Skull Base Surgery of the Posterior Fossa

206

can tolerate ischemia. For that reason, all opera-
tions should be performed under pharmacological
cerebral protection (using barbiturates or propofol)
with mild hypothermia (33 °C) [ 103 – 105 ].

Pharmacological Cardiac Arrest
Circulatory arrest is an important adjunct for the
treatment of cerebrovascular lesions. In properly
selected patients, circulatory arrest greatly facili-
tates the surgical treatment of aneurysms and
AVMs, but it is associated with a high risk of
complications [ 106 ]. As a result, the use of hypo-
thermic circulatory arrest has largely been aban-
doned, particularly due to the introduction of
well-tolerated pharmacological alternatives. The
introduction of adenosine has transformed circu-
latory arrest from an invasive procedure to one
that is much better tolerated, safer, and more tran-
sient than hypothermic circulatory arrest.
Moreover, adenosine produces reliably reproduc-
ible arrest durations. Adenosine allows the sur-
geon to decrease flow into an aneurysm (or a
ruptured AVM), allowing for vascular control
and final intervention for the lesion [ 107 ]. In
aneurysms, the use of adenosine allows the sur-
geon to obtain proximal and distal control, or it
provides the relaxation necessary to allow proper
placement of the clip across the neck of the aneu-
rysm. In AVMs, the use of adenosine allows the
rupture point to be identified and controlled. Akin
to other methods of cardiac arrest, rapid ventricu-
lar pacing allows for a reproducible period of car-
diac arrest, necessary for the final steps of
aneurysm dissection or for identification of
bleeding points, especially in the confines of the
posterior fossa. Unlike adenosine arrest, rapid
ventricular pacing is titratable, and controlled
periods of arrest can be obtained with this
technique.

Approaches and Approach

Selection to Posterior Fossa

Vascular Lesions

The choice of approach for any lesion should
take certain basic parameters into consideration.
These parameters include the shortest approach

(when possible), an approach that allows the sur-

geon to readily visualize the lesion or lesions

while minimally disturbing other eloquent neuro-

vascular structures, thereby limiting patient mor-

bidity, and the approach that is most convenient

to the surgeon due to handedness and experience.

For intrinsic lesions, such as cavernous malfor-

mations of the brainstem, the two-point method

is an excellent starting point for approach selec-

tion but must be combined with knowledge of

safe-entry zones for optimal approach selection

[ 86 ]. The judicious use of skull base approaches

allows for adequate visualization of the contents

of the posterior fossa, without undue risk of

injury to the critical structures (Fig. 14.7). An

important tenet of skull base surgery is the

removal of bone and minimization of tissue and

brain retraction to achieve the necessary visual-

ization and working trajectory. Some general

approach-related considerations are presented

below; however, the specific steps of the various

surgical approaches are not outlined, and the

reader is referred to other material for details of

the approaches [ 85 , 86 , 108 – 110 ].

Approaches to the Ventral Midbrain/

Posterior Fossa

Approaches to the ventral midbrain include the

pterional, orbitozygomatic, and anterior petro-

sectomy approach and their variants. Aneurysms

of the basilar apex, proximal PCA, and SCA can

be readily exposed using the pterional, subtem-

poral, supraorbital, modified orbitozygomatic,

and full orbitozygomatic approaches (Fig. 14.8)

[ 111 ]. These ventrolateral approaches allow the

surgeon to release cerebrospinal fluid from the

basal cisterns and develop working corridors

between the carotid artery, optic nerve, and ocu-

lomotor nerve to arrive at the region of the basilar

apex. These approaches are also well suited for

intrinsic lesions that lie in the ventral midbrain.

Exposure using these surgical corridors leads the

surgeon to two safe-entry zones on the ventral

midbrain, the anterior mesencephalic and the

interpeduncular safe-entry zones, which can be

accessed to remove lesions from the lateral and

M.Y.S. Kalani and R.F. Spetzler