Chapter 31 Intracranial Vascular Anatomy
Describing the intracranial vascular anatomy is a complex and challenging task, particularly if the anatomy has to be depicted from an endoscopic surgical perspective. For this reason, we have decided to use the concept of corridors as a teaching tool in this chapter. By creating one corridor, it is possible to see what lies behind and this model can be reproduced endlessly during dissection. In this sense, the paranasal sinuses (especially the ethmoid and sphenoid sinuses) can be considered the endoscopic gateway to the ventral and lateral skull base. Obviously, knowledge of the surgical landmarks is mandatory to safely perform every single approach to the skull base. However, the endoscopic corridors are followed by a description of the gross anatomy of each vascular structure described in this chapter.
31.1 Internal Carotid Artery
From an endoscopic point of view the internal carotid artery (ICA) can be divided in five segments: (1) parapharyngeal or cervical segment, (2) petrous segment, (3) supralacerum segment, (4) cavernous segment [(a) paraclival and (b) parasellar], and (5) supracavernous segment [(a) clinoidal and (b) cisternal]. In this chapter, only the intracranial segments (3–5) ( Fig. 31.1 ) will be described in detail, while the parapharyngeal and petrous segments will only be mentioned.1
31.1.1 Parapharyngeal and Petrous Segments
The ICA starts at the common carotid artery bifurcation and runs toward the skull base, to enter the external orifice of the carotid canal of the petrous bone. The ICA is seated anteromedial to the vagus nerve and medial to the internal jugular vein inside the carotid sheath. The endoscopic corridor to reach the parapharyngeal or cervical segment of the ICA is the infratemporal one and the anatomic dissection as well as anatomy are described in more detail in Chapter 25. Once in the skull base, the ICA enters the carotid canal and turns from vertical to horizontal inside the petrous bone. Then, the ICA follows a posterior-to-anterior and lateral-to-medial trajectory toward the foramen lacerum (FL). The petrous segment of the ICA and its exposure are described in more detail in Chapter 17.
31.1.2 Supralacerum Segment
Supralacerum Segment of ICA via the Transpterygoidal Corridor
The key structure to reach endoscopically the anterior genu and supralacerum segment of the ICA is the pterygoid or vidian canal. The pterygoid bone lies posterior to the palatine bone. By drilling in a lateral direction where the pterygoid plate joins the basisphenoid, the pterygoid (vidian) canal can be identified.2,3 This canal points toward the ICA, at the level of the supralacerum portion (anterior genu) ( Fig. 31.2 ). The vidian nerve passes through the pterygoid canal running from the anterior genu of the ICA to reach the pterygopalatine ganglion in the upper portion of the pterygopalatine fossa ( Fig. 31.3 ). The artery of the pterygoid canal (vidian artery) is not always present. From an anatomic point of view, there is an anterior (from external carotid artery [ECA] system) and a posterior (from ICA system) vidian artery. However, it is possible that there is no counterpart from the maxillary artery. In this case, the artery (~0.5 mm) has been described to transverse the cartilage-filled FL to enter the pterygoid canal.
Once the ICA exits the petrous bone, it curves upward above the FL, thus giving the anterior genu that is not truly intrapetrous. Anatomically, the FL is an opening in the dry skull that in life is filled by fibrocartilaginous tissue (fibrocartilago basalis) that is firmly attached to the ICA, eustachian tube, clivus, and petrous portion of the temporal bone.4 Its borders are the petrous apex, the body of the sphenoid, and medially the occipital bone. Tiny periosteal branches of the petrous ICA, the meningeal branches of the ascending pharyngeal artery, and small veins pass it.5 The supralacerum segment of the ICA gives origin to the vidian artery that enters and courses into the pterygoid canal to anastomose with the same-named branch of the maxillary artery ( Fig. 31.3 ).6
31.1.3 Cavernous segment
Above the petrolingual ligament, the ICA can be considered intracavernous. The petrolingual ligament connects the petrous apex and the lingula of the sphenoid. It can be considered the border between the horizontal and cavernous portions of the ICA. From an endoscopic anterior viewpoint, we can define two cavernous segments: (1) paraclival segment and (2) parasellar segment, both evident especially in well-pneumatized sphenoid sinuses.
