotid system communicates with the vertebrobasilar system via posterior communicating arteries and occasionally via other embryological arteries, most frequently the trigeminal and the hypoglossal arteries.
SUPRATENTORIAL ARTERIAL ANATOMY
The common carotid artery bifurcates at the level of the angle of the mandible and gives rise to the internal and the external carotid arteries. The external carotid artery gives rise to the superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, superior temporal and maxillary arteries (Fig. 1.1).
Internal Carotid Artery
The internal carotid artery (ICA) supplies most of the ipsilateral cerebral hemisphere, eye and accessory organs, forehead and, in part, the nose. The ICA is divided into five segments: (i) cervical, (ii) petrous, (iii) cavernous, (iv) clinoid and (v) supraclinoid segments. The cervical segment of the ICA starts from the carotid bifurcation and ascends in front of the upper three cervical transverse processes to enter the inferior aperture of the carotid canal in the petrous temporal bone to become the petrous carotid artery. The cervical carotid artery does not present any branch. The petrous segment of the ICA presents a vertical and a horizontal portion; the vertical portion ascends in the carotid canal behind the cochlea, curves anteromedially to become the horizontal portion.
Figure 1.1: Lateral view of the left external and internal carotid arteries. The upper part of the mandible, the attached muscles, the zygomatic arch and the maxilla have been removed. 1, External acoustic canal; 2, Mastoid process; 3, Posterior belly of the digastrics muscle; 4, Maxillary artery; 5, Occipital artery; 6, Superficial temporal artery; 7, Middle meningeal artery entering the foramen spinosum; 8, Vertebral artery; 9, External carotid artery; 10, Posterior auricular artery; 11, Posterior arch of C1; 12, Facial artery; 13, Lingual artery; 14, Internal carotid artery; 15, Ascending pharyngeal and external carotid arteries
Figure 1.2: Frontal view of the internal carotid artery and the elements of the posterior fossa. 1, Cerebellomesencephalic segment of the SCA; 2, Transverse sinus; 3, Lateral pontomesencephalic segment of the SCA; 4, Anterior pontomesencephalic segment of the SCA and P1 segment of PCA; 5, Superior petrosal sinus; 6, Basilar artery; 7, AICA and the labyrinthine artery, (related to the VII and VII nerves); 8, Horizontal portion of cavernous ICA; 9, Ascending portion of cavernous ICA and the horizontal petrous ICA; 10, Anterior medullary segment of vertebral artery; 11, Origin of PICA, contents of the jugular foramen (IX, X, XI nerves) and XII inside the hypoglossal canal; 12, Vertical petrous ICA; 13, Extradural vertebral artery piercing the dura; 14, Dentate ligament; 15, Maxillary artery; 16, Anterior spinal artery; 17, Vertebral artery (between the C3 and C2, and between C2 and C1); 18, cervical segment of ICA
Figure 1.3: Lateral view of the right middle fossa and parasellar region. The trigeminal nerve has been folded downward to expose the petrolingual ligament which extends above the horizontal petrous ICA, just proximal to where the artery enters the cavernous sinus. 1, Pituitary stalk; 2, Optic nerve; 3, Supraclinoid carotid artery and the distal ring; 4, Oculomotor nerve; 5, Posterior bend; 6, Clinoid ICA and the proximal ring; 7, Anterior clinoid process (partially drilled out); 8, Trochlear nerve; 9, Ascending cavernous ICA; 10, Horizontal cavernous ICA; 11, Anterior bend; 12, Abducens nerve; 13, Petrolingual ligament; 14, Horizontal petrous ICA; 15, Greater superficial petrosal nerve; 16, Middle meningeal artery; V1, Ophthalmic division of the trigeminal nerve; V2, Maxillary division of the trigeminal nerve; V3, Mandibular division of the trigeminal nerve
The horizontal portion then continues in the carotid canal, inside the temporal bone, first anteromedially underneath the gasserian ganglion, then superomedially toward the foramen lacerum to enter the cavernous sinus to become the ascending portion of the cavernous carotid artery (Fig. 1.2). The transition between the horizontal petrous carotid artery and the ascending cavernous artery is marked by the presence of the petrolingual ligament (Fig. 1.3); the petrous ICA can send off two branches: (1) the caroticotympanic, and (2) pterygoid arteries; the caroticotympanic artery is small and it enters the tympanic cavity by a foramen in the carotid canal. The pterygoid or vidian artery is inconstant and it enters the pterygoid canal with the vidian nerve.
The cavernous segment of ICA presents four portions: (i) the ascending, (ii) the posterior bend, (iii) the horizontal, and (iv) the anterior bend. The ascending portion of the cavernous ICA exits the foramen lacerum and courses superiorly toward the posterior clinoid process and it is crossed on its lateral wall by the abducens nerve, the sole truly intracavernous cranial nerve, as all other cranial nerves related to the cavernous sinus course on its lateral wall. No branch arises from the ascending portion of the cavernous carotid artery. At the level of, or just before the level of the posterior clinoid process, the cavernous carotid artery will turn anteriorly to constitute the posterior bend. The posterior bend sends off the meningohypophyseal trunk, constituted by three branches: (i) the dorsal meningeal artery that courses posteriorly to supply the dura of the upper clivus area and the abducens nerve; (ii) the tentorial artery (Fig. 1.4)1 runs laterally to supply the tentorium and (iii) the inferior hypophyseal artery that supply the posterior portion of the pituitary gland and its capsule. On the side of the body of the sphenoid bone, the posterior bend continues anteriorly to constitute the horizontal portion of the cavernous ICA; if the cavernous ICA is elongated, the horizontal cavernous carotid artery can become tortuous and it can present a descending course as shown in Figure 1.3.
From the inferior or lateral surface of the horizontal portion, the cavernous ICA sends off the inferolateral trunk, which runs laterally above the abducens nerve to reach the lateral wall of the cavernous sinus. In 8% of the cases, there are branches arising from the medial part of the horizontal portion, called McConnell capsular artery, to supply the capsule of the pituitary gland.2 The anterior part of the horizontal portion and its transition into the anterior bend is related laterally to the abducens nerve and to the ophthalmic division of the trigeminal nerve (V1) and these mark approximately the inferior margin of the anterior portion of the cavernous sinus.
Figure 1.4: Lateral view of the right parasellar region. 1, Supraclinoid ICA; 2, Ophthalmic artery; 3, Distal dural ring; 4, Optic nerve; 5, Oculomotor nerve; 6, Clinoid ICA; 7, Proximal dural ring; 8, Trochlear nerve; 9, Dorsal meningeal artery; 10, Meningohypophyseal trunk; 11, Tentorial artery; 12, Abducens nerve; 13, Inferolateral trunk
Figure 1.5: Medial view of the right parasellar region. 1, Optic nerve; 2, Supraclinoid ICA; 3, Pituitary stalk; 4, Ophthalmic artery and the dural ring; 5, Optic canal; 6, Adenohypophysis; 7, Neurohypophysis; 8, Optico-carotid recess; 9, Cavernous ICA; 10, Superior orbital fissure; BA, basilar artery; V2, Maxillary division of the trigeminal nerve
Just behind the optic strut, a bony pillar that separates the superior orbital fissure from the optic canal, the horizontal cavernous carotid artery turns superiorly to constitute the anterior bend.3 From the anterior bend the ICA courses superiorly and laterally, medial to the anterior clinoid process to pierce the dura to enter the intradural compartment. The portion of the ICA that courses on the medial side of the anterior clinoid process is called the clinoid segment. The clinoid segment is considered a transitional segment between the cavernous and the intradural compartment, yet, evidence has revealed that the clinoid segment is also located inside the cavernous sinus.4 The clinoid segment of the ICA is limited inferiorly by the carotid-oculomotor membrane, also called the proximal dural ring and limited superiorly by the dura mater covering the anterior clinoid process, also called the distal dural ring. The lateral portion of the clinoid segment is seen when the anterior clinoid process is removed; the medial side of the clinoid carotid artery is not covered by the anterior clinoid process (Fig. 1.5).
Due to the lateral deviation of the carotid artery at this level, there might be a space left between the anterior bend and the clinoid segment and the medial bony wall (the body of the sphenoid bone) that might be a potential site of origin for aneurysms.
From the anterior bend, the clinoid segment of the ICA courses superiorly and laterally to pierce the dura to constitute the supraclinoid segment.
The supraclinoid segment of the ICA has been divided into three segments based on the origin of its major branches5: (1) the ophthalmic segment extends from the origin of the ophthalmic artery to the origin of the posterior communicating artery (PCom); (2) the communicating segment extends from the origin of the PCom to the origin of the AChA; and the (3) choroidal segment extends from the origin of the AChA to the bifurcation of the ICA (Figs 1.6A and B). The ophthalmic artery arises under the optic nerve, usually from the medial one-third of the superior surface of the ICA, then it passes anteriorly and laterally to become superolateral to the carotid artery to enter the optic canal and the orbit (Figs 1.5, 1.7A and B). The intraoperative exposure of the ophthalmic artery often requires the retraction of the optic nerve and the removal of the anterior clinoid process (Figs 1.7C and D). The perforating arteries from the ophthalmic segment arise from the posterior or medial or posteromedial aspect of the ICA and are distributed to the stalk of the pituitary gland, the optic chiasm and less commonly to the optic nerve, premamillary portion of the floor of the third ventricle and the optic tract. The superior hypophyseal arteries, which can range from 1 to 5 in number, arise from the ophthalmic segment and pass medially to supplies the pituitary stalk and the anterior lobe of the pituitary gland (Fig. 1.8). The infundibular arteries are another group of arteries that arise from the PCom and supply the same area as the superior hypophyseal artery. The PCom arises from the posteromedial or the posterior or the posterolateral aspect of the ICA and usually passes posteromedially to join the posterior cerebral artery (PCA).
