186
exposure, a clear operative field, and venous
drainage, it is associated with a risk of air emboli
and surgeon fatigue after operating with out-
stretched arms. This approach has largely been
abandoned by the senior surgeon and will not be
further discussed in this chapter. A 3/4 prone posi-
tion or lateral oblique position is preferred by the
senior author for lateral hemispheric cerebellar
lesions; however, in cases in which intraoperative
MRI (iMRI) is used, this position is difficult
because of limitations on the size of the MR bore
(especially in larger patients), and a straight prone
position has been adopted as a compromise. iMRI
is an excellent adjunct to compensate for “brain
shift,” a common phenomenon in posterior fossa
surgery where cerebrospinal fluid (CSF) drainage,
retraction, lesion excision, and brain edema are
frequent occurrences. iMRI can compensate for
these factors and ensure that complete removal of
the lesion has been achieved [ 54 ].
Midline Approach for Vermial or
Medial Hemispheric Lesions
For midline lesions, the patients are positioned
straight prone on the operating table. Special atten-
tion must be given so that the skull is securely
placed in the head-holding fixation device. Military
flexion (axial distraction and head flexion) is
obtained by allowing at least a finger’s breadth
spacing between chin and chest. The venous jugu-
lar drainage and endotracheal tube are checked for
any kinking, and the surgeon and anesthesiologist
together confirm that ventilation and venous out-
flow are intact. For prone cases, a rigid or the so-
called “armored” endotracheal tube may help with
airway patency throughout the case. The table is
elevated 10–15° above horizontal; with the patient’s
head in about 20° of military flexion, there is a
clear line of sight for midline approaches including
C1 and foramen magnum anatomy when working
under the operating microscopes. The patient’s legs
may be flexed and supported with pillows under the
shins. All extremities are padded, and the patient is
securely belted or taped to the bed to allow for table
rotation during the case for more avenues of visual-
ization into the posterior fossa.
For medial lesions of the vermis or medial cer-
ebellar hemispheres, a linear, medial incision is
made beginning 2 cm superior and extending
6 cm inferior to the inion. Dissection of the soft
tissue should occur along the nuchal line, care-
fully dividing it in the midline. Skin hemostasis is
obtained with Raney clips, and the incision is
retracted to provide access to the skull base for
bony opening. The periosteum is reflected off of
the bone with a periosteal elevator, and bone wax
is used to occlude bridging and epiploic veins.
The craniotomy or craniectomy (preferred by the
senior author) is then performed, with special
attention to remove all bone up to the venous
sinuses (Fig. 13.6a). Neuronavigation can be
used in these instances to verify vascular anat-
omy, especially if placing burr holes for a crani-
otomy; however, most of the time, with careful
drilling and dural visualization, injury to the
sinuses can be avoided. Small amounts of
Surgicel or other hemostatic agents can be placed
on any small areas of venous bleeding and with
gentle pressure usually are sufficient to obtain
hemostasis.
The exposed dura is carefully examined for
venous lakes and divided in a “Y”-shaped inci-
sion as possible to avoid these areas. The depth of
the tumor will dictate its visibility at this stage,
but careful surgical planning and extensive
knowledge of posterior fossa anatomy often pre-
cludes excessive use of stereotactic navigation in
many cases. For a deeper, midline lesions affect-
ing the fourth ventricle, the inferior vermis may
be divided at the midline, permitting resection of
the tumor. The major concern with this approach
is the risk of cerebellar mutism. Alternatively, a
telovelar approach can be adopted to avoid split-
ting the vermis during resection of a fourth ven-
tricular lesion. For deeper hemispheric lesions,
the cerebellar cortex should be divided with a
small corticectomy parallel to the cerebellar
folia, and the tumor may then be visualized and
resected. Neuronavigational tools may be useful
here to define acceptable boundaries of resection,
and intraoperative ultrasound can be used to
localize tumors that are not easily visible at the
surface. In select cases, iMRI can confirm ade-
quate removal of tumor.B.D. Weaver and R.L. Jensen