Cavernous Paraclival Segment
Cavernous Paraclival Segment of ICA via the Transpterygoidal Corridor
A wide bilateral sphenoidotomy allows good exposure of the intrasphenoid landmarks (sellar floor, optic nerve, medial and lateral opticocarotid recess, cavernous paraclival and parasellar ICA, vidian crest, V2, clivus) that appear differently depending on whether the pneumatization of the sinus is presellar, sellar, or conchal type.
Exposure of the posterior wall of the maxillary sinus with section of the sphenopalatine, the descending palatine and palatovaginal arteries, allows dissection of the medial portion of the pterygoid and identification of the vidian foramen; thus, safety surgical landmarks can be maintained. Drilling of the sphenoidal floor is then performed, identifying the middle pterygoid lamina, which is also an essential landmark. In fact, if this lamina is drilled perpendicularly to the clivus, it is possible to reach the ICA at its anterior genu, where its petrous tract becomes vertical. Once identified, the paraclival ICA can be skeletonized by thinning down and removing its overlying bone ( Fig. 31.2 ).
The cavernous paraclival segment (posterior vertical segment) of the ICA, endoscopically speaking, represents the lateral border of the transclival corridor. This segment runs upward, at a variable distance from the posterior clinoid process, and bends forward forming the posterior bend of the cavernous segment of the ICA (ICAc). It must be underlined that the configuration of the posterior bend varies significantly and that sometimes it can bulge upward into and deform the dura of the cavernous sinus (CS) roof just lateral to the posterior clinoid process. The meningohypophyseal trunk (MHT) is usually visible at the posterior vertical segment of the ICA and usually arises at the medial aspect of this. The branches of the MHT are the inferior hypophyseal artery (IHA) for the pituitary gland, the dorsal meningeal artery for the dura of the posterior cranial fossa (PCF), and the Bernasconi–Cassinari artery for the tentorium ( Fig. 31.4 ).
Although there is a short paraclival-located supralacerum portion, delimited inferiorly by the petrolingual ligament and superiorly by the maxillary nerve (V2), which is extracavernous, we will consider for convenience the whole paraclival segment as intracavernous.
The paraclival segment corresponds to the posterior vertical segment of the ICAc, extending from the anterior genu over the FL to the posterior bend of the parasellar segment at the level of the posterior clinoid process. The paraclival segment of the ICA presents constantly as a branch, the MHT which typically arises from the posterior bend of the ICA. Two types of the vessel are described: complete and incomplete. In the complete version, there are three branches: the tentorial artery (also called the Bernasconi–Cassinari artery), the dorsal meningeal artery (also called the dorsal clival artery),7 and the IHA ( Fig. 31.4 ).8 The incomplete type demonstrates one or more of these three vessels arising directly from the ICAc. More often, it is the dorsal meningeal artery. Rarely, all three vessels arise directly from the ICA. The IHA travels superior and medially toward the pituitary gland and primarily to its posterior lobe. In some cases, the vessel supplies the anterior lobe, mainly on its periphery.6 The vessel, when approaching the pituitary gland, bifurcates and sometimes trifurcates.9 The IHA and the dorsal meningeal arteries anastomose with one another on both sides, thus forming a circulus arteriosus around the root of the dorsum sellae. The dorsal meningeal artery, or dorsal clival artery, passes posterior toward Dorello′s canal and supplies the dura of the upper clivus and the proximal aspect of the abducens nerve ( Fig. 31.5 ). During its travelling, it contributes, with the Bernasconi–Cassinari artery (which supplies the tentorium), to the blood supply of the proximal part of the cranial nerves inside the CS.