Figures 1.6A and B: (A) Lateral view of the left supraclinoid ICA and its main branches. AChA, Anterior choroidal artery; PCom, Posterior communicating artery; (B) Frontal view of the left supraclinoid ICA. AChA, Anterior choroidal artery; PCom, Posterior communicating artery
Figures 1.7A and B: (A) Superior view of the right orbit. 1,Trochlea of the superior oblique muscle; 2, Lacrimal gland; 3, Optic nerve with its sheath; 4, Nasociliary nerve; 5, Optic nerve; 6, Supraclinoid ICA; IV, Trochlear nerve (B) Superior view of the right orbit. The optic nerve has been sectioned to display the ophthalmic artery and its branches. 1, Anterior ethmoidal artery; 2, Ophthalmic artery; 3, Posterior ethmoidal artery; 4, Medial rectus muscle; 5, Nasociliary nerve; 6, Ophthalmic artery; 7, Supraclinoid ICA
During its trajectory, the PCom stays in a cisternal compartment limited medially by the pituitary stalk, laterally by the anteromedial surface of the uncus, superiorly by the optic tract and the floor of the third ventricle and inferiorly by the roof of the cavernous sinus and the dorsum sellae (Figs 1.8 to 1.12); however, the trajectory of the PCom can be variable: if the supraclinoid carotid artery is short and presents a lateral curve, the origin of the PCom can be rather lateral, close to the uncus and close to the roof of the cavernous sinus (Fig. 1.7C); if the supraclinoid carotid artery is long and straight upward, the PCom can stay in the center of that cisternal compartment, away from the walls of that compartment (Figs 1.13A and B); if the PCom is of fetal type, its trajectory tends to be more lateral, closer to the third nerve; if the supraclinoid carotid artery presents a trajectory that is almost parallel to the roof of the cavernous sinus, consequently, the supraclinoid carotid artery is in close proximity to the posterior bend of the cavernous carotid artery, the PCom can course very close to the roof of the cavernous sinus (Figs 1.14A to E); if the PCA is high and the origin of the PCom is low, the PCom might course posteriorly, medially and superiorly to meet the P1. Therefore, the general anatomy of the PCom is presented here, but readers are advised to pay attention to the trajectory of the supraclinoid ICA and PCom of each particular patient.
In the embryo, the PCom continues as PCA, but in the adult the PCA becomes part of the basilar system. If PCom remains the major origin of the PCA, the configuration of the PCom is termed “fetal”. In 60% of the cases, no perforating arteries arise from the communicating segment of the ICA; when present, the perforating arteries from PCom range 4–14 in number, they arise predominantly from the proximal half of the artery and course superiorly to terminate in the floor of the third ventricle (Fig. 1.15). The largest branch from the PCom is the premamillary artery or “anterior thalamoperforating artery” (Fig. 1.11).
The anterior choroidal artery arises either from the posterolateral or from the posterior aspect of the ICA. The AChA courses posteriorly and laterally below the optic tract, between the uncus and the crus cerebri to enter the temporal horn through the inferior choroidal point, the beginning of the choroidal fissure (Figs 1.10, 1.12, 1.16 and 1.17). During its course through the crural cistern (cisternal segment), the AChA sends off branches to the optic tract, crus cerebri, lateral geniculate body and to the uncus. The AChA supplies the optic radiation, globus pallidus, midbrain, thalamus and the retrolenticular and posterior portion of the posterior limb of the internal capsule. Inside the temporal horn, the plexal segment the AChA anastomoses with the lateral posterior choroidal artery (PLChA).6 Due to its close relationship to the uncus, optic tract and the inferior choroidal point, the AChA is a good angiographic landmark for locating those structures (Figs 1.12 and 1.18A to C).7,85
Figures 1.7C and D: (C) Surgical view. A left pterional approach has been performed. The frontal and the lateral view of the corresponding angiography are displayed. Note that the supraclinoid ICA presents a sharp medial loop where the aneurysm is present, then curves laterally. 1, Anterior clinoid process; 2, Left optic nerve; 3, Supraclinoid ICA; AN, aneurysm; A1, A1 segment of the anterior cerebral artery; M1, sphenoid segment of the middle cerebral artery (D) Same case as shown in Figure 1.7C. The anterior clinoid process has been removed to display the ophthalmic artery and the proximal portion of the aneurysm neck. 1, Left optic nerve; 2, Ophthalmic artery; 3, Supraclinoid ICA; 4, Aneurysm; M1, Sphenoid segment of the middle cerebral artery
Figure 1.8: Superior view of the sellar area. 1, Optic nerve; 2, Ophthalmic artery; 3, Superior hypophyseal artery; 4, Supraclinoid ICA; 5, PCom; 6, P1 segment of the posterior cerebral artery (PCA); 7, P2A segment of the PCA; III, oculomotor. Note that the anterior half of the PCom is related inferiorly to the roof of the cavernous sinus and the posterior half of it is related inferiorly to the interpeduncular cistern
The choroidal segment of the ICA is the most frequent site of perforating arteries (range 1–9); they arise from the posterior aspect of the ICA and terminate in the posterior half of the central region of the anterior perforated substance (APS), optic tract and the uncus (Fig. 1.19).6
The vascular territory of the AChA, PCom and perforators from the supraclinoid carotid artery can be seen in Figures 1.20A to E.9 The supraclinoid segment of the ICA bifurcates under the APS into the middle and the anterior cerebral arteries (Figs 1.17, 1.18B and 1.19).
Middle Cerebral Artery
The middle cerebral artery (MCA) is divided into four segments: (1) M1 or sphenoidal segment, (2) M2 or insular segment, (3) M3 or opercular segment and (4) M4 or cortical segment.10
The M1 or sphenoidal segment of the MCA extends from the bifurcation of the ICA to the limen insulae. It courses first in the carotid cistern then continues in the sphenoidal compartment of the sylvian fissure; the proximal half of the M1 is related posteriorly and inferiorly to the anteromedial surface of the uncus, anteriorly to the lesser wing of sphenoid and superiorly to the APS; the distal half is related inferiorly to the planum polare, anteriorly to the lesser wing of sphenoid, superiorly and posteriorly to the insular pole, which is the inferior portion of the anterior surface of the insula (Figs 1.10, 1.15, 1.17, 1.18B and 1.21A). The M1 presents two types of branches: (1) the lateral lenticulostriate arteries which arise mostly from the superior or posterosuperior aspect of the M1 and penetrate the middle and posterior portions of the lateral half of APS to supply the basal ganglia (Figs 1.21B to E) and the early branches that course toward the temporal lobe to supply the temporal pole (Figs 1.10, 1.21B and C).
Figure 1.9A: Medial view. A sagittal cut has been performed at the midline. The medial side of the ophthalmic segment of ICA is a cisternal space. 1, Foramen of Monro; 2, Massa inter- media; 3, Anterior commissure; 4, Mamillary body; 5, Anterior cerebral artery and lamina terminalis; 6, PCA and superior cerebellar artery (SCA); 7, Chiasmatic recess of the third ventricle; 8, Pituitary stalk; 9, Oculomotor nerve; 10, Optic nerve and Ophthalmic artery; 11, Basilar artery; 12, Dorsum sellae and clivus; 13, Dural ring; 14, Adenohypophysis; 15, Neurohypophysis
The bifurcation of the MCA occurs before the limen of insulae in 86% of the cases (Figs 1.18A and 1.21B).10
The shape of the M1 segment can vary: it initially can loop upward toward the anterior perforates substance then it courses downward toward the limen insulae (Fig. 1.22A); it initially can loop downward toward the temporal lobe then courses upward toward the limen insulae (Fig. 1.18B); it can present a rather straight course from the bifurcation of the supraclinoid carotid artery to the limen insulae without looping (Fig. 1.13B); it also can present a rather early bifurcation (Fig. 1.22B).11
The M2 or insular segment extends from the limen insulae to the superior and inferior circular or limiting sulci of the insula; it runs in the insular compartment of the sylvian fissure and is constituted by the superior and the inferior trunks and their branches. After reaching the superior or inferior circular or limiting sulcus of the insula, the M2 branches enter the opercular compartment and are called M3 segment (Figs 1.10, 1.18B, 1.21B and 1.22C).
Anatomically, the limen insulae is a ridge located at the anterior pole of the insula and it separates the carotid cistern from the sylvian cistern (Figs 1.21B and 1.22D). More evident than the limen insulae is the genu of the MCA, which is the turn made by the MCA around the insular pole, passing from the basal surface to the lateral surface of the cerebrum. The genu of the MCA is located just few millimeters deep and anterior from the pars triangularis of the inferior frontal gyrus.
Figure 1.9B: Same specimen as in Figure 1.9A, but the inferior portion of the floor of the third ventricle and pituitary stalk have been removed.1, Choroidal segment of ICA; 2, PCA; 3, AChA; 4, Communicating segment of the ICA; 5, Ophthalmic segment of the ICA; 6, PCom
Figure 1.10: Superior view. 1, Olfactory tract; 2, Genu of the middle cerebral artery (MCA); 3, Anterior cerebral artery; 4, Supraclinoid ICA; 5, Insular pole; 6, PCom; 7, tentorial edge; 8, AChA; 9, P1 segment of the PCA; 10, P2A segment of the PCA; 11, P2P segment of the PCA and lateral posterior choroidal artery; 12, Sylvian point; 13, Calcarine artery; 14, Parieto-occipital artery; *, Lesser wing of the sphenoid; Early branch, early branch of the MCA; M2, insular segment of the MCA; M3, Opercular segment of the MCA; ICP, Inferior choroidal point
Figure 1.11: Basal view. 1, PCom; 2, Perforating branch of the PCom; 3, Anterior thalamoperforating artery; 4, AChA; 5, P1 segment of the PCA; 6, P2A segment of the PCA; 7, Tuber cinereum; 8, Medial posterior choroidal artery (MPChA); 9, Mamillary body
Figure 1.12: Medial view. The right crus cerebri and part of the floor of the third ventricle have been removed. 1, Inferior choroidal point; 2, Perforating branches of the AChA; 3, P2A segment of the PCA; 4, AChA and the posteromedial surface of the uncus; 5, Anterior cerebral artery; 6, M1 segment of the MCA; 7, Anteromedial surface of the uncus; 8, P1 segment of the PCA; 9, PCom
Figures 1.13A and B: (A) Lateral projection of a carotid angiography. The supraclinoid ICA is relatively elongated and it is far from the posterior bend of the cavernous ICA (meaning that probably the supraclinoid ICA is high above the roof of the cavernous sinus and above the dorsum sellae) (B) The frontal projection of the case shown in Figure 1.13A
That relationship is a useful landmark for locating intraoperatively, a MCA aneurysm (Fig. 1.22E)12: if the aneurysm is proximal to the genu of the MCA, it is located far proximal to the pars triangularis and the splitting of the sylvian fissure can be started well proximal to the pars triangularis; if the aneurysm is located at the level of the genu of the MCA, the splitting of the sylvian fissure can start just proximal to the pars triangularis; if the aneurysm is just a little distal to the genu of the MCA, the splitting of the sylvian fissure can start at the level of the pars triangularis (Figs 1.22F and G).
