anatomy of vertebral artery, spine surgery

1. 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 . 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 transverse 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 magnum.
• -V4: the intracranial segment, from its dural penetration to the vertebrobasilar
junction.
• The vertebral arteries are major arteries of the neck. Typically, the vertebral
arteries originate from the subclavian arteries. Each vessel courses superiorly
along each side of the neck, merging within the skull to form the single,
midline basilar artery. As the supplying component of the vertebrobasilar
vascular system, the vertebral arteries supply blood to the upper spinal
cord, brainstem, cerebellum, and posterior part of brain.

4. • The V1 segment courses vertically from its origin on the posterior aspect of
the subclavian artery to the transverse foramen men of C6. It is accompanied
by 2 veins anteriorly and posteriorly .
• The V2 segment courses vertically from one transverse foramen 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 . At its entrance into the transverse canal at C6, the
periosteal sheath adheres somewhat to the VA, forming a proximal fibrous
ring. Otherwise, the VA is free inside this periosteal sheath.

5. • The V3 segment has a complex course . 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 .

6. • At the C1 transverse foramen it again changes direction, coursing horizontally
in the groove of the posterior arch of the atlas . 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

8. • Branches of the VA ->Embryologically, the VA is formed by the junction of
metameric segments. Therefore, each segment gives rise to similar branches:
one radiculomeningeal branch and one muscular branch .
• 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 vascular 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 . This
vascular network always exists even when not visible on angiography.

9. • Another important branch is the anterior meningeal artery from the radicular
branch of the third interspace (C2-C3) .
• 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.

10. • Anomalies and variations ->In 40% of subjects, one VA is larger than the
contralateral VA. The former is called dominant, the latter is called minor.
• 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.
• Occasionally, the VA is duplicated at its origin, with two trunks that join into
one VA at the entrance of the transverse canal.
• 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.
• Another variation involves the intradural course of the VA from C2 rostrally . In
this variation, VA is composed of an atretic component coursing normally along
the spine between C2 and C1 and a major component piercing the dura
between C1 and C2 and then coursing intradurally.

11. • 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 especially affects the
V3 segment during head rotation . 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
become parallel to the posterior arch of atlas. This effect is of particular
importance when positioning patients for surgery.

12. Vertebral artery injuries in cervical spine surgery
• Injury to the vertebral artery is a potentially devastating complication of
cervical spine surgery. While the overall incidence rate in the cervical spine is
1.4%.
• patients are at the greatest risk when undergoing posterior instrumented
upper cervical spine surgery (4-8% incidence) In contrast, the risk of injury is
only 0.3-0.5% for anterior subaxial cervical spine procedures.
• The clinical sequelae of an iatrogenic vertebral artery injury can vary widely
depending on anatomic variations, circulation dominance, and the presence
of collateral blood flow.
• Patients can have minimal, but noticeable, sequelae, and serious
complications such as lateral medullary (Wallenberg) syndrome,
quadraparesis, and even mortality have been reported.

13. • AVOIDANCE
• Anterior cervical surgery-> Vertebral artery injuries in anterior subaxial
cervical spine surgery are rare and most occur during the decompression.
• An important mode of prevention involves evaluation of a tortuous vertebral
artery on preoperative advanced imaging. Assessment of spinal canal width
can also aid in the avoidance of the vertebral artery.
• Since the vertebral artery is located between 0.8 and 1.6 mm from the lateral
tip of the uncinate process, avoiding an aggressive removal of the uncinate
joint is prudent when pathology is appropriate.
• Finally, one potential pitfall from the use of an operating microscope is the
potential for an asymmetric decompression. An oblique or asymmetrical
decompression during a corpectomy may put the vertebral artery at risk,
especially if a tortuous path is missed.The keys to preventing this complication
include symmetric and stable patient placement on a head holder and
positioning of the microscope perpendicular to and over the center of the
wound

