Microsurgical anatomy of the upper basilar artery and the posterior circle of Willis

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✓ The microvascular anatomy of the posterior part of the circle of Willis, important in surgery of pituitary tumors and basilar aneurysms, was defined in 50 cadaver brains. Significant findings were as follows: 1) Anomalies of the posterior half of the circle of Willis were found in 46% of cases. 2) Hypoplastic P-1 (posterior cerebral segment) and posterior communicating segments gave origin to the same number and size of perforating arteries, having the same termination as normal-sized segments. Thus hypoplastic segments should be handled with care and divided to aid in exposure of the basilar bifurcation only after careful consideration. 3) An average of four perforating branches arose from P-1; most from the superior and posterior surfaces. No branches arose from the anterior surface of the basilar bifurcation. The most proximal P-1 branch originated 2 to 3 mm distal to the basilar bifurcation. It was most commonly a thalamoperforating artery. The largest P-1 branch was usually a thalamoperforating or a posterior choroidal artery. 4) An average of seven branches emerged from the superior and lateral surfaces of the posterior communicating artery. The anterior half was a richer source of perforators than the posterior half. The largest communicating branch in 80% of specimens supplied the premamillary area. 5) The anterior choroidal artery originated from the carotid artery on both sides in all cases. A double anterior choroidal artery was present in 4% of cases.

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Address reprint requests to: Albert L. Rhoton, Jr., M.D., Division of Neurological Surgery, Box J-265 Health Center, University of Florida, Gainesville, Florida 32610.

© AANS, except where prohibited by US copyright law.

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    Superior view of the basilar (B.A.), superior cerebellar (S.C.A.), P-1, and distal segments of posterior cerebral (P.C.A.), posterior communicating (P.Co.A.), internal carotid, and proximal anterior choroidal (A.Ch.A.) arteries. The arterial branches below the posterior perforating substance (P. Perf. S.), mamillary bodies (Mam. B.), optic tracts (O.Tr.), chiasm, and nerves (O.N.) are shown in half tone. The third and fourth nerves (III and IV) course between the superior cerebellar and posterior cerebral arteries. Arterial branches to the upper pons, posterior mesencephalon, interpeduncular fossa, posterior perforating substance, mamillary bodies, tuber cinereum, optic tracts and chiasm arise from the basilar, P-1, posterior communicating and internal carotid arteries. A.C.A. = anterior cerebral artery; C.A. = carotid artery; P.Ch.A. = posterior choroidal artery; Premam. A. = premamillary artery; Th.Pe.A. = Thalamoperforating artery. A: Normal configuration of the posterior half of the circle of Willis; both P-1's are larger than communicating arteries and the latter are not hypoplastic (diameter greater than 1 mm). The right superior cerebellar artery is duplicated. The largest right P-1 branch gives rise to both the thalamoperforating and the posterior choroidal arteries. Only two perforating arteries arise on right P-1. The left posterior choroidal arises on P-2. Both premamillary arteries (largest communicating trunk to premamillary area) arise from the middle one-third of the posterior communicating arteries. Anterior choroidal arteries arise as a single trunk. B: Hypoplastic left communicating artery. Thalamoperforating artery arises on P-1 medial to the posterior choroidal on both sides. The left premamillary artery arises from the posterior and the right from the anterior portion of the posterior communicating artery. The superior cerebellar arteries are duplicated on both sides. C: Posterior communicating arteries are hypoplastic bilaterally. The largest right P-1 branch gives rise to both the thalamoperforating and the posterior choroidal arteries. The thalamoperforating artery arises medial to the posterior choroidal artery on the left P-1. The premamillary artery arises from the anterior one-third of the right posterior communicating artery and from the middle third on the left. The left anterior choroidal arises from the carotid as two trunks.