The M3 or opercular segment runs in the opercular compartment and is related to the frontal and parietal opercula superiorly and to the temporal operculum inferiorly. The loop of the most posterior M3 segment branch that exits from the sylvian fissure is called “M point” or “sylvian point.”13 Anatomically the sylvian point is located behind the insula, above the medial end of the Heschl's gyrus (Figs 1.10, 1.18B and 1.22C). The angiographic sylvian point or “M point” displays the location of the medial end of the Heschl's gyrus, posterior end of the insula and the central core, atrium and the pulvinar of the thalamus.12,14
Figure 1.14A: The frontal projection of a right carotid angiography. In this case, the supraclinoid ICA is short, it presents a lateral curve, an aneurysm and a fetal type of PCom
Figure 1.14B: Lateral projection of the same case shown in Figure 1.14A. Note the proximity of the supraclinoid ICA to the posterior bend of the cavernous ICA
Figure 1.14C: The intraoperative photograph of the case shown in Figures 1.14A and 14B. A right pterional approach has been performed, and the anterior clinoid process has been removed. 1, Optic nerve; 2, Anterior clinoid process (removed); 3, Supraclinoid ICA; 4, PCom; 5, Anteromedial surface of the uncus; 6, M1 segment of the MCA
Figure 1.14D: The supraclinoid ICA has been retracted medially to show the lateral part of the aneurysm, which is attached to the roof of the cavernous sinus. 1, Aneurysm; 2, Roof of the cavernous sinus; 3, PCom (fetal type)
On lateral projection, the M2 and M3 segments form the “sylvian triangle” that depicts the shape of the insula and the anterior, inferior and posterior limits of the central core15 (Fig. 1.22H). The caudate nucleus is the only central core structure projected above the superior level of the sylvian triangle on lateral projection (Fig. 1.22I).12
The fourth segment is the M4 or cortical segment; it extends from the sylvian fissure to the lateral surface of the cerebrum (Figs 1.22C and E).9
Figure 1.14E: The supraclinoid ICA has been retracted laterally to show the medial part of the aneurysm
Figure 1.15: Intraoperative photograph. A left pterional approach has been performed. 1, Oculomotor nerve; 2, Optic nerve; 3, Anteromedial surface of the uncus; 4, Supraclinoid ICA; 5, M1 segment of the MCA; 6, PCom; 7, Perforators from the PCom to the floor of the third ventricle; 8, A1 segment of the anterior cerebral artery
Figure 1.16: Basal view. The floor of the temporal horn has been removed. 1, Supraclinoid ICA; 2, PCom; 3, AChA; 4, Optic tract; 5, Uncus; 6, Lateral geniculate body; 7, Pulvinar of the thalamus; 8, Choroid plexus. The arrow indicates the inferior choroidal point. Note that the cisternal segment of the AChA runs below the optic tract; inside the temporal horn, the cisternal segment of the AChA anastomoses with the lateral posterior choroidal artery
Figure 1.17: Intraoperative photograph. A left pterional approach has been performed. 1, Supraclinoid ICA; 2, Aneurysm; 3, PCom, 4, Uncus; 5, AChA; 6, A1 segment of the anterior cerebral artery (ACA); 7, M1 segment of the MCA
The vascular territory of the MCA can be seen in Figures 1.23A to E.9
Anterior Cerebral Artery
The anterior cerebral artery (ACA) is classified in five segments; (1) A1 segment—extends from the bifurcation of the ICA to the ACom. (2) A2 segment—extends from the ACom to the junction between the rostrum and the genu of the corpus callosum.
Figure 1.18A: Frontal view of a left carotid angiography. The arrowheads indicate the anterior choroidal artery. Please note that the bifurcation of the middle cerebral artery is before the limen insulae
Figure 1.18B: The anatomical correlation of the frontal projection of the angiography. 1, Parieto-occipital artery; 2, Calcarine artery and calcar avis; 3, Sylvian point; 4, P3 segment of the posterior cerebral artery (PCA); 5, P2P segment of the PCA; 6, Crus cerebri; 7, Head of the hippocampus; 8, A1 segment of the anterior cerebral artery (ACA); 9, Supraclinoid ICA; 10, The genu of the middle cerebral artery (MCA); M1 = sphenoid segment of the MCA; M2 = insular segment of the MCA; M3, opercular segment of the MCA; the arrowheads indicate the trajectory of the AChA; the arrow indicates the location of the inferior choroidal point. Please note that the ACom (anterior communicating artery) is located at the level of the planum sphenoidale
Figure 1.18C: Lateral view of the carotid and vertebrobasilar angiography. The arrowheads indicate the trajectory of the AChA. The arrow indicates the inferior choroidal point. Please refer to Figure 1.12 for anatomical correlation
Figure 1.19: Medial view. 1, Perforators from ACA, MCA and ICA to the anterior perforated substance; 2, Perforators to the posterior perforated substance; 3, ACA; 4, Choroidal segment and the bifurcation of the ICA; 5, P1 segment of the PCA; 6, PCom; 7, Communicating segment of the ICA
(3) A3 segment—extends from the genu of the corpus callosum to the point where the artery turns sharply and posteriorly above the genu of the corpus callosum. The A2 and A3 segments together are also called ascending segment. (4) A4 and (5) A5 segments extend above the corpus callosum, from the genu to the splenium. These two segments together are also called horizontal segment and the point bisected in the lateral view close behind the coronal suture separates them. The segment of the ACA distal to the ACom (A2–A5) has also been called pericallosal artery (Fig. 1.24A).
Figure 1.20A: Magnetic resonance imaging, lower axial section of the brain to display the vascular territory of the AChA (at this level the majority of the amygdala is supplied by the AChA, in yellow)
Figure 1.20B: Magnetic resonance imaging, axial section of the brain, at a level slightly higher than the level shown in Figure 1.20A. The perforators of the PCom supply the lower lateral walls of the third ventricle (in brown); the AChA supplies the lower portion of the genu and the posterior limb of the internal capsule (light yellow)
Figure 1.20C: Magnetic resonance imaging, axial section of the brain at a level slightly higher than Figure 1.20B. The perforators of the ICA supply the genu of the internal capsule (orange); the perforators of the PCom supply the anterior portion of the thalamus (brown); the perforators of the AChA supply the posterior limb of the internal capsule (light yellow)
Figure 1.20D: Magnetic resonance imaging, axial section of the brain at a level higher than Figure 1.20C. The perforators of the ICA supply the genu of the internal capsule (orange); the perforators of the PCom supply the anterior portion of the thalamus (brown); the perforators of the AChA supply the posterior limb of the internal capsule (yellow)
The junction of the ACom with the A1 segment occurs above the chiasm in 70% (Fig. 1.24B) and above the nerve in 30%. The shorter A1 segments are usually stretched tightly over the chiasm; the longer ones pass anteriorly over the optic nerve and can be elongated and tortuous and reach either the tuberculum sellae or the planum sphenoidale (Fig. 1.18B).16–18
The medial lenticulostriate perforators, ranging from 1 to 11 branches (average of 6.4), arise from the superior, the posterior, or the posterior-superior aspect of the proximal half of A1 segment and pursue a direct posterior and superior course to enter the medial half of APS (Fig. 1.24C). Embryologically the ACom develops from a multichanneled vascular network that coalesces to a variable degree by the time of birth. Only in 20% of the cases, the ACom communicates two A1 segments of equal size.
Figure 1.20E: Axial section of the brain higher than Figure 1.20D. The vascular territory of the AChA is shown in light yellow and the vascular territory of the perforators of the middle cerebral artery is shown in purple
The ACom complex probably exists as a single channel in about 75% of the cases.18 The perforators from ACom, ranging from 0 to 4 (average 1.6), usually arise from its postero-inferior aspect to supply the infundibulum, the APS, the optic chiasm, the subcallosal area and the preoptic areas of the hypothalamus. The recurrent artery of Heubner of the ACA arises in 78% of the cases from the proximal A2 and it doubles back on its parent vessel, courses anterior to the A1 segment in 60% of the cases and can be seen upon elevating the frontal lobe prior to the visualization of the A1 segment; it is the largest and longest branch directed to the APS. After its origin, it passes above the carotid bifurcation and accompanies the M1 into the medial part of the sylvian fissure before entering the anterior and middle portions of the full mediolateral extent of the APS (Fig. 1.24D). The A2 segment is also the source of the central or the basal perforating arteries, which pass posteriorly to enter the optic chiasm, the lamina terminalis and the anterior forebrain, below the corpus callosum. The two first cortical branches of the ACA supplying the medial surface, the orbitofrontal and the frontopolar arteries usually arise from the A2 segment. The segments A3–A5 give rise to other cortical branches to supply the medial surface of the hemisphere. All the cortical branches arise more frequently from the pericallosal than from the callosomarginal artery. The A2 segment occasionally can be unpaired or azygos (Fig. 1.24E).
From the microsurgical viewpoint, especially for ACom aneurysm surgery, it is important to evaluate the morphology of the A1, whether it is tortuous or straight, short (at the level of the optic chiasm) or elongated (at the level of the optic nerve); it is important to assess the height of the junction between both A1s, because sometimes the ACom complex location can be very high, deeply in the interhemispheric fissure, making its approach difficult, especially when approached from the lateral aspect of the cerebrum as in pterional approach. Also in ACom aneurysm surgery, it is important to evaluate the course of the A2s on the lateral projection in relation to the direction of the fundus of the aneurysm, whether they are parallel, ahead or behind the aneurysm.
Figure 1.21A: Basal view. The inferior portion of the anterior half of the basal ganglia is located immediately above the anterior perforated substance. 1; Frontal horn; 2, Caudate nucleus; 3, Lentiform nucleus; 4, Genu of the MCA; 5, Anterior perforated substance (APS); 6, Supraclinoid ICA; 7, Insular pole; 8, Amygdala; 9, Head of the hippocampus; M1, Sphenoid segment of the MCA
The importance of understanding not only the anatomy, but also the understanding of the angiography and other radiological examinations can be demonstrated in the following illustrative case: 37-year-male presented severe subarachnoid hemorrhage 24 hours before the surgery. The Glasgow coma scale was 13 at the admission. The computed tomography (CT) scan showed a severe subarachnoid hemorrhage (Fig. 1.25A); the carotid angiography was performed and it showed an ACom aneurysm and a left carotid-ophthalmic aneurysm (Figs 1.25B to F); the 3D reconstruction of the angiography also was performed (Fig. 1.25G).
