Vertebral Artery: Surgical Anatomy

Bernard George, MD, and Jan Cornelius, MD

Comprehensive knowledge of surgical anatomy is essential be- fore contemplating surgery around the vertebral artery (VA). In this article, the course and relationships of the 4 segments of the VA are presented, and the relevant features for a well-conducted surgery are given. The most important branches of the VA, especially the anterior radiculomedullary artery and the anterior meningeal artery, are described. There are many variations in the course and branches of the VA; those affecting surgical tech- nique are emphasized. Finally, changes in the course of the third segment (C2 to foramen magnum) of the VA caused by head and neck movements, especially during surgical positioning, are discussed. Copyright �9 2001 by W.B. Saunders Company

S urgical exposure of the vertebral artery (VA) is still consid- ered challenging. However the difficulties in this exposure

are actually overestimated on the basis of old reports, problems with this exposure, especially troublesome venous bleeding. A comparison can be made with the cavernous sinus. The VA and the internal carotid artery (ICA) 1 are surrounded by a venous plexus before penetrating the dura, which causes difficulty when exposing these 2 vessels. A better understanding of the anatomy of this region has permitted many surgeons to develop techniques for surgery in and around the cavernous sinus. The same principle should apply to VA surgery.

General Anatomy of the Vertebral Artery

The VA courses along the spine, is deeply located in the neck, and is crossed on its posterior aspect by the cervical nerve roots (Fig 1). In standard descriptions 2,3, the VA is divided into 4 segments:

- VI: the proximal or ostial segment, from its origin to its entry into the transverse canal at the C6 level.

- V2: the transversary segment, from its entry into the trans- verse canal at the C6 level to the transverse foramen of C2.

- V3: the suboccipital segment, from the transverse foramen of C2 to its dural penetration at the level of the foramen mag- num.

- V4: the intracranial segment, from its dural penetration to the vertebrobasilar junction.

The V1 segment courses vertically from its origin on the posterior aspect of the subclavian artery to the transverse fora-

From the Department of Neurosurgery, H6pital Lariboisiere, Paris, France.

Address reprint requests to Bernard George, MD, Department of Neurosurgery, H6pital Lariboisiere, 2, rue Ambroise Pare, 75010 Paris, France. E-maik bemard.george@lrb.ap-hop-paris.fr

Copyright �9 2001 by W.B. Saunders Company 1092 -440X/01/0404- 0002535.00/0 doi:l 0.1053/otns.2001.30168

men of C6. It is accompanied by 2 veins anteriorly and poste- riorly (Fig 2). It passes under a layer of fat and lymphatic elements that are relatively important; on the left side the tho- racic duct crosses the VA in this layer.

The V2 segment courses vertically from one transverse fora- men to the next. It is surrounded by the perivertebral venous plexus and enclosed in a periosteal sheath that is continuous with the periosteum of the foramina of the transverse processes (Fig 2). At its entrance into the transverse canal at C6, the periosteal sheath adheres somewhat to the VA, forming a prox- imal fibrous ring. Otherwise, the VA is free inside this periosteal sheath. The transverse process of C6 is at the same depth as those of C4 and C5; the C7 transverse process is located more posteriorly because of the lordosis of the spine. This relative position exists even in the operative position with the head extended and with a cushion placed under the shoulders. Be- cause of the posterior position of C7, the VA courses at some distance anterior to the C7 transverse process and reaches the C6 transverse process with no change of direction.

The V3 segment has a complex course 4,5 (Fig 3). First, it runs horizontally from the C3 to the C2 transverse foramen. Unlike the other transverse processes, which are perpendicular to the vertebral bodies, the C2 transverse process is oblique inferiorly and laterally. Moreover, the C2 transverse process is longer than the other transverse processes. Therefore, the VA must turn laterally and course almost horizontally to reach the C2 transverse process. Then it courses vertically" from C2 to C1 (Fig 4A and B). At the C1 transverse foramen it again changes direction, coursing horizontally in the groove of the posterior arch of the atlas (Fig 5). The end of this groove is often clearly indicated by an increase in the height of the posterior arch of the atlas. At the end of this groove, the VA turns obliquely and vertically and medially toward the dura. It pierces the dura mater, invaginating the dura and the periosteal sheath 3 to 4 mm, making a double furrow around the VA. At this level the VA is also adherent to the periosteal sheath forming a distal fibrous ring. Otherwise, the VA is free inside the periosteal sheath, still surrounded by the venous plexus, s

Branches of the VA

Embryologically, the VA is formed by the junction of meta- meric segments. Therefore, each segment gives rise to similar branches: one radiculomeningeal branch and one muscular branch (Fig 6). The muscular branches are connected with the muscular branches of the ascending cervical artery, the deep cervical artery, and the external carotid artery, forming a vas- cular network that may become enlarged in some cases. When the VA is occluded proximally, this network allows the VA to refill with blood distally to compensate for the decrease in vertebrobasilar flow (Fig 7A and B). This vascular network always exists even when not visible on angiography. A more

1 6 8 Operative Techniques in Neurosurgery, Vol 4, No 4 (December), 2001: pp 168-181

Fig 1. Scheme of the relationship of the VA to the cervical muscles and nerve roots. (A) and (B) Frontal plane on the right side. (C) Horizontal plane. 1 = Longus Capitis Muscle. 2 = Longus Colli Muscle. 3 = Scalenus Muscle. 4 = Nerve Roots.

selective or high-pressure injection may be necessary to iden- tify it (Fig ~ and B).