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    (continued)D: Fetal origin of the right posterior cerebral artery. The thalamoperforating artery on the right arises near the basilar bifurcation. The right posterior choroidal artery arises on P-2. The left posterior choroidal artery arises medial to the thalamoperforating artery. The right premamillary artery arises from the anterior portion of the communicating artery. The left premamillary area is supplied by a group of nearly equal-sized arteries. The anterior choroidal artery bifurcates immediately after origin on the left. E: Bilateral fetal origin of the posterior cerebral artery. The right posterior choroidal artery arises lateral to the thalamoperforating artery. The largest P-1 branch gives rise to the left thalamoperforating and choroidal arteries. The right premamillary artery arises from the middle portion of the communicating artery. A premamillary arterial complex is present on the left. F: Fetal type of right posterior cerebral origin and hypoplastic left communicating artery. The right posterior choroidal arises lateral to the well developed thalamoperforating artery. No thalamoperforating branches are present on the left. The right premamillary artery arises from the anterior and the left from the posterior portion of the communicating artery. G: Superior view of the suprasellar area. Arterial branches stretched around the superior extension of a pituitary tumor. Anterior cerebral arteries send branches to the superior surface of the optic nerves and chiasm. The posterior communicating, internal carotid, and posterior cerebral arteries send branches into the area below and behind the chiasm. Recurrent arteries arise just distal to the anterior communicating artery.

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    Inferior view of the circle of Willis. Fetal type of left posterior cerebral artery (P.C.A.). The left third nerve (III) courses under P-1, right under P-2. The left thalamoperforating (Th.Pe.A.) and the posterior choroidal (P.Ch.A.) arteries originate from P-1. The right premamillary artery (Premam. A.) emerges from the anterior and the left from the middle third of the posterior communicating artery (P.Co.A.). Small communicating branches course superiorly and medially and terminate in the premamillary area. The left thalamoperforating and right premamillary arteries are well developed in spite of the small trunk of origin. No branches arise on the anterior surface of the basilar artery (B.A.). Perforating branches arise from the anterior cerebral (A.C.A.) and anterior communicating arteries. C.A. = carotid artery; M.C.A. = middle cerebral artery; O.N. = optic nerve.

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    Inferior view of the circle of Willis with bilateral hypoplastic posterior communicating arteries (P.Co.A.). O.N. = optic nerve; O.Tr. = optic tract; A-1, A-2 = anterior cerebral artery. Upper: Well developed premamillary arteries (Premam. A.) arise from the communicating arteries, regardless of the hypoplastic trunk vessels. Right anterior choroidal artery (A.Ch.A.) arises from the carotid (C.A.) above and swings lateral to the communicating artery. Lower: The basilar artery (B.A.) bifurcates at the pontomesencephalic junction. The superior cerebellar arteries (S.C.A.) bifurcate on the level of the anterior pontine segment. Thalamoperforating branches (Th.Pe.A.) originate from P-1 near bifurcation. Well developed premamillary arteries originate from the posterior communicating arteries. No branches arise from the anterior surface of the basilar artery.

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    Superior view of the basilar bifurcation and midbrain. There are no perforators on the superior surface of the basilar bifurcation. The upper portion of the basilar artery (B.A.) is a rich source of paramedian branches to the interpeduncular fossa and pontine area. S.C.A. = superior cerebellar artery; P-1 = posterior cerebral artery.

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    Superior view of the area of basilar bifurcation and P-1. The posterior part of the optic chiasm is split at junction with optic tracts (O.Tr.) to give this view. A: Anterior cerebral arteries (A-1) pass above the chiasm; the anterior third ventricle lies above the mamillary bodies. The thalamoperforating arteries (Th.Pe.A.) terminate in the retromamillary area and interpeduncular fossa. The right posterior choroidal artery (P.Ch.A.) originates from the distal portion of P-1. Small branches arise from the posterior cerebral artery (P.C.A.) and terminate in the peduncle. The left third nerve (III) courses medial to the posterior communicating artery (P.Co.A.). The right posterior communicating joins P-1 medial to the third nerve. The left anterior choroidal artery (A.Ch.A.) passes to the peduncle and optic tract. B: The left superior cerebellar artery (S.C.A.) bifurcates just lateral to its origin. Both third nerves course below the posterior communicating artery. The right thalamoperforating, with its many branches, and posterior choroidal arteries arise from P-1. Perforating arteries originate from the superior surface of the left posterior communicating artery, course superiorly and terminate in the premamillary area, optic tract, and tuber cinereum. Both anterior choroidal arteries branch to the optic tract and peduncle. Both third nerves pass between the posterior cerebral and superior cerebellar arteries and course below the posterior communicating artery. C: Fetal type of right posterior cerebral origin. One trunk, arising from P-1, gives rise to both the thalamoperforating and posterior choroidal arteries on both sides. Both third nerves pass between the superior cerebellar and posterior cerebral arteries. D: Several thalamoperforating branches arise from the superior and posterior surfaces of P-1, creating a complicated plexus of vessels that terminate in the interpeduncular fossa. The largest branch is referred to as the thalamoperforating artery. The left posterior choroidal artery originates from the posterior cerebral artery just distal to the communicating artery. The fifth nerve lies below the posterior cerebral arteries bilaterally. The posterior communicating arteries pass medial to the third nerve on both sides. Ped = peduncle; S. Nigra = substantia nigra.