Figure 1.21B: Coronal view. 1, Anterior commissure; 2, Lamina terminalis; 3, Supraclinoid ICA; 4, A1 segment of the anterior cerebral artery (please note that it is rather elongated and courses above the optic nerve); 5, Early branch of the MCA; 6, Optic nerve; 7, Anterior communicating artery (ACom); 8, Olfactory tract. The white arrows indicate the cisternal portion of the lateral lenticulostriate arteries, and the black arrows indicate the lateral lenticulostriate arteries inside the basal ganglia. The white arrowheads indicate the limen insulae. M1; Sphenoid segment of the MCA; M2, Insular segment of the MCA
Figure 1.21C: Right pterional view. 1, Optic nerve; 2, Olfactory tract; 3, Early branch of the MCA; 4, Supraclinoid ICA and AChA; 5, PCom; M1, sphenoid segment of the MCA; A1 segment of the ACA; the arrows indicate the lateral lenticulostriate arteries
Figure 1.21D: Frontal projection of a right carotid angiography. The arrows indicate the lateral lentriculostriate arteries from M1. Note that from their origin from M1 to the basal ganglia, the lenticulostriate arteries present three characteristic curves: (1) first medial, (2) then lateral and (3) finally medial again. On the right carotid angiography, those three curves of the lenticulostriate artery depict the shape of letter “S”. The white arrowhead indicates the sylvian point
Figure 1.21E: Lateral projection of a carotid angiography. The lenticulostriate arteries fan out anteriorly, superiorly and posteriorly (arrows)
A left pterional approach was performed (due to the presence of the carotid-ophthalmic aneurysm). The interpretation of the radiological examinations was: (1) the ACom aneurysm is the one that bled; (2) The ACom aneurysm is hidden in the interhemispheric fissure, therefore, the intraoperative removal of the rectus gyrus is necessary; (3) the right A1 is the predominant one and it joins the ACom complex from behind and because of this, it will be more difficult to expose it for the purpose of controlling the parent vessel, if the aneurysm is approached from the opposite side; (4) one of the ACom aneurysm is projected anteriorly, therefore in the surgery it will be projected superiorly and the other ACom aneurysm is projected posteriorly, therefore, it will be projected inferiorly in the surgery; (5) the left A2 is more anterioly located than the right A2, therefore, it will be found first when a left pterional approach is performed and it might block the access to the aneurysm.14
Figure 1.22C: Lateral view of the left hemisphere. The sylvian fissure has been split wide to display the branches of the MCA over the insula. The white dotted line indicates the limiting or circular sulcus of the insula that separates the insula from the adjacent frontal, temporal and parietal opercula. M1, Sphenoid segment of the MCA; M2, linsular segment of the MCA; M3, Opercular segment of the MCA; M4, Cortical segment of the MCA. The white arrow indicates the sylvian point
Figure 1.22D: Anterior view of the insula. 1, Anterior orbital gyrus; 2, Medial orbital gyrus; 3, Rectus gyrus; 4, Pars orbitalis; 5, Posterior orbital gyrus; 6, Pars opercularis; 7, Insular apex; 8, Anterior perforated substance. The arrowheads indicate the limen insulae
All the observations mentioned above cannot be obtained only by knowing the general microsurgical anatomy, but they can be obtained from the information provided by the radiological examinations of the patient; knowing the general microsurgical anatomy can make the extraction of the radiological information much easier (Fig. 1.25H).
The vascular territory of the ACA can be seen in Figures 1.26A to E.9
Posterior Cerebral Artery
Embryologically the posterior cerebral artery (PCA)17 arises as a branch of the ICA, but up to birth it is most often origin is the basilar artery.
Figure 1.22E: Left pterional view. The white arrow indicates the genu of the MCA. The black arrow indicates the tip of the pars triangularis of the inferior frontal gyrus. Please note the cortical branches of the MCA (M4 segment) spreading out over the cerebral hemisphere
Figure 1.22F: Frontal view of a carotid angio CT. The white arrow indicates the aneurysm; the white arrowhead indicates the genu of the MCA; the black arrow indicates the location of the pars triangularis; the double arrowheads indicate the sylvian point. Note that in this case, the aneurysm is located proximal to the genu of the MCA
Figure 1.22G: The intraoperative photograph of the case shown in Figure 1.22F. Left pterional view. The white arrow indicates the genu of the MCA and the black arrow indicates the tip of the pars triangularis
Figure 1.22H: Lateral projection of the carotid angiography. The arrows indicate approximately the location of the superior, anterior and the inferior limiting sulci of the insula
The PCA is classified in four segments: (1) the P1 segment extends from the basilar bifurcation to the site where the PCom joins the PCA. (2) The P2 segment extends from the PCom to the posterior aspect of the midbrain. The P2 segment is further divided into P2A (anterior) and P2P (posterior) segments. P2A begins at the PCom and courses around the crus cerebri. The P2P begins at the posterior margin of the crus cerebri and runs laterally to the tegmentum of the midbrain within the ambient cistern, parallel and inferiorly to the basal vein, inferolaterally to the geniculate bodies and pulvinar and medially to the parahippocampal gyrus to enter the quadrigeminal cistern. (3) The P3 segment begins under the posterior part of the pulvinar in the lateral aspect of the quadrigeminal cistern and ends at the anterior limit of the anterior calcarine sulcus. The P3 segment is often divided into its major terminal branches: the calcarine and the parieto-occipital arteries before reaching the anterior limit of the anterior calcarine sulcus. The point where the PCA's from each side are closer to each other is called collicular or quadrigeminal point.
Figure 1.22I: Magnetic resonance imaging, coronal view of the hemisphere. The arrow indicates the superior limiting sulcus of the insula and the arrowhead indicates the caudate nucleus. Please note that the caudate nucleus is higher than the superior limiting sulcus of the insula
Figure 1.23A: The approximate vascular territory of the MCA in the basal aspect of the brain. At this level, the MCA supplies the orbital surface of the frontal lobe, the anterolateral portion of the basal surface of the temporal lobe and the anterior part of the amygdala
Figure 1.23B: At this level, the MCA supplies the inferior and lateral aspect of the lentiform nucleus, insula and part of the frontal and temporal lobes
Figure 1.23C: Axial cut, higher than the level shown in Figure 1.23B. At this level the MCA supplies the lateral part of the frontal and the temporal lobe, insula, part of the putamen and lateral part of the globus pallidus
It marks the posterior limit of the midbrain on angiography. (4) The P4 segment is the cortical branches of the PCA (Figs 1.8, 1.10, 1.12, 1.18B and 1.27A to C).19
The main branches arising from the PCA are; the posterior thalamoperforating, the direct perforating, the short and long circumflex, the thalamogeniculate, the MPChA and the LPChA, the inferior temporal, the parieto-occipital, the calcarine and the posterior pericallosal arteries. The posterior thalamoperforating arteries which arise from P1 and enter the brain through the posterior perforated substance, interpeduncular fossa and medial crus cerebri, supply the anterior and part of the posterior thalamus, hypothalamus, subthalamus, substantia nigra, red nucleus, oculomotor and trochlear nuclei, oculomotor nerve, mesencephalic reticular formation, pretectum, rostromedial floor of the third ventricle and the posterior portion of the internal capsule (Fig. 1.27C). The direct perforating arteries to the crus cerebri arise mainly from the P2A segment and supply the crus cerebri.
Figure 1.23D: Axial cut, higher than the level shown in Figure 1.23C. At this level, the MCA supplies part of the frontal and the temporal lobe, insula, lentiform nucleus, anterior limb of the internal capsule and the head of the caudate nucleus
Figure 1.23E: Axial cut. At this level, the MCA supplies the majority of the lateral surface of the frontal and the temporal lobe, the upper part of the head of the caudate nucleus and the corona radiata
Figure 1.24B: Anterosuperior view. 1, Right A2 segment of the ACA; 2, Left A2 segment of the ACA and the recurrent (Heubner) artery; 3, Right A1 segment of the ACA; 4, Anterior communicating artery (ACom) and the lamina terminalis; 5, Left A1 segment of the ACA; 6, Right supraclinoid ICA; 7, Left supraclinoid ICA; 8, Right optic nerve; 9, Pituitary stalk; 10, Left optic nerve
The short and long circumflex arteries to the brainstem arise mainly from the P1 and less frequently from the P2A; the short circumflex artery courses around the midbrain and terminates at the geniculate bodies; the long circumflex artery courses around the midbrain and reaches the colliculi. The thalamogenicute arteries arise equally from the P2A or the P2P segments, perforate the inferior surface of the geniculate bodies and supply the posterior half of the lateral thalamus, posterior limb of the internal capsule, and the optic tract (Fig. 1.27D). The medial posterior choroidal arteries (MPChA) arise mainly from the P2A and less frequently from the P2P and P1 segments and course around the midbrain, medial to the main trunk of the PCA, turn around the pulvinar of the thalamus to proceed superiorly at the lateral side of colliculi and pineal gland, to enter the roof of the third ventricle through the velum interpositum and finally course through the foramen of Monro to enter the choroid plexus in the lateral ventricle (Figs 1.27B and E).20–22 The MPChA supplies the crus cerebri, tegmentum, geniculate bodies (mainly the medial), the colliculi, pulvinar, pineal gland and medial thalamus. Angiographically, on lateral projection, the MPChA describes the shape of the number “3”. The inferior curve of the “3” is when it turns around the pulvinar and the superior curve is when it contours the colliculi before entering the roof of the third ventricle (Fig. 1.27F).
Figure 1.24C: Left pterional view. 1, Left supraclinoid ICA; 2, Left optic nerve; 3, A1 segment of the ACA; 4, Lamina terminalis; 5, Left olfactory tract; 6, Medial lenticulostriate arteries; 7, M1 and the lateral lenticulostriate arteries; 8, Left optic tract
Figure 1.24D: Basal view. 1, Right olfactory tract; 2, Right rectus gyrus; 3, Right A2 segment of the ACA; 4, ACom; 5, Right A1segment of the ACA (cut); 6, Recurrent (Heubner) artery
Figure 1.25A: Computed tomography (CT) scan of the patient. There is a subarachnoid hemorrhage (Fisher III) and hydrocephalus
Figure 1.25B: Right carotid angiography. There is an ACom aneurysm (arrow), arising from the junction of right A1, ACom and right A2. Note that the both A2 are apart, probably indicating that there is a hematoma in that location
Figure 1.25D: Frontal projection of the left carotid angiography. The left A1 is elongated but presents a smaller caliber than the right A1; the PCom is fetal type. There is a carotid-ophthalmic aneurysm on this side
Figure 1.25F: Lateral projection of the right carotid angiography. Both A2s are filled via right A1. Please note that the left A2 is more anteriorly located than the right A2 (the one with the pericallosal aneurysm is the right A2). It is difficult to see the projection of the ACom aneurysm due to the superimposing vessels
Figure 1.25G: Medial view of the right carotid angiography (3D reconstruction). There are two ACom aneurysms, one is projected anteriorly and the other one is projected posteriorly; the right A1 is joining the ACom complex from behind; the left A2 is more anteriorly placed than the right A2
Figure 1.25H: Intraoperative photograph; a left pterional approach has been performed and the left rectus gyrus removed to display the ACom complex. The carotid-ophthalmic aneurysm already has been clipped. 1, ACom aneurysm and the surrounding clot; 2, Left A2; 3, Left optic nerve; 4, Left A1; 5, right A1. Authors still have to find the right A2 before clipping the aneurysm (it is located more posteriorly as shown in the preoperative angiography and angio CT)
Figure 1.26B: The ACA supplied the rectus gyrus and its vicinity area, and also supplies the inferior portion of the head of the caudate nucleus
Figure 1.26C: The ACA supplies the medial part of the frontal lobe, the inferior portion of the head of the caudate nucleus, the inferior portion of the anterior putamen and globus pallidus, and the inferior portion of the anterior limb of the internal capsule
Figure 1.26D: The ACA also supplied the medial portion of the frontal lobe and the septum pellucidum
Lateral posterior choroidal arteries (LPChA) arise mainly from the P2P, and less frequently from the P2A segment and pass laterally to enter the ventricular cavity directly through the choroidal fissure, to supply the choroid plexus in the atrium and the temporal horn. It anastomoses with the AChA (Figs 1.10 and 1.18B). Inferior temporal arteries are distributed to the basal surface of the temporal and occipital lobes. They include hippocampal artery and three groups of temporal arteries, namely, anterior, middle and posterior temporal arteries (Fig. 1.27G). The anterior temporal artery arises mainly from the P2A segment, while the middle and posterior temporal arteries arise mainly from the P2P segment. The hippocampal arteries usually arise from P2 segment and are carried by the arachnoid membrane of the hippocampal sulcus into the hippocampal formation (Figs 1.27H and I).7 Parieto-occipital and calcarine arteries are usually terminal branches of the PCA; they arise predominantly from P3 segment, however, sometimes may also arise from the P2P segment and course respectively in parieto-occipital fissure and the calcarine fissure. As the calcarine fissure reaches laterally to bulge into the medial wall of the atrium and the occipital horn, the calcarine artery also follows laterally into the depth of the calcarine fissure (Figs 1.10, 1.18B and 1.27B). Splenial or posterior pericallosal artery supplies the splenium of the corpus callosum and arises from the parieto-occipital artery in 62% of the cases, but it also can arise from calcarine, MPChA, posterior temporal, P2P, P3 and LPChA (Figs 1.27B and F). The vascular territory of the PCA and its perforating branches to the cerebrum can be seen in Figures 1.28A to E.9
The vascularization of the basal ganglia and the thal- amus can be seen in Figure 1.29A and the angiographic correlation of the PCA can be seen in Figure 1.29B; please correlate it with Figure 1.18B.
