6
from the junction of the superior petrosal, basilar,
and cavernous sinuses anteriorly (Fig. 1.2a). Thus,
the superior and inferior petrosal sinuses, along
with the sigmoid sinus and jugular bulb, form a
venous ring around the medial face of the petrous
bone (Fig. 1.2a). The petrosal and sigmoid por-
tions of the jugular foramen empty into the jugular
bulb, which occupies the lateral pars vascularis of
the jugular foramen (Fig. 1.2c). The basilar sinus
extends across the clivus, completing the anterior
venous anastomosis of the posterior fossa. The
posterior cavernous sinus of the middle fossa com-
municates with the posterior fossa venous circula-
tion through the superior and inferior petrosal
sinuses as well as the basilar sinus. This conflu-
ence of sinuses is often referred to as the petro-
clival venous confluence, through which the
abducens nerve travels as it passes below the pet-
rosphenoidal ligament (Fig. 1.2a).
Obstacles to Surgery
Several structures within the posterior fossa
enclosure can result in significant morbidity if
injured; therefore, these obstacles must be con-
sidered when designing surgical approaches.
Smaller venous sinuses, such as the superior
petrosal sinus, are sacrificed with rare conse-
quences, but larger sinuses such as the transverse
or sigmoid pose greater risk. Therefore, as an
obstacle, the sigmoid sinus is a significant bound-
ary between approaches even though it can be
mobilized and retracted to increase exposure
(Fig. 1.6b). Division of the tentorium rarely
causes morbidity if care is taken to preserve the
trochlear nerve and adequate venous outflow.
Traversal of neural foramina should be avoided
if preservation of nerve function is desired. In
addition, the course of the facial nerve in the tem-
poral bone must be appreciated as it passes
through the tympanic cavity and then inferiorly
into the anterior aspect of the mastoid (Fig. 1.8d).
Manipulation of the greater superficial petrosal
nerve or geniculate ganglion can also result in
facial palsy when working through the middle
fossa floor to access the posterior fossa
(Fig. 1.11d). If hearing preservation is desired,
then the labyrinth, which lies posterior to the
internal auditory canal, should be preserved
(Fig. 1.8c–d). The cochlea is immediately ante-
rior to the fundus of the internal auditory canal
(Fig. 1.9c). The internal carotid artery enters the
petrous bone anterior to the jugular foramen and
runs vertically for a short distance before turning
anteriorly below the cochlea into a horizontal ori-
entation until it exits at the petrous apex
(Figs 1.1d, 1.2d, and 1.9h). This presents a sig-
nificant obstacle during anterior petrosectomy.
Finally, the cavernous segment of the internal
carotid artery can block lateral access from trans-
nasal approaches (Fig. 1.12b, c).Contents of the Posterior Fossa
The cerebellum and brainstem occupy most of
the posterior fossa volume. The brainstem has
three morphologically distinct regions in the
posterior fossa: the midbrain, pons, and medulla
oblongata. Each of these is bordered by arachnoid
cisterns [ 4 ]. The mesencephalon (midbrain) is a
transition from the functionally reflexive spinal
cord and medulla to the correlative and associa-
tive diencephalon and forebrain. It extends from
the thalamus to the pontomesencephalic sulcus.
Anteriorly, the cerebral peduncles represent the
array of corticospinal, corticobulbar, and cortico-
pontine fibers descending from the internal cap-
sule (Figs. 1.3a and 1.4a). These peduncles are
separated by a gap, called the interpeduncular
fossa, which contains a cistern of the same name
that is bounded anteriorly by Liliequist’s mem-
brane. The lateral walls of this cistern give rise
to the oculomotor nerves, and the posterior part
contains the basilar apex and posterior cerebral
artery perforators entering the posterior perfo-
rated substance (Fig. 1.3c). Relative to the dor-
sum sellae, the basilar apex can be high or low
riding, which can influence the choice of surgi-
cal approach. The lateral midbrain is bordered by
the crural cistern around the cerebral peduncle
and the ambient cistern posterior to the peduncle
(Fig. 1.4a). Posteriorly, the superior and infe-
rior colliculi are bordered by the quadrigeminal
cistern.J. Basma and J. Sorenson