Cavernous Parasellar Segment
Cavernous Parasellar Segment of ICA via the Transpterygoidal Corridor
Removal of the anterior sphenoidal wall and drilling of the sphenoidal floor and pterygoid allow exposure of the lateral recess of the sphenoid sinus (medial wall of the CS). Complete exposure of the medial portion of the CS requires removal of the bony wall that covers the ICAc and thereafter the lateral wall of the sphenoid as far as the orbital apex. In very pneumatized sphenoid sinuses, removal may involve the medial portion of the great wing of the sphenoid. To extend the access to the lateral portion of the CS and to the floor of the middle cranial fossa, the removal of the medial portion of the posterior wall of the maxillary sinus and lateralization of the contents of the pterygoid-maxillary fossa are necessary to expose the vidian canal (with cauterization of the vidian artery), as well as the foramen rotundum (with the V2). At this point in the dissection, drilling the pterygoid and sphenoid bases enables the view to be extended to the lateral portion of the CS and to the floor of the middle cranial fossa ( Fig. 31.6 ).
A very close relationship between the cavernous parasellar portion of the ICA and the pituitary gland is present, and it is quite variable even in the absence of pathology. Endoscopically, it is not easy to differentiate the medial cavernous wall from the pituitary capsule, as contrariwise reported in microscopic dissection.10 Below the body of the pituitary gland, it is possible to see the terminal ramification of the IHA of both sides.
The inferolateral trunk (ILT) is slightly more distal to the MHT and nearly always passes above the abducens nerve (VI cn), ending more or less in the Meckel′s cave area ( Fig. 31.7 ). Endoscopically, different fibrous–dural septations can be observed within the CS, thus apparently creating a common space with different venous lacunae, in which the terminal branches of the ILT run with variable shapes. While the ILT is easy to see, the McConnell′s capsular artery, when present, is somewhat difficult to identify. On the roof of the CS, the close relationship with the optic nerve is very evident. The ophthalmic artery usually arises at the supraclinoid portion of the ICA above the dural roof of the CS, but in less than 10% of the cases, it can originate from the cavernous parasellar portion.
The parasellar segment forms the shape of a C with medial concavity. Following the posterior bend of the ICAc, the artery usually runs horizontally for a short distance (horizontal segment) and curves upward, thus giving the anterior bend that reaches the lower and upper dural rings at the level of the anterior clinoid process (clinoid, infraclinoid, or paraclinoid segment) and then turns to the anterior vertical segment (cisternal or supraclinoid segment). Looking through a transsphenoidal corridor, the parasellar ICA is positioned on the side of the pituitary gland, thus producing the parasellar ICA prominence on the sphenoid sinus wall. This prominence corresponds, more or less, to the distal part of the horizontal segment, the anterior bend, and the clinoid segment.
The parasellar segment of the ICA presents different branches, the most constant being the ILT (also called the artery of the inferior CS). The ILT arises from the central one-third of the inferior or lateral surface of the horizontal segment of the ICAc, distal to the origin of the MHT (3–13 mm; Fig. 31.7 ).11 Usually, it crosses over the abducens nerve and then divides into two branches.12 The superior one curves posteriorly along the trochlear nerve and the edge of the tentorium to supply the proximal portion of cranial nerves III, IV, and V1. The second branch is the artery of the superior orbital fissure (SOF), which runs under the first two trigeminal branches and provides vascularization for the foramen ovale and rotundum. The SOF artery is critical for the blood supply of the distal portion of cranial nerves III, IV, ophthalmic (V1), and VI.
Other branches are the McConnell′s capsular artery, the persistent trigeminal artery (PTA), the ophthalmic artery, the superior hypophyseal artery (arteries) (SHAs), the recurrent artery of the FL, and the artery of the Gasserian ganglion.
In more details:
The McConnell′s capsular artery is not always present (30–50% of the cases).13 The vessel may come from the most superior segment of the parasellar ICAc. In less than 10% of cases, it arises from the medial aspect of the horizontal segment of the parasellar ICAc. When present, it supplies the inferior and peripheral aspect of the anterior lobe of the pituitary gland and the diaphragma sellae.6 McConnell′s arteries may give branches that penetrate the sella turcica to enter the sphenoid sinus via the craniopharyngeal canal (when present).