INFRATENTORIAL ARTERIAL ANATOMY
The infratentorial arteries are located in the posterior fossa. The posterior fossa is characterized by the rule of three, where the brainstem presents three parts (midbrain, pons and medulla) and the cerebellum presents three surfaces (petrosal, tentorial and suboccipital), three cerebellar peduncles (superior, middle and inferior), three fissures (cerebellomesencephalic, cerebellopontine and cerebellomedullary), three main arteries [superior cerebellar artery (SCA), anterior inferior cerebellar artery (AICA) and posterior inferior cerebellar artery (PICA)] and three main venous draining groups (petrosal, galenic and tentorial). The SCA and AICA usually arise from the basilar artery and the PICA from the vertebral artery. The upper complex includes the oculomotor, trochlear and trigeminal nerves that are related to the SCA. The middle complex includes the abducens, facial and vestibulocochlear nerves that are related to the AICA. The lower complex includes the glossopharyngeal, vagus, accessory and hypoglossal nerve that are related to the PICA.23,24
In summary, the upper complex includes the SCA, midbrain, cerebellomesencephalic fissure, superior cerebellar peduncle, tentorial surface of the cerebellum and the oculomotor, trochlear, and trigeminal nerve. The SCA arises in front of the midbrain, passes below the oculomotor and trochlear nerve, and above the trigeminal nerve to reach the cerebellomesencephalic fissure, where it runs on the superior cerebellar peduncle and terminates by supplying the tentorial surface of the cerebellum (Fig. 1.30).22
Figure 1.27A: Basal view. The floor of the temporal horn has been removed. 1; PCom; 2, P1 segment of the PCA; 3, P2A segment of the PCA; 4, Medial posterior choroidal artery (MPChA); 5, Posterior perforating arteries; 6, lateral mesencephalic sulcus; 7, P2P segment of the PCA; 8, P3 segment of the PCA; 9, Choroid plexus; 10, P4 segment of the PCA; III, oculomotor nerve
The middle complex includes the AICA, pons, middle cerebellar peduncle, cerebellopontine fissure, petrosal surface of the cerebellum and the abducens, facial and vestibulocochlear nerve. The AICA arises from the basilar artery at the pontine level, courses in relationship to the abducens, facial and vestibulocochlear nerve to reach the surface of the middle cerebellar peduncle, where it courses along the cerebellopontine fissure and terminates by supplying the petrosal surface of the cerebellum (Fig. 1.31).
The lower complex includes the PICA, medulla, inferior cerebellar peduncle, cerebellomedullary fissure, suboccipital surface of the cerebellum and glossopharyngeal, vagus, spinal accessory and hypoglossal nerves. The PICA arises from the vertebral artery at the medullary level, encircles the medulla, passing in relationship to the glossopharyngeal, vagus, accessory and hypoglossal nerves to reach the surface of the inferior cerebellar peduncle, where it dips into the cerebellomeduallary fissure and terminates by supplying the suboccipital surface of the cerebellum (Fig. 1.32).
Figures 1.27B and C: (B) Medial view. The retractors have been placed over the parieto-occipital sulcus superiorly and over the posterior calcarine sulcus inferiorly. 1, Parieto-occipital sulcus; 2, MPChA; 3, Parieto-occipital artery; 4, Posterior pericallosal artery; 5, P3 segment of the PCA; 6, Calcarine artery; 7, P2P segment of the PCA; 8, Anterior calcarine sulcus; 9, Inferior temporal arteries; 10, P2A segment of the PCA; (C) Anterosuperior view. 1, Posterior thalamoperforating artery; 2, AChA; 3, P1 segment of the PCA; 4, PCom; 5, M1 segment of the MCA; 6, A1 segment of the ACA; 7, Supraclinoid ICA; III, Oculomotor nerve; P2A, P2A segment of the PCA
Vertebral Artery
The paired vertebral arteries arise from the subclavian arteries, ascend through the transverse processes of the upper six cervical vertebrae, pass behind the lateral masses of the atlas, enter the dura mater behind the occipital condyles, ascend through the foramen magnum to the front of the medulla and join its mate to form the basilar artery at the pontomedullary junction. Each artery is divided into intradural and extradural parts.23
Figure 1.27D: Basal view. 1, P2A; 2, Inferior temporal artery; 3, MPChA; 4, Direct perforating arteries; 5, P2P; 6, Short circumflex arteries; 7, Long circumflex arteries; 8, P3; III, oculomotor nerve
Figure 1.27E: Superior view. The columns of the fornix have been sectioned and the fornix has been folded backward to display the roof of the third ventricle. 1, Anterior septal vein; 2, Foramen of Monro; 3, Thalamostriate vein; 4, Internal cerebral vein; 5, MPChA; 6, Fornix
Figure 1.27F: Lateral projection of a left vertebral artery angiography. The blue arrowheads indicate the MPChA; the red arrowheads indicate the LPChA; the green arrowhead indicates the posterior pericallosal artery; the yellow dotted line indicates approximately the posterior limit of the brainstem
Figure 1.27G: Basal view to display the inferior temporal arteries. 1, Anterior group; 2, Middle group; 3, Posterior group
Figure 1.27H: Intraoperative photograph of the right hippocampus. 1, Head of the hippocampus; 2, Hippocampal arteries; 3, Body of the hippocampus; 4, Hippocampal branches arising from the P2A segment of the PCA; 5, Fimbria of the fornix; 6, Choroid plexus of the temporal horn
Figure 1.27I: Intraoperative photograph of the left hippocampus. 1, Hippocampus; 2, P2A; 3, Fimbria of the fornix; 4, Hippocampal arteries; 5, Transverse hippocampal vein; 6, Choroid plexus. Please note that the hippocampal arteries and veins course within the arachnoid membrane of the hippocampal sulcus to and from the hippocampal formation
Figure 1.28B: The vascular territory of the branches from the posterior choroidal arteries is in light blue and the vascular territory of the PCA is in blue
The extradural part is divided into three segments. The first segment extends from the origin of the vertebral artery in subclavian artery to the entrance into the lowest foramen, usually at the C6 level. The second segment ascends through the transverse foramina of the upper six cervical vertebrae in front of the cervical nerve roots. After exiting from the transverse foramen of the C2, the vertebral artery deviates laterally to enter the laterally placed transverse foramen of C1. The third segment, the one most intimately related to the foramen magnum, extends from the foramen in the transverse process of the atlas to the site of passage through the dura mater (Fig. 1.33A). This segment passes medially behind the lateral mass of the atlas and atlanto-occipital joint and is pressed into the groove on the upper surface of the lateral part of the posterior arch of the atlas, where it courses along the floor of the suboccipital triangle.
Figure 1.28C: Vascular territories of the PCA and its branches. In purple, thalamogeniculate arteries; in violet, thalamoperforating arteries; in light blue, posterior choroidal arteries; in blue, PCA
Figure 1.28D: In purple, thalamogeniculate arteries; in violet, thalamoperforating arteries; in light blue, posterior choroidal arteries; in blue, PCA
Figure 1.28E: In purple, thalamogeniculate arteries; in violet, thalamoperforating arteries; in blue, PCA
It enters the vertebral canal by passing anteriorly to the lateral border of the atlanto-occipital membrane. The terminal extradural segment of the vertebral artery gives rise to the posterior meningeal artery, posterior spinal artery, muscular branches to the deep cervical musculature and infrequently the PICA (Fig. 1.33A).
The intradural segment begins at the dural foramina just inferior to the lateral edge of the foramen magnum. The dura in this region is much thicker than in other areas and it forms a funnel-shaped foramen around the artery. The first cervical nerve exits the spinal canal and the posterior spinal artery enters the spinal canal through this dural foramen with the vertebral artery. These three structures are bound together at the foramen by fibrous dural bands. The initial intradural segment of the vertebral artery passes just superiorly to the dorsal and ventral roots of the first cervical nerve and just anteriorly to the posterior spinal artery, the dentate ligament and the spinal portion of the accessory nerve (Fig. 1.33B). The intradural segment of the vertebral artery is divided into the lateral medullary and anterior medullary segments before joining its contralateral mate to form the basilar artery. The lateral medullary segment begins at the dural foramen and passes anteriorly and superiorly along the lateral medullary surface to terminate at the preolivary sulcus. The anterior medullary segment begins at the preolivary sulcus, courses in front of, or between, the hypoglossal rootlets and crosses the pyramid to join with the contralateral vertebral artery at or near the pontomedullary sulcus to form the basilar artery (Figs 1.2, 1.31, 1.33B and 1.34). The anterior medullary segment rests on the clivus. The branches arising from the vertebral artery in the region of the foramen magnum are the posterior spinal, anterior spinal, PICA, and anterior and posterior meningeal arteries.
Posterior Inferior Cerebellar Artery
The PICA has the most complex, tortuous and variable course, and area of supply of the cerebellar arteries. The PICA is intimately related to the cerebellomedullary fissure, the inferior half of the ventricular roof, the inferior cerebellar peduncle and suboccipital surface of the cerebellum.25
This artery arises from the vertebral artery near the inferior olive and passes posteriorly around the medulla. The PICA origins at the level of the olive are either lateral or anterior to the olive.