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    Right anterior and lateral views of the suprasellar area. A: Right anterolateral view. Perforating arteries pass from the carotid to terminate in the optic chiasm and tract and the anterior hypothalamus. Right posterior communicating artery (P.Co.A.) is medial to the carotid artery (C.A.). Anterior choroidal artery (A.Ch.A.) arises from the carotid and passes above the posterior cerebral artery (P.C.A.). A-1 branches enter chiasm. Anterior communicating branches (A.Co.A.) pass to hypothalamus. The third nerve (III) lies below the right posterior cerebral artery. The right recurrent artery of Heubner (Rec.A.) arises from A-1. O.N. = optic nerve; M.C.A. = middle cerebral artery. B: Right lateral view with the temporal lobe removed. The tentorium is intact (Tent. Edge = medial tentorial edge). The third nerve courses lateral to the posterior communicating artery, passing between the posterior cerebral and superior cerebellar (S.C.A.) arteries. The middle cerebral artery ends as a blind stump above the carotid artery and optic tract (O.Tr.). Perforating branches pass from the anterior communicating artery to the hypothalamus. The third nerve enters dura lateral to the clivus. Anterior choroidal artery passes lateral to the communicating artery and above the posterior cerebral artery. C: Right lateral view with the tentorium removed. Note relationship of the anterior and posterior communicating arteries to the hypothalamus (Hypo.) and foramen of Monro (For.M.). Fetal type of posterior cerebral artery. Both communicating arteries send branches to the hypothalamus. The posterior communicating artery passes under the optic tract medial to the third nerve. Note the double anterior choroidal artery and the entrance of the third and fourth (IV) nerves into the cavernous sinus. D: Right lateral view. Premamillary (Premam.A.) arises from the posterior communicating artery. Thalamoperforating arteries (Th.Pe.A.) arise from P-1. The optic tract passes around the peduncle to the lateral geniculate. Arterioles pass from the carotid to the optic nerve. Basilar artery (B.A.) medial to third nerve. E: Lateral view. The lateral wall of right cavernous sinus (Cav.S.) is removed, exposing the third, fourth, fifth (V1, V2, V3), and sixth (VI) nerves and intracavernous carotid in the sinus. The anterior choroidal artery arises distal to the posterior communicating and gives branches to the optic tract and peduncle. The anterior communicating branches pass to the hypothalamus. Clin = anterior clinoid.

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    Right lateral view of the anterior and middle fossa with the tentorium and lateral wall of cavernous sinus (Cav.S.) removed. The third (III) and fourth (IV) nerves course between the posterior cerebral (P.C.A.) and superior cerebellar (S.C.A.) arteries, lateral to the posterior communicating and anterior choroidal (A.Ch.A.) arteries. The olfactory nerve is in the anterior fossa above the optic nerve (O.N.). The optic nerve continues posteriorly as the optic tract (O.Tr.), which runs above the posterior part of the circle of Willis. The third nerve penetrates the dura lateral to the dorsum. The third, fourth, and fifth (V1, V2, V3) nerves pass forward in the lateral wall of the cavernous sinus. The anterior choroidal artery passes below the optic tract, and the basilar artery (B.A.) passes between the peduncle (Ped.) and dorsum. Clin = anterior clinoid; Gr.P.N. = greater petrosal nerve; Pe.A. = perforating artery; V.Ga. = Gasserian ganglion.

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