14. • Posterior cervical spine-> Instrumented posterior surgery of the upper cervical
spine places the vertebral artery at the highest risk for injury.
• Avoidance measures start with preoperative evaluation of the vessel course
on advanced imaging, preferably a CT scan. In 18-23% of patients, at least one
side will not accommodate a transarticular (Magerl) screw, and in up to 6%, a
transarticular screw cannot be placed in either side.
• Vertebral artery injuries during posterior exposure can also be avoided. When
exposing the posterior ring of the atlas, lateral dissection should be limited to a
point 15 mm lateral to the midline. Furthermore, since the vertebral artery
typically runs along the superior aspect of C1 posterior ring, dissection. along
the inferior aspect is safest. No dissection should occur on the superior edge of
C1 more than 8 mm lateral to the midline.
• all patients who sustained a vertebral artery injury after the placement of
transarticular screws had a poorly reduced C1/C2 joint when screw placement
occurred. The authors argued that with an anatomically aligned C1/C2 joint
and use of biplanar fluoroscopy, the safety of placement of a transarticular
screw increases significantly.

15. • During placement of a C1 lateral mass screw, avoidance of a vertebral artery
injury hinges upon use of an appropriate starting point, which is the junction
of lateral mass and inferior aspect of posterior arch ,The screw is then angled
10-15 degrees medially to avoid the vertebral artery, which courses laterally.
• Similarly, a properly placed C2 pedicle screw has minimal risk to the vertebral
artery. The C2 pedicle screw is placed utilizing a starting point slightly
superior and medial to the center of the lateral mass and aimed 10 degrees
medially and 15 degrees cephalad . Placing subaxial cervical spine lateral
mass screws can also put the vertebral artery at risk. Techniques such as the
Magerl, Roy-Camille, and Anderson all describe a laterally directed screw (10-
25 degrees) from the midpoint of the lateral mass to avoid the vertebral
artery in the foramen transversarium .

16. • ACTION-> Once a vertebral artery injury has occurred, the surgeon must
always keep the treatment goals in mind in the appropriate order:
• (1) Achieve control of the hemorrhage.
• (2) Prevent acute central nervous system ischemia.
• (3) Prevent postoperative complications such as embolism and
pseudoaneurysm
• Control of vertebral artery bleeding can be achieved in three different ways:
Primary repair, bypass surgery, or sacrifice.
• Primary repair->when available, remains the best option. After vascular
surgery consultation is obtained and aggressive intravenous access for fluid
resuscitation has been communicated to the anesthesia team, the first step
should be to ensure that the head is in a neutral position, as cervical extension
and axial rotation can lead to occlusion of the contralateral vertebral artery.
Digital pressure can often be used to obtain hemostasis, followed by large
pieces of hemostatic agents such as thrombin soaked gel foam and cottonoids.
It is important to use only large items that cannot accidentally embolize. After
proper exposure, a temporary aneurysm clip or vessel loop is placed proximal
and distal to the injury site. The injury can then be directly repaired with 7-0 or
8-0 Prolene (Ethicon Sumerville, NJ).[7,10] Prior to completion of the repair,
the temporary clips should be removed to prevent air embolism or the
propagation of other emboli.

17. • If direct repair is not possible, the remaining options are to bypass the injury
site or vessel sacrifice. Ligation of the vessel should only occur if there is
good retrograde flow, as this may be a sign of considerable
contralateral/collateral flow. To evaluate this, place an aneurysm clip
proximal to the injury on the vertebral artery and look for significant back
flow from the cephalad end.
• The decision to sacrifice the vessel should be a last resort, as the severe
neurologic complication rate can be as high as 43%. If the vessel cannot be
repaired, and there is poor retrograde flow, bypass is indicated.
Alternatively, the vascular surgeon may choose to perform traditional bypass
surgery. Tamponade alone is not effective in achieving hemostasis, as
multiple reports exist of complications after use of this method.
• Postoperatively the patient should be started on an antiplatelet medication
after 6 hours and should be evaluated with conventional angiography, as
often times there may be significant artifact with an MR angiogram.

18. • Symptomsvariable in presentation and time of onset
• vertebrobasilar insufficiency manifests with
– dizziness
– vertigo
– nausea
– diplopia
– blindness
– ataxia
– bilateral weakness
– oropharyngeal dysfunction