Figure 1.29A: The vascularization of the basal ganglia and the thalamus. The anterior and inferior portion of the head of the caudate nucleus are supplied by perforating branches from recurrent artery and medial lenticulostriate arteries from A1; the middle portion of the caudate nucleus is supplied by the lateral lenticulostriate arteries from M1; the posterior portion of the caudate nucleus is supplied by branches from LPChA. The anterior portion of the putamen is supplied by perforating branches from recurrent artery and A2; the middle portion of the putamen is supplied by the lateral lenticulostriate arteries from M1; the posterior portion of the putamen is supplied by perforating branches from AChA. The anterior portion of the globus pallidus is supplied by the medial lenticulostriate arteries from A1; the middle portion of the globus pallidus is supplied by the lateral lenticulostriate arteries from M1; the posterior portion of the globus pallidus is supplied by perforating branches from AChA. The optic tract is supplied by AChA. The amygdala is supplied by AChA and perforating branches from the choroidal segment of the ICA. The anterior portion of the thalamus is supplied by the AChA; the anteroinferior portion of the thalamus is supplied by perforating branches from PCom; the inferior portion of the thalamus is supplied by the posterior thalamoperforating and thalamogenicute branches from PCA; the superior and the posterior portions of the thalamus are supplied by LPChA; the medial portion of the superior and the posterior portions of the thalamus are supplied by MPChA. APS, anterior perforated substance; PPS, posterior perforated substance
Figure 1.29B: Frontal projection of a vertebrobasilar angiography to display the segments of the PCA. From “a” to “b”, P1; From “b” to “c”, P2A; From “c” to “d”, P2P; From “d” to “e”, P3; Distal to “e”, P4
Figure 1.30: Anterosuperior view of the posterior fossa. 1, Left transverse sinus; 2, Vermian branches of the SCA; 3, Hemispheric branches of the SCA; 4, Cerebellomesencephalic segment of the SCA; 5, Lateral pontomesencephalic segment of the SCA; 6, PCA; 7, Anterior pontomesencephalic segment of the SCA; 8, Superior petrosal vein and superior petrosal sinus; 9, Basilar artery; III, Oculomotor nerve; IV, Trochlear nerve; V, Trigeminal nerve; VI, Abducens nerve; VII, Facial nerve; VIII, Vestibulocochlear nerve
The PICA origin is anterior to the olive if the vertebral artery pursues its usual course anterior to the olive (Figs 1.2 and 1.31), but if the vertebral artery is tortuous and kinked posteriorly, the origin of PICA is lateral to the olive (Figs 1.2 and 1.34A).
At the anterolateral margin of the medulla, the PICA passes rostral or caudal to or between the rootlets of the hypoglossal nerve and at the posterolateral margin of the medulla, it courses rostral to or between the fila of the glosspharyngeal, vagus and accessory nerves. If the vertebral artery is elongated or tortuous and courses lateral to the olive, it stretches the hypoglossal rootlets dorsal over its posterior surface. The PICA commonly passes from the lateral to the posterior aspect of the medulla by passing between the rootlets of the glossopharyngeal, vagus and accessory nerves (Fig. 1.34A). After passing these nerves, it courses around the cerebellar tonsil and enters the cerebellomedullary fissure and passes posterior to the lower half of the roof of the fourth ventricle. On exiting the cerebellomedullay fissure, its branches are distributed to the vermis and hemisphere of the suboccipital surface. Its area of supply is the most variable of the cerebellar arteries.26
Figure 1.31: Anterior view of the brainstem and cerebellum. 1, PCA; 2, SCA; 3, Pontine artery; 4, Basilar artery; 5, Flocculus; 6, Olive; 7, PICA; 8, Pyramid; V, Trigeminal nerve; VI, Abducens nerve; VII, Facial nerve; VIII, Vestibulocochlear nerve; IX, Glossopharyngeal nerve; X, Vagus nerve; XI, Spinal accessory nerve; XII, Hypoglossal nerve; VA, Vertebral artery. The white arrowhead indicates AICA. Please note the direct branches from the posterior wall of the basilar artery to the brainstem
Figure 1.32: Suboccipital view of the cerebellum. 1, Transverse sinus; 2, Inferior vermian veins; 3, Vermian branch of PICA; 4, Sigmoid sinus; 5, Tonsil; VA, Vertebral artery. The white arrowheads indicate the hemispheric branches of PICA
Figure 1.33A: Posterior view of the craniocervical junction. 1, Posterior spinal artery and the spinal accessory nerve; 2, Dura mater and the first triangular process of the dentate ligament; 3, Vertebral artery above the posterior arch of C1 and behind the lateral mass of C1; 4, Dentate ligament; 5, C1 nerve; 6, Transverse process of C1; 7, Dorsal ganglion of C2; 8, Vertebral artery exiting from the foramen transversarium of C2
Figure 1.33B: A magnified view of the transition between the extradural and the intradural vertebral artery. 1, Spinal accessory nerve; 2, Dorsal root of C1; 3, First triangular process of the dentate ligament; 4, Posterior spinal artery; 5, Dura; 6, Second triangular process of the dentate ligament; 7, Posterior arch of the C1; 8, Dorsal roots of the C2. Please note that the spinal accessory nerve runs behind the dentate ligament
The PICA gives off perforating, choroidal and cortical arteries.
PICA Presents the Following Segments
The anterior medullary segment—this segment lies anterior to the medulla. It begins at the origin of the PICA anterior to the medulla and extends backward past the hypoglossal rootlets to the level of the rostrocaudal line through the most prominent part of the inferior olive that marks the boundary between the anterior and lateral surface of the medulla. An anterior medullary segment is more likely to be present if the PICA arises from the superior part of the vertebral artery, because the vertebral artery courses from the lateral side of the medulla below to the anterior surface of the medulla above. From its origin, the PICA usually passes posteriorly around or between the hypoglossal rootlets, but occasionally loops upward, downward, laterally or medially before passing posteriorly around or between the hypoglossal rootlets (Figs 1.2, 1.31 and 1.34A). The lateral medullary segment—this segment begins where the artery passes the most prominent point of the inferior olive and ends at the level of the origin of the glossopharygeal, vagus and accessory rootlets. This portion is present in most PICAs (Figs 1.2, 1.31, 1.34A and B).28
Figure 1.34A: Frontal view of the posterior fossa. 1, PCA; 2, SCA; 3, Basilar artery; 4, Superior petrosal sinus; 5, AICA; 6, Median branches to the brainstem; 7, Anterior medullary segment of the vertebral artery; 8, Lateral medullary segment of the vertebral artery; 9, Preolivary sulcus; 10, Anterior spinal artery. The black arrow indicates the labyrinthine artery. The white arrow indicates the origin of the PICA. V, Trigeminal nerve; VI, Abducens nerve; VII, Facial nerve; VIII, Vestibulocochlear nerve; IX, Glossopharyngeal nerve; X, Vagus nerve, XI, Accessory nerve, XII, Hypoglossal nerve
The tonsillomedullary segment—this segment begins where the PICA passes posterior to the glossopharyngeal, vagus and accessory nerves and extends medially across the posterior aspect of the medullar near the caudal half of the tonsil. It ends where the artery ascends to the midlevel of the medial surface of the tonsil. The proximal portion of this segment usually course near the lateral recess and then posteriorly to reach the inferior pole of the tonsil. This segment commonly passes medially between the lower margin of the tonsil and the medulla before turning rostrally along the medial surface of the tonsil. The loop passing near the lower part of the tonsil, referred to as the caudal loop, has been reported to form a caudally convex loop that coincides with the caudal pole of the tonsil.
The telovelotonsillar segment—this is the most complex of the segments. It begins at the midportion of the PICA's ascent along the medial surface of the tonsil toward the roof of the fourth ventricle and ends where it exits the fissures between the vermis, tonsil and cerebellar hemisphere to reach the suboccipital surface of the cerebellum. In most, but not at all, hemispheres, this segment often forms a loop with convex rostral curve, called the cranial loop.27–29
This loop is located caudal to the fastigium between the cerebellar tonsil below and the tela choroidea, and inferior medullary velum above. The apex of the cranial loop usually overlies the central part of the inferior medullary velum, but its location varies from the superior to the inferior margin and from the medial to the lateral extent of the inferior medullary velum (Figs 1.34A to C). The apex of the cranial loop is inferior to the level of the fastigium of the fourth ventricle in most cases, but may also extend to the level of the fastigium. This segment gives rise to branches that supply the tela choroidea and the choroid plexus of the fourth ventricle. The cortical segment— this segment begins where the trunks and branches leave the groove between the vermis medially and tonsil and the hemisphere laterally, and includes the terminal branches. The bifurcation of the PICA often occurs near the origin of this segment. The cortical branches radiate outward from the superior and lateral border of the tonsil to the vermis and hemisphere (Fig. 1.32).
Figure 1.34B: Left posterolateral view. 1, Inferior colliculus; 2, SCA; 3, Trigeminal nerve; 4, Superior cerebellar peduncle; 5, Middle cerebellar peduncle; 6, Internal acoustic meatus and AICA; 7, Jugular foramen; 8, Lateral mass of C1; VA, Vertebral artery. Proximal to “a”, Lateral medullary segment of PICA; from “a” to “b”, Tonsillomedullary segment of PICA; from “b” to “c”, Telovelotonsillar segment of PICA. The white arrow indicates the cranial loop of PICA; the black arrow indicates the caudal loop of PICA. Floc, Flocculus
The site of the origin of the PICA from the vertebral artery varies from below the foramen magnum to the vertebrobasilar junction. PICA may arise from the vertebral artery in an extradural location.3029
Figure 1.34C: Suboccipital view. The biventral lobule and both tonsils have been removed to display the roof of the fourth ventricle. 1, Inferior vermian vein; 2, Pyramid; 3, Inferior medullary velum; 4, Uvula and the vermian branch of PICA; 5, Vein of the lateral recess of the fourth ventricle; 6, Vermian branch of PICA; 7, Cranial loop of PICA; 8, Tela choroidea and inferior medullary velum (the telovelar junction has been split) and branch of PICA to the inferior medullary velum; 9, Choroidal branches of PICA to tela choroidea; 10, Flocculus and the jugular foramen; 11, Hemispheric branch of PICA (cut) and the caudal loop of PICA; 12, Posterior spinal artery
Branches from Posterior Inferior Cerebellar Artery
The PICA arises from the posterior or lateral surface of the vertebral artery more often than anterior or medial surface. On leaving the parent vessel, the initial course of the PICA is posterior, lateral, or superior more often than anterior, medial or inferior. The most common site of the bifurcation is in the telovelotonsillar fissure as the artery courses around the rostral pole of the tonsil. The medial trunk usually ascends in the vermohemispheric fissure to reach the vermis and the lateral trunk passes laterally out of telovelotonsillar fissure to reach the hemispheric surface.