Among the radiculomeningeal branches, the anterior radicu- lomedullary branch (Fig 8) supplies the spinal cord. It may

originate from one of the VAs or from another branch of the subclavian artery. When it arises from a VA, it always arises below C3.

Another important branch is the anterior meningeal artery from the radicular branch of the third interspace (C2-C3) (Fig 9). It courses along the lateral aspect of the odontoid process and connects with the opposite anterior meningeal artery, de- scribing an arch circumscribing the odontoid process. It pro- vides the main supply to the anterior part of the dura of the foramen magnum and therefore to meningiomas in this loca- tion.

The last interesting branch is the posterior meningeal artery, which originates from the VA just after its dural penetration or occasionally from the occipital artery. The posterior meningeal artery supplies the dura of the cerebellar convexity and the posterior part of the foramen magnum.

Anomal ies and variat ions

There are many anomalies and variations in the anatomy of the VA that must be identified before surgery is performed in this region.

Size

In 40% of subjects, one VA is larger than the contralateral VA. The former is called dominant, the latter is called minor (Fig 10). In some cases, the minor VA is very small but joins the contralateral VA to form the basilar trunk, in which case it is considered hypoplastic. When a minor VA does not join the contralateral VA, it is considered atretic and may terminate at other vessels. Typically, it terminates at the posterior inferior cerebellar artery (PICA), but it also may terminate at the occip- ital artery (Fig 11).

Course

The most important variation in the course of the VA concerns level of entry into the transverse canal. Instead of entering the transverse foramen of C6, the VA may enter at C5, C4, or even C3 (Fig 12A, B and C). In these cases, the VA courses anterior to the lower transverse processes (C6, C5, C4) and then enters the transverse foramen. Therefore, it courses between the pre- vertebral muscles and the bone of the transverse process and

C5 C6

Fig 2. Scheme of the Vl and V2 segments of the VA (V). On the right, the Vl segment is accompanied by two veins. Then the V2 segment runs from one transverse process (P) to the other, surrounded by the periosteal sheath (S). C5 and C6 indicate the lateral aspect of the C5 and C6 vertebral bodies.

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r II

Fig 3. Scheme of the V3 segment (frontal view). Dotted lines indicate the lateral l imits of the dural sac. Notice the relation of the VA (V) with the Cl -C2 joint (arrow).

Fig 4. Coronal view of CT (A) and MR (B) images showing the course of the VA at the junction be- tween V2 and V3. 1 = at- las. 2 = axis. P = trans- verse process of atlas. Arrows: vertebral artery. C = condyle. T = jugular tubercle. White aster- isk = internal jugular vein. The CT image is a postoperative view after resection of a neurinoma through an oblique cor- pectomy of C2. Repro- duced with the permis- sion from Management of the Vertebral Artery in Surgery of the Skull Base. Donald PJ (ed). Lippincott-Raven, Phila- delphia, PA, 1998, pp 533-553

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Fig 5. Operative view of the VA (star) in the groove of the atlas. Reproduced with permission from: Management of the Vertebral Artery in Surgery of the Skull Base. Donald PJ (ed). Lippincott-Raven, Philadelphia, PA, 1998, pp 533-553

Fig 6. Atretic VA ending at the occipital artery (O) with a tiny connection with the PICA (P). Note the radicular branches (arrows). Reproduced with permission from: The Verte- bral Artery. Pathology and Surgery. George B, Laurian C. Springer-Verlag Wien, New York, NY 1987, p 258

Fig 7. (A) Vascular net- work of muscular branches refilling the distal VA (ar- rowheads) in a case of proximal occlusion. (B) Vascular network refill- ing the distal VA (black triangles) through the oc- cipital artery when the filling pressure is suffi- cient (right side). On the left side, the tip of the catheter is in the external carotid artery (arrow) and the VA is not refilled. On the right side, superse- lective catheterization of the occipital artery (ar- row) provides sufficient pressure to refill the VA. Reproduced with permis- sion from: The Vertebral Artery. Pathology and Sur- gery. George B, Laurian C. Springer Verlag Wien, New York, NY 1987, p 258

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Fig 8. Anterior radiculomedullary artery (black arrow) in a case of tumoral occlusion of the VA (open arrow). Reproduced with permission from: The Vertebral Artery. Pathology and Surgery. George B, Laurian C. Springer-Verlag Wien, New York, NY 1987, p 258

Fig 9. Anterior meningeal artery (arrows) in a case of foramen magnum meningioma.

