15(Fig. 1.8e). The superior lip of the IAC, known
as the suprameatal tubercle, may also be drilled
to enlarge the ostium of Meckel’s cave, which
is helpful for trigeminal tumors that extend
from the posterior fossa into the middle fossa
[ 10 – 12 ]. It also allows mobilization of the tri-
geminal nerve with further drilling of the petrous
apex toward the petroclival fissure, similar to
Kawase’s approach. Extensive drilling risks
injury to the posterior and superior semicircu-
lar canals, and their common crus, as well as
the petrous carotid artery. Although this exten-
sion may open the space anteriorly to the ventral
brainstem, the working area around the prepon-
tine cistern remains limited.
Far-Lateral Approach
The retrosigmoid exposure may be extended
more inferiorly by adding C1 and C2 hemilami-
nectomies so that the foramen magnum and spi-
nal cord are visualized (Fig. 1.7f, g). Variants
of the far-lateral approach provide exposure of
the hypoglossal canal, vertebral artery, posterior
inferior cerebellar artery (PICA), lower cra-
nial nerves, lower clivus, and cervicomedullary
junction [ 13 , 14 ]. The suboccipital musculature
is typically mobilized inferolaterally as a single
flap. The exposure can be tailored by removing
additional bone from the occipital and atlan-
tal condyles to allow a more lateral to medial
perspective. This often entails transposition of
the vertebral artery out of the C1 transverse
foramen (Fig. 1.7g). Removal of the posterior
third of the condyle exposes the lateral aspect
of the hypoglossal canal, but extensive condyle
removal may destabilize the atlanto-occipital
joint. Paracondylar bone can be drilled to access
the posterior jugular foramen. Drilling of the
jugular tubercle widens the surgical view of the
lower clivus.
Posterolateral: Presigmoid
A trajectory through the mastoid bone anterior to
the sigmoid sinus provides exposure of the brain-
stem with minimal or no retraction of the cerebel-
lum [ 15 ]. Because the petrous bone is directed
anteriorly and medially toward the brainstem, the
resulting angle of attack allows visualization of
the anterior and medial aspects of the cranial
nerves as they emerge from the brainstem better
than a retrosigmoid approach. Additionally, the
working distance to the brainstem is shorter.
Once a mastoidectomy has been performed,
removal of the labyrinth and cochlea further
improves the view of the anterior brainstem and
petroclival region but at the cost of hearing loss
and facial nerve weakness. The retrolabyrinthine
approach is often adequate, and it spares these
structures.
The operative corridor is bounded inferiorly
by the jugular bulb, which is sometimes located
very close to the labyrinth, limiting the surgical
corridor. The posterior boundary is the sigmoid
sinus, but this can be retracted further posteriorly
by additional bone removal. Further posterior
mobilization of the sigmoid sinus is possible by
dividing the superior petrosal sinus and tento-
rium (Fig. 1.6b). Division of the tentorium allows
a combination of infratentorial and supratentorial
access. Hence, optimal presigmoid approaches
are often not purely infratentorial, because the
corridor anterior to the sigmoid sinus is often
very narrow unless it is retracted.Mastoidectomy
The cortical bone in the area between the exter-
nal auditory meatus, the supramastoid crest, and
the mid-mastoid tip is progressively drilled [ 16 ]
(Fig. 1.8). Alternatively, the superficial cortical
layer may be disconnected and preserved for a
more cosmetic closure. Drilling just below the
supramastoid crest exposes the middle fossa dura,
and the sigmoid sinus is skeletonized inferiorly to
the level of the jugular bulb. The posterior fossa
dura is found between the transverse-sigmoid
junction (TSJ), the superior petrosal sinus, jugu-
lar bulb, and the labyrinth. Drilling deep in the
suprameatal triangle opens the mastoid antrum,
which is the biggest mastoid air cell. From there,
the head of the incus can be identified in the epi-
tympanic recess 1 cm deep to the spine of Henle,
as well as the lateral semicircular canal (SCC).1 Surgical Anatomy of the Posterior Fossa