Figure 1-12 Vertebrobasilar arterial system and major arteries with common variations of the cortical branches of the posterior cerebral artery. A, Lateral view. B, Anteroposterior view. Vertebral artery (VA) and basilar artery (BA) branches: anterior inferior cerebellar artery (AICA); posterior inferior cerebellar artery (PICA); superior cerebellar artery (SCA). Posterior cerebral artery (PCA) and its branches: calcarine artery (CalcA); parieto-occipital artery (ParOccipA); posterior choroidal artery (PChoA); posterior temporal (PTempA); posterior communicating artery (PCoA). (Illustration b y Dave Peace.)
The distal 2 sets of circumferential arteries consist of the superior cerebellar artery (SCA) followed by the PCA, representing the terminal branches of the BA at the level of the midbrain. Perforators from the proximal SCA partially supply the nucleus of CN III and its fascicles. In addition, small branches often supply the CN V root. CN III exits between the SCA and PCA, where it may be compressed by an aneurysm.
Perforators from the proximal PCA (P1 segment) supply the rostral portion of the midbrain (involved in vertical gaze) and part of the LGN. A large branch, the artery of Percheron, often supplies both sides of the midbrain from one of the PCAs. Because thalamostriate arteries originate from P1, infarcts related to the internal ICA–MCA spare the thalamus. The P1 segment ends with the PCoA, which joins the vertebrobasilar circulation to the carotid circulation anteriorly. The connecting PCoA parallels the course of CN III, which explains the high frequency of CN III palsy with PCoA aneurysms (see Chapters 2 and 8). As the distal PCA courses around the brainstem, it gives off a parieto-occipital branch before terminating in the calcarine branch, which supplies the primary visual cortex (Fig 1-13).
Figure 1-13 The occipital cortex and its blood supply. Areas V1, V2, and V3 keyed by color. CalcA = calcarine artery; CC = corpus callosum; MCA = middle cerebral artery; PCA = posterior cerebral artery. (Illustration b y Craig A. Luce.)
Venous System
Ocular venous outflow begins in the arcade retinal veins, which exit into the central retinal vein (CRV) and in the choroidal veins, which exit the sclera through the vortex veins. Anteriorly, the episcleral venous plexus collects both blood from the anterior uveal circulation and aqueous percolating through the Schlemm canal. These 3 primary venous drainage pathways (Fig 1-14) empty mainly into the superior ophthalmic vein, which runs posteriorly within the superior medial orbit to the orbital apex, where it crosses laterally to enter the cavernous sinus posterior to the superior orbital fissure.
Figure 1-14 Venous drainage of the orbit. A, Anterior view of the superficial venous system of the eyelids. B, Sagittal view of the venous circulation of the orbit and globe. v = vein. (Illustrations b y Christine Gralapp.)
Microscopic collaterals variably exist between these venous beds. In rare instances, shunts connecting retinal veins to choroidal veins are present within the retina. More commonly (usually in association with central retinal vein occlusion or optic nerve sheath meningioma), optociliary shunt vessels (retinochoroidal collateral vessels) may appear on the disc surface. At a more macroscopic level, the superior ophthalmic vein is variably connected anteriorly to the angular and facial veins and inferiorly to the inferior ophthalmic vein and pterygoid venous plexus. These collaterals may become important, particularly in patients with elevated venous pressure (usually related to a carotid cavernous fistula).
Intracranially, the superficial cortical venous system drains mainly superiorly and medially to the superior sagittal sinus running in the sagittal midline (Fig 1-15). In addition to the cortical drainage, the superior sagittal sinus absorbs CSF through the arachnoid villi and the pacchionian granulations. Thus, obstruction to venous outflow results in decreased CSF absorption and elevated intracranial pressure. The superior sagittal sinus runs posteriorly to terminate at the torcular Herophili (confluence of the venous sinuses) at the level of the tentorium separating the cerebellum from the occipital lobes. The transverse sinuses run anteriorly from the connection of the tentorium and the skull to the petrous pyramid, where they turn to run caudally as the sigmoid sinus down to the jugular bulb, where the internal jugular vein exits the skull.
Figure 1-15 Illustration of the cerebral venous sinus system. (Illustration b y Christine Gralapp.)
Inferior superficial cortical venous drainage is carried directly down to the transverse and sigmoid sinuses through the vein of Labbé and the basilar vein of Rosenthal. The deep drainage of the supratentorial diencephalon and mesencephalon begins as deep draining veins (often in relationship to the ventricular system). These deep veins drain together to form the vein of Galen,
which runs posteriorly to drain into the straight sinus. This runs within the tentorium to drain, along with the superior sagittal sinus, into the torcular Herophili.
Some anterior cerebral venous drainage may access the cavernous sinus. The 2 cavernous sinuses are joined by variable connections through the sella and posteriorly through a plexus of veins over the clivus. The cavernous sinus drains primarily caudally into the jugular bulb via the inferior petrosal sinus, which traverses the Dorello canal with CN VI under the petroclinoid ligament. Alternatively, drainage may occur laterally along the petrous apex through the superior petrosal sinus to the junction of the transverse and sigmoid sinuses (Fig 1-16). Small veins may drain through the foramen rotundum and foramen ovale as well as through the pterygoid plexus to anastomose with the facial venous system (external jugular vein).
Figure 1-16 Anatomy of the cavernous sinus drainage system. (Illustration b y Christine Gralapp.)
Veins of the eyelids anastomose medially between the angular vein and branches of the superior ophthalmic vein (particularly at the superior medial orbit in the region of the trochlea). Facial veins drain inferiorly and laterally to form the external jugular vein, which eventually joins the internal jugular in the neck.
Kupersmith MJ. Neurovascular Neuro-Ophthalmology. New York: Springer-Verlag; 1992.
Lasjaunias P, Berenstein A, ter Brugge KG. Surgical Neuroangiography. 2nd ed. New York: Springer; 2001. Clinical Vascular Anatomy and Variations; vol 1.