Cerebral vasculature

Embryology

Embryological variants of cerebral vascular anatomy are commonly encountered and appear to be associated with an increased incidence of vascular pathology, probably as a result of increased flow. Variants include a persistent fetal posterior communicating artery supplying the posterior cerebral artery, occurring in approximately 20% of the popul­ation, and much rarer persistent primitive communications between the carotid and basilar circulation in the form of a trigeminal artery, an otic artery, a hypoglossal artery or a proatlantal intersegmental artery. Embryological variants of the anterior cerebral artery include a solitary azygos A ₂ segment.

Surgical surface anatomy

The pterional approach is most frequently used to access the anterior circulation. This is performed via a curvilinear incision from the pretragal region to the upper forehead behind the hairline. The pretragal access enables visualization of the superior aspect of the zygomatic arch, permitting the inclusion of the lower temporal bone in the crani­otomy flap. This provides good visualization of the Sylvian fissure. Superiorly, the scalp flap must be retracted to enable visualization of the readily palpable frontozygomatic process.

For posterior fossa approaches, the occipital protuberance indicates the level of the confluence of the venous sinuses. A posterior extension of the level of the zygomatic arch provides a marker for the level of the transverse sinus. The spinous process of the second cervical vertebra is readily palpable. Landmarks for lateral approaches to the foramen magnum are considered in Chapter 23 . The lateral mass of the first cervical vertebra is usually palpable in this approach.

Clinical anatomy

Arterial supply

The cranial compartment is supplied by paired internal carotid arteries (ICA) feeding the anterior circulation and paired vertebral arteries (VA) supplying the posterior circulation. Considerable individual variability in the circulation exists and there are many well-described anatom­ical variants. The anterior and posterior circulations both contribute to the circulus arteriosus (circle of Willis), lying in the basal cisterns and giving rise to the anterior, middle and posterior cerebral arteries ( Figs 26.1 and 26.2 ).

Anterior circulation

Extradural internal carotid artery (segments C1–C3)

The ICA is a terminal branch of the common carotid artery (CCA), which arises from the aortic arch on the left and the brachiocephalic trunk on the right. The ICA, described using the Gibo/Rhoton classification, is divided into four main parts: C1–C4, which are referenced to enable accurate locations of pathological lesions. The C1 (cervical) segment arises at the bifurcation of the CCA at the level of the third cervical vertebra, giving rise to the carotid sinus soon after its origin. The artery ascends within the anterior triangle of the neck, passing deep to the parotid gland, hypoglossal nerve, digastric muscle and structures attached to the styloid process, and posterior to the pharyngeal branch of the vagus nerve and the glossopharyngeal nerve; these structures are all at risk during exposure of the vessel in the neck. It then enters the carotid canal within the petrous temporal bone, where it is referred to as the C2 (petrous) segment. It ascends for a short distance, separated by a thin film of lamellar bone from the tympanic cavity (middle ear) and the cochlea posteriorly, and then turns anteromedially toward the non-patent foramen lacerum. As the artery enters the horizontal portion of the canal, it lies caudal to the floor of the trigeminal ganglion (Gasserian ganglion, Meckel's cave). This is often deficient, in which case the artery is separated from the trigeminal ganglion by a thin fibrous membrane. The C2 segment gives rise to the Vidian artery (artery of the pterygoid canal) and the carotico­tympanic artery. As it crosses under the petrolingual ligament, the C2 segment enters the cavernous sinus, which envelops the C3 (cavernous) segment. It ascends towards the posterior clinoid process, before turning in an anterior direction within the sinus. The C3 segment gives rise to the meningohypophysial trunk, the inferolateral trunk and the medial trunk. The meningohypophysial trunk further divides into the tentorial artery of Bernasconi and Cassinari, which supplies the tentorium; the inferior hypophysial artery, which supplies the neuro­hypophysis; and the dorsal meningeal artery, which supplies part of the clivus and the abducens nerve.

The ICA next ascends at the level of the anterior clinoid process, passing medial to it and through the proximal and distal dural rings. The transitional stage between the proximal and distal rings, referred to as the clinoidal segment in the modified Fischer/Bouthillier class­ification, is clinically relevant because aneurysms arising here are not intradural and do not give rise to subarachnoid haemorrhage. As the artery passes through the distal ring it becomes intradural and is referred to as the C4 (supraclinoid) segment.