The medial trunk terminates by sending branches over the inferior part of the vermis and adjacent part of the tonsil and hemisphere. The lateral trunk divides into a larger hemispheric trunk that gives off multiple branches to the hemisphere and smaller tonsillar branches that supply the posterior and inferior surfaces of the tonsil. The division of the lateral trunk into tonsillar and hemispheric branches may occur at various sites in relation to the tonsil, but is most commonly located near the posterior margin of the medial surface of the tonsil. The trunks passing through the tonsillomedullary fissure send branches to the medullar and the trunk passing through the telovelotonsillar fissure send ascending branches to the dentate nucleus (Figs 1.32 and 1.34C).31
Perforating Arteries
The perforating arteries are small arteries that arise from the three medullary segments and terminate in the brainstem. The direct type pursues a straight course to enter the brainstem. The circumflex type passes around the brainstem before terminating in it. The circumflex perforating arteries are divided into short and long types.
The perforating branches of the PICA intermingle and overlap with those arising from the vertebral artery. The segment of the vertebral artery distal to the origin of the PICA more frequently gives rise to perforating arteries than the segment proximal to the PICA origin.
Choroidal Arteries
The PICA gives rise to branches that supply the tela choroidea and choroid plexus of the fourth ventricle, usually supplying the choroid plexus near the midline of the roof of the fourth ventricle and in the medial part of the lateral recess.32 More choroidal branches arise from the tonsillomedullary and telovelotonsillar segment than from the lateral or anterior medullary segment. The AICA usually supplies the portion of the choroid plexus not supplied by the PICA (Figs 1.34B and C).
Cortical Arteries
The most common area supplied by the PICA includes the majority of the ipsilateral half of the suboccipital surface of the cerebellum. The cortical branches are divided into hemispheric, vermian and tonsillar groups. The vermian branches usually arise from the medial trunk and the hemispheric and tonsillar branches from the lateral trunk. Each half of the vermis is divided into median and paramedian segments and the hemisphere lateral to the vermis is divided into medial, intermediate and lateral segments (Fig. 1.32).
The angiographic correlation (frontal projection) of the vertebral artery and PICA can be seen in Figure 1.34D. Please correlate it with Figures 1.2, 1.31, 1.34A and B for vertebral artery and Figures 1.2, 1.31, 1.34A, B and E for PICA.
The lateral projection of the angiographic correlation of those arteries can be seen in Figure 1.34F. Please correlate it with Figure 1.34B.
Basilar Artery
The basilar artery is formed by the junction of the two vertebral arteries.
Figure 1.34D: The frontal projection of a vertebrobasilar angiography. From “a” to “b”, lateral medullary segment of PICA; from “b” to “c”, tonsillomedullary segment of PICA; from “c” to “d”, telovelotonsillar segment of PICA. The single arrow indicates the probable entry site of the extradural vertebral artery into the intradural compartment; note the slight reduction in the lumen of the vertebral artery at this point. The single arrowhead indicates the foramen transversarium of C2. The double arrowheads indicate the foramen transversarium of C1
As a single vascular trunk, it usually courses superiorly in a shallow median groove on the anterior surface of the pons. It extends from the pontomedullary sulcus, which demarcates the rostral medulla and caudal pons, to the level of the emerging oculomotor nerves from the caudal midbrain. The basilar artery is often tortuous and may be slightly curved in its rostral course along the brainstem. The basilar artery may deviate a short distance off the midline, but a few will deviate laterally as far as the origin of the abudcens nerve or the facial and vestibulocohlear nerves.33,34 Its length varies from 2.5 to 4.0 cm and after passing between the two oculomotor nerves, the artery terminates by dividing into the two posterior cerebral arteries (Figs 1.2, 1.31 and 1.34A).
In its course the basilar artery gives off the following branches on both sides; pontine, labyrinthine, anterior inferior cerebellar, superior cerebellar and posterior cerebral arteries. The pontine branches of the basilar artery consist of a number of small vessels that supply the pons and midbrain. Median vessels arise at right angles from the posterior aspect of the basilar and enter the pons along its anterior median groove (Figs 1.34A). The transverse branches arise from the posterolateral and lateral aspects of the basilar artery and in their circumferential course around the brainstem, they send penetrating vessels into the pons.
Figure 1.34E: Frontal view. The brainstem has been removed to display the segments of PICA in relation to the roof of the fourth ventricle. From “a” to “b”, lateral medullary segment of PICA; from “b” to “c”, tonsillomedullary segment of PICA; from “c” to “d”, telovelotonsillar segment of PICA. 1, Superior cerebellar peduncle; 2, Superior medullary velum, 3, Nodule (covered by inferior medullary velum); 4, Middle cerebellar peduncle. VII, Facial nerve; VIII, Vestibulocochlear nerve; IX, Glossopharyngeal nerve; X, Vagus nerve; XI, Accessory nerve
Figure 1.34F: The lateral projection of a vertebrobasilar angiography. From “a” to “b”, lateral medullary segment of PICA; from “b” to “c”, tonsillomedullary segment of PICA; from “c” to “d”, telovelotonsillar segment of PICA. 1, Basilar artery; 2, Vermian branch of PICA; 3, Pyramidal loop (vermian branch of PICA coursing around the pyramid); 4, Hemispheric branch of PICA; 5, Vertebral artery behind the lateral mass of C1. Please note that the hemispheric branch of PICA is closer to the skull than the vermian branch
Anterior Inferior Cerebellar Artery
The anterior inferior cerebellar artery generally arises from the lower third of the basilar artery (Figs 1.2, 1.31, 1.34A and B). The AICA courses through the central part of the cerebellopontine angle (CPA) near the facial and vestibulocochlear nerve.
The AICA is intimately related to the pons, lateral recess, foramen of Luschka, cerebellopontine fissure, middle 31cerebellar peduncle, and petrosal surface of the cerebellum. The AICA originates from the basilar artery, usually as a single trunk and encircles the pons near the abducens, facial and vestibulocochlear nerve. After coursing near and sending branches to the nerves entering the internal acoustic meatus and to the choroid plexus protruding from the foramen of Luschka, it passes around the flocculus on the middle cerebellar peduncle to supply the lips of the cerebellopontine fissure and the petrosal surface of the cerebellum.
The AICA is divided into the rostral and caudal trunks near the facial-vestibulocochlear nerve complex. After crossing the nerves, the rostral trunk usually courses laterally above the flocculus to reach the surface of the middle cerebellar peduncle and the petrosal fissure to be distributed to the superior lip of the cerebellopontine fissure and to the adjoining part of the petrosal surface. The caudal trunk supplies the inferior part of the petrosal surface, including a part of the flocculus and the choroid plexus and is frequently related to the lateral portion of the fourth ventricle. The AICA gives rise to perforating branches to the brainstem, choroidal branches to the tela choroidea and choroid plexus and to the nerve related arteries, including the labyrinthine, recurrent perforating and subarcuate arteries.35
The AICA presents the following segments; anterior pontine, lateral pontine, flocculonodular and cortical segments.
The anterior pontine segment begins at the origin and ends at the level of a line drawn through the long axis of the inferior olive and extending upward on the pons and is located between the clivus and the belly of the pons; the AICA usually lies in contact with the rootlets of the abducens nerve.
The lateral pontine segment begins at the anterolateral margin of the pons and passes through the CPA, above, below or between the facial and vestibulocochlear nerves and is intimately related to the internal auditory meatus, to the lateral recess and to the choroid plexus protruding from the foramen of Luschka. This segment gives rise to the nerve-related branches that course near or within the internal acoustic meatus in close relationship to the facial and vestibulocochlear nerves. These nerve-related branches are the labyrinth artery, which supplies the facial and vestibulocochlear nerves and the labyrinth (Fig. 1.34A); the recurrent perforating arteries, which pass toward the meatus, but turn medially to supply the brainstem; and the subarcuate artery which enters the subarcuate fossa to reach the center of the superior semicircular canal (Fig. 1.35A).
The flocculonodular segment begins where the artery passes rostral or caudal to the flocculus to reach the middle cerebellar peduncle and the cerebellopontine fissure. The trunks that course along the peduncle may be hidden beneath the flocculus or in the lips of the cerebellopontine fissure (Fig. 1.35B).
The cortical segment is composed of the cortical branches to the petrosal surface of the cerebellum.
Branches of the Anterior Inferior Cerebellar Artery
The AICAs arising as a single trunk usually bifurcate into a rostral and a caudal trunk. The segment proximal to the bifurcation is the main trunk and the two trunks formed by the bifurcation are the rostral and the caudal trunks. If the bifurcation is proximal to the facial and vestibulocochlear nerve, the caudal trunk courses caudal to the flocculus to supply the inferior part of the petrosal surface, including a part of the flocculus and the choroid plexus.
Figure 1.35A: Left retrosigmoid view. 1, Transverse sinus; 2, Superior petrosal vein; 3, Subarcuate artery; 4, Flocculus; 5, PICA looping above the rootlets of the hypoglossal nerve. V, Trigeminal nerve; VI, Abducens nerve; VIII, Vestibulocochlear nerve; IX, Glossopharyngeal nerve; X, Vagus nerve; XI, Spinal accessory nerve
Figure 1.35B: Left anterolateral view of the petrosal surface of the cerebellum and the brainstem. 1, SCA; 2, Transverse pontine artery; 3, Basilar artery; 4, Flocculus; 5, AICA; 6, Choroid plexus extruding from the foramen of Luschka. Please note the numerous median arteries arising from the posterior wall of the basilar artery to the brainstem. The white arrowheads indicate the meatal loop of AICA
If the bifurcation is distal to the nerves, the caudal trunk courses posteriorly in the inferior limb of the cerebellopontine fissure near the foramen of Luschka. The distal branches of the caudal trunk often anastomose with the PICA and those from the rostral trunk anastomose with the SCA. The AICA gives rise to perforating arteries to the brainstem, choroidal branches to the lateral segment of the choroid plexus and the nerve-related arteries.
