Skull Base Surgery of the Posterior Fossa

11

Three pairs of cerebellar arteries arise from the
vertebrobasilar system. The course of the cere-
bellar arteries often includes loops and turns that
are not confined to a single plane. Nonetheless, it
can be useful to conceptualize three zones along
the vertical axis of the posterior fossa defined by
these three arteries (Fig. 1.3).
The upper zone follows the superior cerebel-
lar artery (SCA) and includes the midbrain, upper
pons, and superior cerebellum. After arising near the
basilar apex, the SCA travels immediately inferior
to the oculomotor nerve before curving around the
brainstem close to the pontomesencephalic junc-
tion. It typically bifurcates into rostral and caudal
branches. Laterally, these branches enter the cer-
ebellomesencephalic fissure where they travel with
the trochlear nerve. Before entering this fissure, one
of its branches may loop inferiorly to contact the
trigeminal nerve, possibly causing trigeminal neu-
ralgia. After coursing posteriorly around the supe-
rior cerebellar peduncle (Fig. 1.4b), the branches
emerge from the cerebellomesencephalic fissure to
supply the vermis, superior cerebellar hemispheres,
and dentate nucleus.
The middle zone follows the anterior inferior
cerebellar artery (AICA) and includes the middle
pons and cerebellopontine angle. The AICA usu-
ally originates from the lower half of the basilar
artery (Figs. 1.3a–b). As it courses around the
brainstem, it may pass near any of the cranial
nerves emerging from the pontomedullary sul-
cus – the abducens, facial, cochlear, and vestibu-
lar nerves. Compression of the root entry zone of
the facial nerve may cause hemifacial spasm.
AICA enters the cerebellopontine angle to supply
the middle cerebellar peduncle and the portion of
the cerebellum facing the petrous temporal bone
(Fig. 1.3a). Often, a loop of AICA can extend
into the internal auditory canal (IAC) and impinge
upon the nervus intermedius to cause geniculate
neuralgia (Fig. 1.7e). AICA may also pass
between the nerves of the 7/8 complex before
they enter the IAC.
The lower zone follows the posterior inferior
cerebellar artery (PICA) and includes the medulla
and inferior cerebellum. PICA typically arises
from the distal vertebral artery along the antero-
lateral medulla (Fig. 1.3a, b) but can arise from

any part of the vertebral artery. Its origin can also

be extradural. The PICA courses posteriorly from

the anterior medulla above or below the hypo-

glossal nerve rootlets. As it reaches the lateral

medulla, it turns inferiorly before encountering

the cerebellar tonsil. It then passes anterior or

posterior to the glossopharyngeal, vagus, and spi-

nal accessory nerves in a variable manner

(Fig. 1.3a). After forming its caudal loop, the

PICA ascends along the inferior cerebellar

peduncle, deep to the cerebellar tonsil, before

bifurcating into branches supplying the inferior

vermis and inferior hemisphere (Fig. 1.5c, e, f).

Veins

The veins of the posterior fossa are more variable

than the arteries [ 3 ]. Deep veins include the vein

of the cerebellomesencephalic fissure, the vein of

the cerebellopontine angle, and the vein of the

cerebellomedullary fissure. Also, one can com-

monly identify the veins of the superior, middle,

and inferior cerebellar peduncles. The superficial

cerebellar veins drain the tentorial, petrosal, sub-

occipital surfaces. The brainstem has a variable

network of veins which are longitudinal or trans-

verse. There are three major groups of bridging

veins. The galenic group drains the internal cere-

bral vein and the basal vein of Rosenthal. The

petrosal group includes the inferior petrosal sinus

and jugular foramen, as well as the superior

petrosal sinus that drains Dandy’s vein. The ten-

torial group drains into the transverse sinus or

directly into the tentorium.

Special Regions

Cavernous Sinus

The cavernous sinus borders the sella and sphe-

noid sinus laterally in the middle fossa. It pro-

vides a conduit for cranial nerves traveling from

the posterior fossa into the orbit. These are situ-

ated laterally, and the carotid artery is located

medially. The internal carotid artery enters the

cavernous sinus after exiting the carotid canal of

1 Surgical Anatomy of the Posterior Fossa