Intradural internal carotid artery (segment C4)

The carotid cistern contains the C4 (supraclinoid) segment of the ICA. This cistern lies immediately inferior to the anterior perforated substance, which is delineated anteriorly by the medial and lateral olfactory striae arising from the olfactory tract; this serves as a useful surgical landmark. As the vessel emerges from the distal dural ring, it projects posteriorly, coursing in a parallel trajectory to the C3 segment, to produce a loop referred to as the carotid siphon. The supraclinoid segment is further subdivided into three parts: ophthalmic, communicating and choroidal, based on the origin of its major branches.

The ophthalmic segment extends from the origin of the supraclinoid ICA to the origin of the posterior communicating artery (PComm); it gives off the ophthalmic artery, just distal to the distal dural ring, usually from the medial third of the superior surface of the ICA, and the superior hypophysial artery. The ophthalmic artery follows the inferior surface of the optic nerve into the optic canal and orbit. Variants can arise from the clinoidal or cavernous segments and, rarely, from the external carotid artery. Perforating arteries from this segment supply the pituitary stalk, optic chiasma/tract and the premammillary portion of the third ventricle. The communicating segment extends from the origin of the PComm artery to the origin of the anterior choroidal artery. The PComm artery usually arises on the postero­medial aspect of C4 and courses posteromedially to the posterior cerebral artery. If the fetal variant persists, the artery often takes a course lateral to the oculomotor nerve, supplying the posterior cerebral artery. This segment often does not give rise to perforators (60%) ; when present, perforators have the same distribution as those in the ophthalmic segment and may perforate the anterior and posterior perforated substances. The choroidal segment runs from the origin of the anterior choroidal artery to the terminal bifurcation of the ICA. The anterior choroidal artery usually arises from the posterolateral aspect of the ICA and runs posteromedially, crossing inferior to the optic tract from lateral to medial, to reach the lateral aspect of the cerebral peduncles. At the lateral geniculate nucleus, it turns laterally and then continues superior to the uncus to enter the choroidal fissure. This is the most common site of origin of perforators from the ICA, typically from the posterior aspect of the vessel; they supply the anterior perforated substance, optic tract and uncus. Anterior perforated substance perforators supply both limbs and genu of the internal capsule, the basal ganglia and the anterior portion of the thalamus. Apart from direct perforators arising from the ICA, contributions also arise from the anterior chor­oidal, middle cerebral and anterior cerebral vessels.

Middle cerebral artery

The middle cerebral artery (MCA) is the largest of the terminal divisions of the ICA and lies in the Sylvian fissure. It consists of four principal segments, M1–M4, which are described in relation to the adjacent anatomical structures. Knowledge of this anatomy is required when clipping MCA aneurysms and undertaking resection of tumours in the insula.

The M1 (sphenoidal) segment extends from the ICA bifurcation to the limen insulae, the most anterior aspect of the insular cortex. M1 runs laterally, about 9 mm posterior to the sphenoidal wing, inferior to the anterior perforated substance, and gives off several lateral lenticulostriate arteries and temporal polar branches. The lenticulostriate vessels penetrate the anterior perforated substance to supply the anterior limb, genu and posterior limb of the internal capsule; they need to be preserved in aneurysm surgery and represent the medial extent of insular tumour resection. The M1 segment usually bifurcates proximal to the limen; the bifurcation can be equal or asymmetric. A temporal branch often arises from the M1 prior to its bifurcation. At the limen, the two trunks of the MCA are referred to as the M2 (insular) segments. They course over the insular surface, giving rise to anterior and posterior cortical branches. On reaching the circular sulcus, the arteries enter the M3 (opercular) segment, where branches are closely adherent to the frontoparietal and temporal operculae. The M3 vessels emerge at the surface of the Sylvian fissure, where they become the M4 (cortical) segments. Arteries projecting to the frontoparietal cortex exhibit two 180° bends, at the circular sulcus and at the cortical surface of the operculum; these are visible on anteroposterior angiographic projections. The largest M4 vessels supply the temporo-occipital and angular regions, and make effective targets for extracranial–intracranial (EC–IC) bypass ( Fig. 26.3 ).

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