14
Suboccipital, Telovelar
The suboccipital craniotomy is one of the most
common approaches in posterior fossa surgery
(Fig. 1.5c). The occipital bone is removed to
expose the suboccipital surface of the cerebel-
lum as well as the transverse and sigmoid sinuses
if wide exposure is needed. The cisterna magna
is often opened to allow drainage of CSF before
further dissection is done. The vermis presents an
apparent surgical barrier to the fourth ventricle,
and transvermian approaches were once com-
mon, but not without morbidity. Careful study
of this anatomy led to the development of the
telovelar approach, whereby the fourth ventricle
is entered by cutting the tela choroidea and infe-
rior medullary velum, which form the inferior
roof of the fourth ventricle [ 6 ]. These thin mem-
branes are exposed by separating the tonsil and
uvula (Fig. 1.5e, f). This approach can expose the
entire floor of the fourth ventricle, including the
lateral recess and the cerebral aqueduct without
morbidity.
Posterolateral: Retrosigmoid
A posterolateral perspective into the posterior
fossa can be obtained through a presigmoid or ret-
rosigmoid approach. The retrosigmoid approach
is far more common, and it can provide exposure
extending from the tentorium to the foramen mag-
num as well as cranial nerves IV through XII [ 7 ]
(Fig. 1.7a, b). The asterion typically lies posterior
to the sigmoid-transverse junction, which is often
drilled early in the approach, though it has not
been found to be a consistently reliable landmark
(Fig. 1.6a). Another landmark for the transverse
sinus is a line joining the occipital eminence, or
the inion, with the upper margin of the zygomatic
arch or the upper margin of the middle third of
the ear. The transverse-sigmoid junction may
also be predicted to be just posterior to the upper
level of the mastoid notch, which can be easily
palpated. Navigation technology has made local-
ization of the sinuses a much easier task. Because
this perspective is posterior to several cranial
nerves and the lateral to medial trajectory is lim-
ited by the sigmoid sinus, access to the anterior
brainstem is hindered (Fig. 1.7a, b). Additional
bone removal over the sigmoid sinus allows more
retraction of the sinus to increase anterior and
medial visualization [ 8 ]. The superior petrosal
vein may obscure the trigeminal nerve, so it is
often divided, though a venous infarct may occa-
sionally occur.
The retrosigmoid approach can be tailored for
specific pathologies, such as vestibular schwan-
nomas involving the internal auditory canal [ 9 ]Fig. 1.6 Sigmoid sinus and related landmarks. (a) Lateral
view of the left sigmoid sinus. The asterion represents the
junction of the occipitomastoid, lambdoid, and parieto-
mastoid sutures. It typically lies posterior to the sigmoid
sinus and inferior to the transverse sinus, though it is not
always a reliable localizing landmark. The retrosigmoid
approaches are posterior to the sigmoid sinus and involve
some retraction of the cerebellum. Note that the presig-
moid space between the sigmoid sinus and the labyrinth
can be very limited. (b) Division of the superior petrosal
sinus allows the sigmoid sinus to be mobilized posteriorly,
significantly enlarging the presigmoid corridorJ. Basma and J. Sorenson