Skull Base Surgery of the Posterior Fossa

56

lost functional hearing, a total petrosectomy

(with drilling of the labyrinth and sometimes

cochlea) can be considered.

• Posterior petrosal meningiomas, which can
  arise anterior or posterior to the internal audi-
  tory canal (IAC) but are lateral to the trigemi-
  nal nerve. Those arising anteriorly can be
  addressed either via a zygomatic approach or
  a petrosal approach depending on size,
  whereas those confined to the posterior petro-
  sal surface can be addressed via a transmas-
  toid approach to the cerebellopontine angle
  (CPA) [ 7 ].


• Medial tentorial meningiomas, which can
  originate anywhere along the tentorium.
  Those with significant infra- and supratento-
  rial components can be approached via the
  petrosal approach which also allows for exci-
  sion of the affected dura.

In addition to extra-axial pathology, other
lesions are amenable to resection via a petrosal
approach including suprasellar lesions such as
retro-chiasmatic craniopharyngiomas [ 8 ], tri-
geminal schwannomas, posterior circulation
aneurysms [ 9 ], brainstem vascular malforma-
tions [ 10 ], and large epidermoids spanning the
posterior and middle cranial fossae.

Preoperative Planning

The main consideration in selection of surgical
approach is based on careful study of the lesion
as well as the surrounding normal anatomy. In
particular, the structure and pattern of venous
drainage should be considered especially when
the approach involves planned transection of the
tentorium and ligation of the superior petrosal
sinus (SPS) [ 11 ]. Areas deserving of special
attention include the point of insertion of the vein
of Labbe, the dominance of the SPS, and the
presence of other venous structures such as tento-
rial veins that would prohibit the approach.
Hearing evaluation should be performed pre-
operatively in order to establish a baseline and
evaluate candidacy for total petrosectomy.

Furthermore, attention should be paid to tumor

involvement of the temporal bone and in particu-

lar the petrous apex; the approach selected must

allow for removal of involved bone.

Role of Intraoperative Monitoring

The incidence of cranial nerve injury can be miti-

gated by using intraoperative neuromonitoring

[ 12 ]. Intraoperative neuromonitoring is employed

for all skull base cases in our institution and

includes, at minimum, somatosensory evoked

potentials (SSEP), brainstem auditory evoked

potentials (BAER), and motor evoked potentials

(MEP). Additional cranial nerves are monitored

depending on the anticipated extent of involve-

ment with the lesion. For extensive sphenopetro-

clival lesions, we routinely monitor the trigeminal

(V), oculomotor (III), abducens (VI), and facial

nerves (VII). The lower cranial (IX, X, XI) nerves

are also monitoring should the lesion extend near,

or extend past, the jugular fossa.

Petrosal Approach (or Posterior

Petrosal Approach)

Patient Positioning

The patient is placed supine with the operating

table slightly flexed to elevate the head slightly

above the level of the heart. The head is fixed

in a three-pin head holder and turned to the

side contralateral to the tumor with the vertex

tilted toward the floor and the head tilted

slightly toward the contralateral shoulder. A

shoulder roll is required to achieve the optimal

position. Intraoperative navigation is routinely

used, with preoperative magnetic resonance

(MR) and computed tomography (CT) imaging

co-registered, for confirmation of anatomical

landmarks. Intraoperative neuromonitoring is

also routinely used as described in the preced-

ing section. An area of the abdomen wall is

marked and prepped for anticipated fat graft

harvest.

D. Aum et al.