169difficult to visualize during the debulking of epi-
dermoids. The abducens nerve can be identified
below the seventh and eighth cranial nerve com-
plex from its brainstem origin before going back
up to its clival intradural fold before it enters
Dorello’s canal. The trochlear nerve should be
sought just medial to the dural free margin at the
incisura. Enough debulking is achieved once the
4th, 5th, 6th, 7th, and 8th complexes; the 9th,
10th, 11th, and 12th cranial nerves; the basilar
artery; and the anterior inferior cerebellar artery
are identified and decompressed. At this stage,
the arachnoid-epidermoid cyst wall adhesions are
identified and divided sharply, and the cyst wall
is removed. In cases where the cyst wall is very
adherent to the brainstem, cranial nerves, or
blood vessels, the adherent part of the cyst is left
behind to prevent neurological or vascular injury.
Attention now is turned toward the supraten-
torial middle fossa portion of the cyst. The tento-
rial surface posterior to the superior petrosal
sinus and medial to the petrosal vein is divided
sharply using a sharp blade and extended toward
the tentorial free margin using a microscissor.
The trochlear nerve is at risk during this step and
must be visualized and protected before complet-
ing the tentorial cut, which is performed posterior
to where the trochlear nerve enters the tentorial
edge. Bleeding from the tentorium can be con-
trolled with hemostatic agent and bipolar cautery.
The supratentorial portion of the cyst can now be
debulked, and the third cranial nerve, posterior
cerebellar artery, and posterior communicating
artery can be identified and freed using microsur-
gical techniques. The endoscope can be brought
at this stage and 30-, 45-, and 70-degree angled
scopes can be introduced into the surgical field so
the remaining pieces of the lesion can be visual-
ized and removed using curved suction and
microinstruments.
In cases where the supratentorial portion of
the cyst proves difficult to resect microscopi-
cally or endoscopically from the retrosigmoid
window, the skin incision can be extended supe-
riorly by curving forward and then inferiorly to
the level of the root of the zygoma just anterior
to the tragus, and a separate subtemporal bone
window is made. Once the bone window is ele-
vated, the inferior margin of the temporal bone
is drilled away until the root of the zygoma is
flush with the middle fossa floor. The dura is
opened in an inverted U shape and reflected
inferiorly. The temporal lobe is elevated, and the
tentorium is followed medially to the free mar-
gin. The arachnoid over the ambient cistern is
divided sharply, and the lesion should be evi-
dent at this stage. In a similar fashion, the epi-
dermoid is debulked, and the oculomotor and
trochlear nerves, the posterior cerebellar artery,
and the posterior communicating artery should
be identified and freed (Fig. 12.3).
The dura is closed in a watertight fashion
either primarily or using a dural substitute. Bone
wax is used to seal the middle fossa and mastoid
bone to ensure obliteration of any opened air
cells. Muscle and fascia, galea, and skin are
closed in a multilayered fashion.Complication Avoidance
Surgical position-related neuropathy is not
uncommon, especially in an obese patient with a
short neck when lateral positioning is used. In
our experience, somatosensory and motor evoked
potentials may provide invaluable information
after final positioning and may be used to adjust
the extremity position of neck position before
starting the procedure.
To avoid sigmoid or transverse injury during
surgical exposure and to ensure adequate bony
removal before opening the dura, a neuronaviga-
tion system is a useful surgical adjunct when
available. Careful attention to the sigmoid and
transverse dominance in the preoperative imag-
ing is important in all posterior fossa approaches.
When sinus injury occurs during surgery, every
effort is made to repair the injured sinus primar-
ily using a vascular stitch; depending on the
extent of the injury, postoperative antiplatelet
treatment using aspirin may be considered. When
a subtemporal window is planned, the superficial
sylvian vein drainage patterns and vein of Labbé,
especially in the dominant temporal lobe, should
be identified in the preoperative imaging and pro-
tected during surgery.12 Epidermoid Cyst