Skull Base Surgery of the Posterior Fossa

19 6

Surgical Vascular Anatomy

of the Posterior Circulation

The vascular anatomy of the posterior circulation
consists of the paired vertebral arteries (VAs), the
basilar artery (BA), the posterior inferior cerebel-
lar arteries (PICAs), the anterior inferior cerebel-
lar arteries (AICAs), the superior cerebellar
arteries (SCAs), and the posterior cerebral arter-
ies (PCAs) [ 2 ].
The VA consists of four segments (Fig. 14.1).
The V1 segment originates from the subclavian
artery and enters into the transverse foramina of
the cervical vertebrae, most commonly at C6.
The V2 segment of the vessel begins as the VA
enters the transverse foramina and continues

until C1. The V3 segment of the VA is the portion

of the vessel from its emergence from the C1

transverse foramen to the point that it penetrates

the dura of the posterior fossa at the level of the

foramen magnum. The V4 segment of the VA

constitutes its intracranial course until its union

with the opposite VA at the vertebrobasilar junc-

tion to form the BA. The intracranial VAs give

rise to medullary perforators before joining to

form the BA. The first intracranial branch of the

VA is the posterior spinal artery, although this

vessel may arise from the V3 segment of the VA

in exceptional cases. The anterior spinal artery is

another major branch of the VA that is responsi-

ble for supplying the spinal cord. It is the third

major branch of the VA before it joins the contra-

lateral VA at the vertebrobasilar junction.

The PICA is the second branch vessel to arise

from the VA (Fig. 14.2) [ 3 ]. This vessel is divided

into five segments, designated p1 through p5. The

p1 segment (the anterior medullary segment)

arises from the VA and travels anterior to the

medulla to the hypoglossal rootlets at the medial

edge of the olive. The p2 segment (the lateral

medullary segment) courses from the medial edge

of the olive to the rootlets of the lower cranial

nerves at the lateral edge of the olive. The p3 seg-

ment (the tonsillomedullary segment) begins at

the lateral edge of the olive and extends to the

inferior extent of the cerebellar tonsil, coursing

rostrally to the midpoint of the medial tonsil. The

p4 segment (the telovelotonsillar segment) begins

at the midpoint of the medial tonsil, courses past

the roof of the fourth ventricle, and extends to the

tonsillobiventral fissure. The p5 segment (the cor-

tical segment) is the remainder of the artery distal

from the tonsillobiventral fissure and is responsi-

ble for irrigating the cerebellar hemispheres.

At the pontomedullary junction, the VAs join

to form the BA (Fig. 14.3) [ 4 ]. The BA extends

from the pontomedullary junction to the mesen-

cephalopontine junction where it bifurcates to

form the PCAs. In addition to a rich array of per-

forating vessels arising from the dorsal surface of

the basilar apex, the BA gives rise to large pon-

tine vessels, including the pontomedullary arter-

ies, the posterolateral arteries, and the long lateral

pontine branches that supply the brainstem. The

Fig. 14.1 Illustration showing the four segments of the
vertebral artery (V1-V4) as well as its relationship to adja-
cent structures. BA basilar artery, PICA posterior inferior
cerebellar artery, SA subclavian artery (Used with permis-
sion from Barrow Neurological Institute, Phoenix, Arizona)

M.Y.S. Kalani and R.F. Spetzler