Nerve-related branches: The nerve-related branches are those that course in or near the porus of the meatus and related to the facial and vestibulocochlear nerves. In AICA's course through the CPA, the nerve-related trunks give rise to three types of branches: (i) the labyrinthine arteries that are the one or more branches of the AICA that enter the internal auditory canal and send branches to the bone and dura lining the internal auditory canal, to the nerve within the canal and terminate by giving rise to the vestibular, cochlear and vestibulocochlear arteries that supply the organs of the inner ear (Fig. 1.34A).35 (ii) The recurrent perforating arteries arise from the nerve-related vessels and often travel from their origin toward the meatus, occasionally looping into the meatus before taking a recurrent course along the facial and vestibulocochlear nerves to reach the brainstem. They send branches to these nerves and to the brainstem surrounding the entry zone of those nerves. (iii) The subarcuate artery usually originates medial to the porus, penetrates the dura covering the subarcuate fossa and enters the subarcuate canal. This artery supplies the petrous bone in the region of the semicircular canals (Fig. 1.35A).36
The subarcuate canal is recognized as a potential route of extension of infections from the mastoid region to the meninges and to the superior petrosal sinus.37
Cortical branches: The most common pattern is for the AICA to supply the majority of the petrosal surface of the cerebellum, but the cortical area of the supply is quite variable. It can vary from a small area on the flocculus and adjacent part of the petrosal surface to include the whole petrosal surface and adjacent part of the tentorial and suboccipital surfaces of the cerebellum. After crossing the nerves, the rostral trunk usually courses above the flocculus to be distributed to the superior lip of the cerebellopontine fissure and the caudal trunks course caudal to the flocculus to supply the inferior part of the petrosal surface (Figs 1.31 and 1.35B).
The angiographic correlation of AICA (frontal projection) can be seen in Figure 1.35C; please correlate it with Figures 1.2, 1.31, 1.34A, 1.34B and 1.35B. The lateral projection of the angiographic correlation is seen in Figure 1.35D; please correlate it with Figure 1.35B.
Superior Cerebellar Artery
The SCA is intimately related to the cerebellomesencephalic fissure, the superior half of the fourth ventricular roof, the superior cerebellar peduncle and tentorial surface of the cerebellum.33,34 The SCA arises in front of the midbrain, usually from basilar artery near the apex and passes below the oculomotor nerve; the SCA may infrequently arise from the proximal PCA and pass above the oculomotor nerve. The height of the bifurcation of the basilar artery is an important determinant of the initial course.19,38
The level of the bifurcation of the basilar artery is normal if the bifurcation occurs at the pontomesencephalic junction, high if it occurs anterior to the mesencephalon and low if it is anterior to the pons. Nearly two-thirds of SCAs have a point of contact with the oculomotor nerve, usually on the inferior surface. The point of contact usually involves the main trunk or, less commonly, the rostral trunk if there is an early bifurcation. The trochlear nerve has points of contact with the SCA trunks in almost all cases. The SCA encircles the brainstem above the trigeminal nerve, making a shallow caudal loop on the lateral side of the pons. Contact occurs between the SCA and the trigeminal nerve in those cases with the most prominent caudally projecting loops. The point of contact with the SCA is usually on the superior or superomedial aspect of the trigeminal nerve (Fig. 1.36A).39
The tentorium incisura, the opening through the tentorium cerebella, presents triangular shape with the base facing the clivus.40 The other two limbs are formed by the right and left free edges that join at an apex located between the colliculi below the occipital lobes above (Fig. 1.36B). The proximal portion of the SCA courses medial to anterior third of the free edge of the tentorium cerebelli.
Figure 1.35C: The frontal projection of a vertebrobasilar angiography. The red letters refer to SCA and the blue letters refer to AICA. AICA: From “a” to “b”, Anterior pontine segment; From “b” to “c”, Lateral pontine segment; From “c” to “d”, Flocculonodular segment; distal to “d”, cortical segment. SCA: From “a” to “b”, Anterior pont- omesencephalic segment; From “b” to “c”, Lateral pontomesencephalic segment; From “c” to “d”, Cerebellomesencephalic segment; distal to “d”, cortical segment
Figure 1.36A: Superolateral view. 1, Basilar artery; From “a” to “b”, Anterior pontomesencephalic segment of the SCA; From “b” to “c”, Lateral pontomesencephalic segment of the SCA; Distal to “c”, Cerebellomesecephalic segment of the SCA. IV, Trochlear nerve; V, Trigeminal nerve. Please note that the lateral pontomesencephalic segment of the SCA touches the trigeminal nerve
Figure 1.36B: Superior view of the tentorial surface of the cerebellum. 1, Tentorial edge; 2, Superior petrosal sinus; 3, Lateral hemispheric branch of the SCA; 4, Intermediate hemispheric branch of the SCA; 5, Vermian and paravermian branches of the SCA; 6, Medial hemispheric branch of the SCA; IV, Trochlear nerve; V, Trigeminal nerve; From “c” to “d”, cerebellomesencephalic segment of the SCA
Distally, the SCA loops caudally and passes beneath, sometimes contacting the middle third of the free edge of the tentorium, making it the most rostral of the infratentorial arteries. Further distally, branches pass medial to the posterior third of the free edge as they enter and exit the cerebellomesencephalic fissure.
After passing above the trigeminal nerve, it enters the cerebellomesencephalic fissure, where its branches make several sharp turns and give rise to the precerebellar arteries, which pass to the deep cerebellar white mater and the dentate nucleus. On leaving the cerebellomesencephalic fissure where its branches are again medial to the tentorial edge, its branches pass posteriorly under tentorial edge and are distributed to the tentorial surface of the cerebellum. The SCA can arise as a single trunk and bifurcates into a 34rostral and caudal trunk. The SCA gives off perforating branches to the brainstem and cerebellar peduncles. The rostral trunk supplies the vermian and paravermian area and the caudal trunk supplies the hemisphere on the suboccipital surface. The SCA presents the following segments:
The anterior pontomesencephalic segment extends from the origin of the SCA to the anterolateral margin of the brainstem. It is located between the dorsum sella and upper brainstem. Its lateral part is medial to the anterior half of the free tentorial edge. The lateral pontomesencephalic segment begins at the anterolateral margin of the brainstem and frequently dips caudally onto the lateral side of the upper pons. Its caudal loop projects toward and often reaches the root entry zone of the trigeminal nerve at midpontine level. The trochlear nerve passes above the midportion of this segment. The anterior part of this segment is often visible above the tentorium. This segment terminates at the anterior margin of the cerebellomesencephalic fissure. The basal vein and the PCA course above and parallel to this segment of SCA. The cerebellomesencephalic segment courses within the cerebellomesencephalic fissure. The branches of SCA enter the shallowest part of the fissure located above the trigeminal root entry zone and again course medial to the tentorial edge with its branches intertwined with the trochlear nerve. The fissure in which the SCA proceeds progressively deepens medially and is the deepest in the midline behind the superior medullary velum. The SCA loops deeply into the fissure and passes upward to reach the anterior edge of the tentorial surface of the cerebellum. The trunks and branches of the SCA are held in the fissure by branches that penetrate the fissure's opposing walls. The cortical segment includes the branches distal to the cerebellomesencephalic fissure that pass under the tentorial edge and are distributed to the tentorial surface of the cerebellum. The branches of the rostral trunk supplies medial area and the branches of the caudal trunk supply the lateral area of the tentorial surface of the cerebellum (Figs 1.2, 1.30, 1.31 and 1.34A).
The branches of the SCA are; perforating, precerebellar and cortical branches.
The perforating branches are divided into direct and circumflex type. The direct type pursues a straight course to enter the brainstem. The circumflex type winds around the brainstem before terminating in it. The circumflex perforating arteries are subdivided into short and long types. The short circumflex type travels 90 degrees or less around the circumference of the brainstem.
The perforating branches usually arise from the main, rostral and caudal trunks. The most common type of perforating artery arising from the main trunk is the long circumflex type, but it also gives off direct and short circumflex branches. The branches from the rostral and caudal trunk are most frequently circumflex. The basilar artery also gives rise to multiple perforating branches to the brainstem. Those arising near the origin of the SCA intermingle with the direct perforating branches arising from the proximal SCA. Those arising above the origin of the SCA enter the interpeduncular fossa.
The precerebellar arteries arise from the trunks and cortical branches within the cerebellomesencephalic fissure to supply the deep cerebellar nuclei. These branches tether the distal parts of the trunks and the proximal parts of the cortical arteries in the fissure. The precerebellar arteries consist of a medial group of small branches that pass between the superior medullary velum and the central lobule and a lateral group of larger branches that course between the superior and middle cerebellar peduncles and the wings of the central lobule (Figs 1.36C and D).
The most constant cortical supply of the SCA is to the tentorial surface. The cortical territory of the SCA is more constant than that of the AICA and PICA, but is reciprocal with them. The cortical branches are divided into hemispheric and vermian groups. The cortical surface of each half of the vermis is divided into median and paramedian segments and each hemisphere lateral to the vermis is divided into medial, intermediate and lateral segments.
The hemispheric branches arise from the rostral and caudal trunk in the depths of the cerebellomesencephalic fissure. After leaving the fissure, the hemispheric branches proceed to supply the tentorial surface lateral to the vermis. The most common pattern consists of three hemispheric branches: (1) lateral, (2) intermediate, and (3) medial, corresponding to the third of the hemispheric surface that they supply. The vermian arteries occasionally overlap onto the medial hemispheric segment and marginal artery overlaps the lateral hemispheric segment.
The vermian branches arise from the rostral trunk within the cerebellomesencephalic fissure. The rostral trunk most commonly gives rise to two vermian arteries. The most common pattern is two vermian arteries: (1) one distributed to a median strip bordering the midline, and (2) one distributed to a paramedian strip bordering the hemispheric surface (Fig. 1.36B).
About half of the proximal SCA trunks give rise to a marginal branch to the adjacent petrosal surface. When present, the marginal branch is the first cortical branch. It usually arises from the lateral pontomesencephalic segment and does not enter the cerebellomesencephalic fissure, as do the other cortical branches, but passes from its origin to the cortical surface. Its most constant supply is to the part of the petrosal surface adjoining the tentorial surface.
The angiographic correlation of the SCA can be seen in Figure 1.35C; please correlate it with the Figures 1.2, 1.30, 1.31, 1.34A, 1.35B, 1.36A and B.35
Figure 1.36C: Posterolateral view of the tectal and cerebellomesencephalic fissure region. 1, Thalamus; 2, Pulvinar of the thalamus; 3, Pineal gland; 4, Choroid plexus; 5, Superior colliculus; 6, Inferior colliculus; 7, Tentorial edge; 8, Superior cerebellar peduncle; 9, Middle cerebellar peduncle; IV, Trochlear nerve; V, Trigeminal nerve. Please note that the trochlear nerve is coursing among branches of the SCA at the cerebellomesencephalic fissure and this can make the intraoperative identification of the trochlear nerve difficult
Figure 1.36D: Posterolateral view of the bottom of the cerebellomesencephalic fissure. The lingual and part of the superior cerebellar peduncle have been removed to display the fourth ventricle. Please note the red dots, which are the precerebellar arteries penetrating the bottom of the cerebellomesencephalic fissure to supply the deep cerebellar nuclei. 1, Superior cerebellar peduncle; 2, Central lobule; 3, Culmen; 4, Nodule; 5, Superior pole of the tonsil covered by the inferior medullary velum; 6, Dentate nucleus; IV, Trochlear nerve
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