Skull Base Surgery of the Posterior Fossa

8

mids, the inferior olive is bounded by the
preolivary sulcus, where the hypoglossal root-
lets emerge. Cranial nerves IX, X, and XI exit
from the postolivary sulcus. The floor of the
fourth ventricle at this level contains from
medial to lateral: the hypoglossal nucleus, the
dorsal nucleus of the vagus nerve, and the ves-
tibular nuclei (Fig. 1.5g). The medulla is
bounded anteriorly by the premedullary cistern
and laterally by the cerebellomedullary cistern
(Fig. 1.4e).

Arteries and Rule of Three

Dr. Albert Rhoton Jr. divided the neurovascular

contents of the posterior fossa into three groups

of structures, each organized around a major pos-

terior fossa artery. These zones are stacked verti-

cally, defining the main longitudinal axis of the

posterior fossa. A separate important organizing

scheme divides the CSF-filled spaces surround-

ing the brainstem into cisterns (Fig. 1.4). The

brainstem and cerebellum are supplied by

Fig. 1.3 Neurovascular relationships of the brainstem
and cerebellum. (a) Anterior view of the brainstem and
cerebellum. Following Rhoton’s rule of three, the upper
group of structures includes the mesencephalon, superior
cerebellar artery (SCA), oculomotor nerve, and trigeminal
nerve. The middle group includes the pons, anterior infe-
rior cerebellar artery (AICA), cranial nerves VI–VIII, and
the middle cerebellar peduncle. The inferior group holds
the posterior inferior cerebellar artery, lower cranial
nerves, and medulla. (b) Lateral view of the brainstem and
cerebellum. Branches of the SCA are seen coursing
around the midbrain and the superior cerebellar peduncle
before supplying the superior surface of the cerebellum.

The SCA passes below the oculomotor nerve, which exits

from the lateral aspect of the interpeduncular fossa. A

caudal loop of the SCA may impinge upon the trigeminal

nerve. The AICA supplies the middle cerebellar peduncle

and the petrosal surface of the cerebellum. (c) Superior

view of the midbrain and cerebellum. The SCA supplies

the superior surface of the cerebellum, which conforms to

the shape and slope of the posterior tentorium. The inter-

peduncular fossa is crowded by perforating branches of

the basilar bifurcation. (d) Inferior view of the medulla

and cerebellum. The suboccipital surface of the cerebel-

lum and its tonsils are supplied by branches of PICA. The

PICA forms a caudal loop near the cerebellar tonsil

J. Basma and J. Sorenson