The PTA, when present, arises from the central middle third of the posterior bend of the ICAc. Its incidence is described as between 0.06 and 0.6%.14,15 In more than half of the cases, the PTA penetrates the sella turcica near the clivus to join the basilar artery (BA). In the remaining cases, the artery travels lateral to the sella turcica.16 There are two types of PTA, according to the relationship to the abducens nerve: a lateral or petrosal and a medial or sphenoidal. If the artery arises from the posterolateral aspect of the ICAc, it runs lateral to the abducens nerve intradurally and inferior to it within the CS and thus displaces the nerve superiorly. When the PTA arises from the posteromedial aspect of the ICAc, it pierces the dura over the dorsum sellae and courses medial to the abducens nerve.17
In rare cases, the SHAs originate from the parasellar segment of the ICA (more commonly from the paraclinoid segment). These are perforating branches for the superior aspect of the pituitary gland. It is also possible to observe several branches, some arising in the intracavernous segment and others coming from the cisternal one.
The recurrent artery of the FL seems to supply the pericarotid autonomic nervous plexus.21 It forms an anastomosis with the ascending pharyngeal artery. It can be considered a periosteal branch.
The branches of the artery of the Gasserian ganglion cross the abducens nerve within the CS as they travel laterally.
31.1.4 Supracavernous Segment
Above the inferior (also known as lower or proximal or Perneczky′s) dural ring, the ICA can be considered to be supracavernous. The dura lining the inferior aspect of the anterior clinoid process forms the lower dural ring, which can be considered the border between the intracavernous and supracavernous portions of the ICA.22 From an endoscopic anterior viewpoint, we can define two segments of the supracavernous ICA: (1) clinoidal segment and (2) cisternal segment.
Supracavernous Clinoidal Segment of ICA via the Transpterygoidal Corridor
The dura lining the upper and lower surface of the ACP extends medially to form the upper (also known as superior or distal) and lower dural rings that border the clinoidal segment of the ICA ( Fig. 31.9 ). Note that the clinoid segment of the ICA is against the optic strut. The lateral opticocarotid recess (lOCR) corresponds to the pneumatization of the optic strut.22 The superior and inferior border of the lOCR identify the position of the upper and lower dural rings.23 The roof of the CS is given by the extension of the dura just beneath the anterior clinoid process. This dura forms the caroticoclinoid ligament, lying between the anterior and middle clinoid process. This ligament fixes the vessel to the carotid sulcus. So, the middle clinoid process can be considered as marking the inferior border of the paraclinoid segment on its medial wall. As a whole, the medial and antero-inferior venous compartments of the CS are separated from the paraclinoid compartment. Just underneath the proximal dural ring, III, IV, and V1 can be found running toward the SOF. The lower dural ring is often incomplete on the medial side and often a venous channel can follow the paraclinoidal ICA to the superior dural ring which is given by the extension of the dura covering the superior surface of the anterior clinoid process. This ring is not closely attached to the ICA, and a small cuff of arachnoid can surround the artery.13 Posteriorly the upper dural ring joins the lower dural ring to form the apex of the clinoidal triangle of the roof of the CS.24 The dura of the upper ring leaves a small posteromedial space called the carotid cave, and not rarely the SHA may arise in this space.
Supraclinoid Segment of the ICA via
(a) Transtuberculum–Transplanum Suprasellar Corridor
The opening of the ethmoido-sphenoidal planum allows access to the anterior skull base and the suprasellar cistern without passing through the sella turcica ( Fig. 31.10 ). In this case, the transsphenoid access is accompanied by selective opening of the posterior ethmoid to reveal the posterior ethmoidal arteries, which are considered the anterior margins of this approach, to avoid damaging the olfactory neuroepithelium. This is followed by the bony removal between the lOCR, which corresponds to the tuberculum sellae area on the intracranial side. Opening of the dura mater here enables control over the optic chiasm and, thereafter, the pituitary stalk and the gland.
A rich arterial network coming from the internal carotid arteries is present all around the pituitary stalk. This network is given by the SHAs, which usually arise from the supraclinoid portion of the ICA ( Fig. 31.11 ). Rarely, these vessels can arise from the cavernous portion of the ICA. Sometimes it is possible to see a “crown” of arteries around the superior aspect of the pituitary stalk, from which coaxial vessels run downward to reach the body of the anterior lobe of the pituitary gland ( Fig. 31.12 ). On the inferomedial surface of the optic nerves, it is possible to see the ophthalmic artery, which normally arises as the first branch of the supraclinoid ICA, which enters the optic canal and orbital cavity after a short distance ( Fig. 31.13 ).
