149then debulk the center of the tumor. The amount of
removal of the interior aspect of the tumor is deter-
mined by the size of the tumor on the scan, with
the objective of leaving a margin of the tumor
along the tumor capsule to enable the capsule to be
dissected from the surrounding cerebellum, brain-
stem, and nerves.
Attention is then paid to activity of the facial
and vestibulocochlear nerves as measured by the
electromyographic activity of the facial nerve
and the brainstem auditory evoked responses
(BAERs) of the cochlear nerve. If the tumor is
large and the hearing is nonserviceable, vestibu-
locochlear nerve monitoring is not performed.
We then start a dissection around the capsule of
the tumor to identify the CPA and the location of
the vestibulocochlear and facial nerves at the
brainstem. The vestibulocochlear nerve is more
posteriorly located in the brainstem and is usu-
ally found without difficulty on the medial aspect
of the tumor. If hearing preservation is not a goal,
the vestibulocochlear nerve is stimulated to
ensure there is no aberrant course of the facial
nerve and then is divided sharply. We then con-
tinue to rotate the capsule of the tumor laterally
and identify the facial nerve, which is located
anterior to the root entry zone of the vestibuloco-
chlear nerve. The root exit zone of the facial
nerve is identified visually and verified electro-
physiologically with stimulation from the Prass
electrode. Once the facial nerve is identified at
the brainstem, dissection is then continued with
debulking of the tumor, removal of the capsule,
and slow dissection of the capsule in a medial-to-
lateral direction from the facial and possibly ves-
tibulocochlear nerves if hearing preservation is a
goal (Fig. 11.1). The most adherent aspect of the
tumor to the facial nerve is usually at the region
of the lip of the porus acusticus. The nerve often
flares in dimension at this location, but in most
cases, judicious dissection from medial to lateral
can be performed and the nerve dissected free of
the tumor. The remaining tumor is removed from
the IAC, and careful dissection is performed to
sharply divide the vestibular nerve lateral to the
attachment to the nerve. At this point, we verify
functional continuity of the facial nerve by stimu-
lating the nerve at the brainstem to assess its
function. The BAER is recorded carefully after
resection of the tumor.
The drilled posterior aspect of the IAC is
inspected carefully to see if any air cells have
been entered. If air cells have been entered, bone
wax is used to obliterate them, and a small piece
of abdominal fat is placed over the drilled area
and held in place with fibrin glue. Hemostasis is
obtained, and a small piece of Surgicel is placed
over the region of the brainstem where the tumor
has been dissected free. The dura is closed in a
watertight fashion using dural substitute if neces-
sary. AlloDerm is our preferred material. The
bone is carefully waxed over the region of the
mastoid where any air cells have been entered.
We either replace the bone flap at this point or
place a small titanium or MEDPOR (Stryker,
Kalamazoo, MI) cranioplasty over the defect.
The muscle is closed in separate layers, and the
skin is closed with a 3-0 nylon suture.Fig. 11.2 (continued) Images showing steps of a retrosi-
gmoid approach for resection of a left large VS. (a) Axial
T1-weighted MRI with gadolinium demonstrating large
tumor. (b) Axial T2-weighted MRI demonstrating CSF
cleft between tumor, brainstem, and cerebellar peduncle.
(c, d) Coronal T1-weighted MRI with gadolinium demon-
strating large tumor. (e) Dural opening with flaps of dura
adjacent to transverse and sigmoid sinuses. (f) Tumor
exposure in the CPA and the petrous surface dural open-
ing is made in a semilunar fashion behind Fig. 11.2 (con-
tinued) the posterior lip of the IAC. (g, h) Posterior lip of
IAC is drilled. (i) IAC is opened and the tumor is identi-
fied and the facial nerve is located. (j) The ultrasonic aspi-
rator is used to debulk the tumor. (k) The vestibular nerve
is identified entering the tumor. It is then divided sharply
at the brainstem. (l) The facial nerve is identified at its root
exit zone from the brainstem. It is verified with the stimu-
lator. (m) Continued dissection of the tumor from the
facial nerve at the IAC. (n) The capsule of the tumor is
dissected from the facial nerve from the brainstem in a
medial-to-lateral direction. (o) The remaining portion of
tumor is dissected off the facial nerve near the lip of the
porus acusticus. (p) A small MEDPOR cranioplasty is
used to cover the craniectomy defect. (q, r) Postoperative
axial MRI with gadolinium demonstrating complete
resection of the tumor11 Vestibular Schwannomas