Fig 10. Right dominant VA (big arrowhead) and left atretic VA (small arrowhead) that fail to join. Reproduced with permis- sion from: The Vertebral Artery. Pathology and Surgery. George B, Laurian C. Springer-Verlag Wien, New York, NY 1987, p 258

Fig 11. Atretic VA ending at the occipital artery and weakly filling the V3 segment (arrows).

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C

Fig 12. The VA enters the transverse canal at an abnormal level (C4) (arrow). (A) coronal view. (B) Sagittal v iew showing the bayonet-shaped course of the VA and (C) corresponding drawing. P = Transverse process. M = Muscles. V = Vertebral Artery. In A, note the similar course of the contralateral VA.

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Fig 13. Duplication of the right VA; one component follows the normal course (curved arrow) and one component is intradural (arrow). In (A) the two components are the same size; in (B) the extradural component is atretic and not visible. Reproduced with permission from: The Vertebral Artery. Pathology and Surgery. George B, Laurian C. Springer-Verlag Wien, New York, NY 1987, p 258

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Fig 14. Ossification of the occipitoatlantal membrane tums the VA groove into a tunnel. (A) Standard radiograph. (B) Operative view; the asterisk indicates the end of the VA groove. Reproduced with permission from: The Vertebral Artery. Pathology and Surgery. George B, Laurian C. Springer-Verlag Wien, New York, NY 1987, p 258

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standard radiography an appearance similar to hourglass neuri- nomas. These loops are sometimes related to osteophytes that displace the VA laterally. In any case, the loops are seldom symptomatic (Fig 15). 15,16,17,18 (See the article, Compression of and by the VA, in this issue).

Occasionally, the VA is duplicated at its origin, with two trunks that join into one VA at the entrance of the transverse canal (Fig 16). 19

Congenital anastomoses between the carotid and vertebro- basilar systems can be formed by the trigeminal, otic, or hypo- glossal arteries. Most of these anastomoses are located intracra- nially. One of them, however, the proatlantal artery, connects the VA with either the external or internal carotid artery and is located extracranially (Fig 17) . 20'21'22

Branches

The most common variation is an extracranial origin of the PICA, which may be present in 20% of the population. PICA may arise from the VA above C1, piercing the dura just beside the VA, or between C1 and C2, or even lower in the neck (Fig 18A and B). z3'24

The posterior meningeal artery usually originates from the VA just after it penetrates the dura; however, it may originate just before it enters the dura.

Fig 15. Tortuosity of the V2 segment of the VA.

assumes a bayonet shape before it enters the transverse fora- m e n .

Another variation involves the intradural course of the VA from C2 rostrally (Fig 13). In this variation, VA is composed of an atretic component coursing normally along the spine be- tween C2 and C1 and a major component piercing the dura between C1 and C2 and then coursing intradurally. 6'7,s,9,~~ This anomaly may explain some of the hemorrhagic accidents that have followed lateral suboccipital puncture when perform- ing myelography.lZ

In the groove of the atlas, the VA is covered by the occipi- toatlantal membrane, which may sometimes be calcified or even ossified. In this case, the VA groove becomes a tunnel, causing difficulty when exposing the VA (Fig 14A and B). 13.14

The V2 sometimes becomes tortuous and forms loops. These loops can enlarge the intervertebral foramina, producing on

Fig 16. Double trunk at the origin of the VA (arrowhead). Reproduced with permission from: The Vertebral Artery. Pa- thology and Surgery. George B, Laurian C. Springer-Verlag Wien, New York, 1987, p 258

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Fig 17. Proatlantal artery (arrow) connects the external ca- rot id artery and the VA at C1.

Fig 18. Abnormal origin of the PICA, at the level of dural penetrat ion of the VA (A) and at C1-C2 (B).

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Fig 19. Relationship of the V3 segment to the atlas while the head is straight (A) and while rotated toward the contralat- eral side (B). The VA is f igured as a red wire, the venous plexus as blue wires, and the accessory nerve as a yel low wire. In (A), the C l -C2 segment is vertical and the superior C1 segment is horizontal. In (B), the two segments are stretched and almost parallel to the posterior arch of the atlas. 1 = A t l a s . 2 = Axis. Notice the C l -C2 joint in (A) (arrow).

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Dynamic changes

The VA is free inside the periosteal sheath along the V2 and V3 segments. Therefore, during movements of the head and neck, the VA may be stretched or compressed. This mechanism espe- cially affects the V3 segment during head rotation (Fig 19A and B). During this movement, the atlas follows the movements of the head, rotating around the odontoid process. As a result, the 2 parts of the contralateral V3 segment are stretched and be- come parallel to the posterior arch of atlas. This effect is of particular importance when positioning patients for surgery.

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