Superolaterally to the optic chiasm, the supraclinoid segment of the ICA can be seen running upwards on the lateral side of the optic tract, where it then divides into the anterior cerebral artery (ACA) and the middle cerebral artery (MCA) just below the anterior perforate substance ( Fig. 31.14 ). There is a large arterial complex at this level, where the branches are overall known as anterior perforating arteries. By definition, this is a group of arteries that enter the anterior perforate substance and they originate from the ICA, the ACA, the MCA, and the anterior choroidal artery (AchA). The veins from the frontal lobes (such as the olfactory veins and the orbital veins) converge here.
Above the chiasm, in the midline, the anterior communicating artery (AcomA) complex with the first segments of ACA (A1 and A2) as well as the lamina terminalis can be seen ( Fig. 31.15 ). In a slightly more anterior position lies the posterior portion of the olfactory tracts, as well as the rectal gyrus. The olfactory vein is usually identifiable close to the olfactory tract, while the anterior cerebral vein, which has been joined by the paraterminal veins coming from the interhemispheric fissure, is generally visible over the chiasm. At this level, the anterior communicating vein can be seen as well.25
(b) Transcribriform Corridor
By removing the bone of the anterior cranial base, in the midline, from orbit to orbit and from the posterior wall of the frontal sinus to planum sphenoidalis, the dura becomes apparent ( Fig. 31.16 ). The crista galli in the midline is also visible and can extend to a variable depth into the cranial cavity, and is situated between the frontal bones. Usually it is a thick bony structure and is rarely pneumatized. To remove the crista galli, it is necessary to drill internally until it becomes eggshell thin and can be removed easily. The falx cerebri can be seen in close proximity to the crista galli. This structure is a large fold of the dura in the sagittal plane, running in the midline between the cerebral hemispheres. As stated, it is attached anteriorly to the crista galli, mainly on its posterior border, and to the adjacent anterior skull base. At this level, the beginning of the superior sagittal sinus is located just behind the frontal sinuses. When drilling the crista galli, venous bleeding from the most inferior part of the superior sagittal sinus or from the venous vessels lying on the cribriform plate can take place. From this point, the superior sagittal sinus runs superiorly in a shallow groove of the inner surface of the calvaria, with its sidewalls given by two laminae of the falx cerebri. In rare cases, it may communicate with the veins of the nasal cavity through the foramen cecum. At this level, it is sometimes possible to see the anterior falcine artery, a branch of the anterior ethmoidal artery. By removing the falx cerebri and the dural plane, the basal surface of the frontal lobes and the interhemispheric fissure become evident. In the paramedian position, it is possible to locate the olfactory bulbs and, more posteriorly, the olfactory tract ( Fig. 31.17 ).26 The basal surface of the frontal lobes is divided by the olfactory sulcus into a medial compartment, the gyrus rectus, and a larger portion, the orbital gyri. More laterally, the orbital sulcus divides this latter portion into anterior, medial, lateral, and posterior groups. Endoscopically, this last division is of limited significance. In close relationship to the olfactory bulb, there is rich vascularization coming from the ACA. The latter supplies the medial part of the orbital gyri, the gyrus rectus, and the olfactory bulb and tract on the basal surface.27 From an endoscopic point of view, only the orbitofrontal and frontopolar arteries are truly significant. These vessels usually arise from A2. The orbitofrontal artery is almost constantly present and arises usually from A2; it seldom arises from A1. From its point of origin, it passes down and forward toward the floor of the anterior cranial fossa to reach the level of the planum sphenoidalis. The frontopolar artery arises usually from A2 and passes anteriorly along the medial surface of the hemisphere toward the frontal pole. The venous vascular network develops in a far less constant way. It is usually possible to recognize the frontopolar, anterior and posterior orbitofrontal veins, and the olfactory veins. All together, these veins drain the basal surface of the frontal lobe. The anterior orbitofrontal veins empty into the anterior part of the superior sagittal sinus. The posterior orbitofrontal veins empty into the veins below the anterior perforated substance that converges on the anterior end of the basal vein.27