Skull Base Surgery of the Posterior Fossa

205

detecting acute hemorrhage [ 91 ]. Fine-cut fast
imaging employing steady-state acquisition
(FIESTA) MRI sequences can help identify vas-
cular compression syndromes in the confines of
the posterior fossa [ 92 , 93 ].
The gold standard for workup of vascular
pathologies in the posterior fossa is digital sub-
traction angiography (DSA). Biplane rotational
DSA with three-dimensional reconstruction pro-
vides dynamic views of blood flow, which can
assist surgeons with surgical planning. DSA can
evaluate collateral circulation, and by applying
selective and sequential compression or tempo-
rary occlusion of inflow vessels to the brain, by
using the Alcock’s test, for example, can provide
essential information for planning treatment. In
patients with AVMs, DSA can provide important
information regarding the extent of shunting,
flow-related aneurysms, and evidence of venous
outflow obstruction. In some cases, DSA can be
combined with endovascular intervention as a
part of a multimodality treatment strategy [ 62 ],
for example, combined endovascular and micro-
surgical treatment of AVMs and aneurysms.

Operative Adjuncts

Neuromonitoring and Mapping

We routinely use intraoperative neuromonitoring
for neurovascular surgery in the posterior fossa,
including monitoring of somatosensory evoked
potentials, motor evoked potentials, and cranial
nerve-specific monitoring, depending on the
location of the lesion in the posterior fossa [ 94 ,
95 ]. Intraoperative neuromonitoring provides
real-time output of the function of pathways that
may be injured during the operation. Caution
should be used, however, because of the depen-
dence of these techniques on the type and depth
of anesthesia. A neuroanesthesia team that is pro-
ficient with the use of monitoring is critical to
properly perform the operation. In addition to
monitoring the function of pathways, electro-
physiological mapping of critical nuclei is possi-

ble, especially during operations on the brainstem

for intrinsic lesions.

Intraoperative Evaluation of Blood

Flow

Several well-accepted adjuncts exist for evalu-

ating intraoperative blood flow. The most com-

mon modalities include intraoperative

angiography, which provides similar informa-

tion to traditional DSA, indocyanine green

angiography (ICG), and intraoperative ultraso-

nography. ICG is now widely used in operating

theaters and has replaced the need for intraop-

erative angiography in many cases [ 96 – 98 ]. In

this technique, a fluorescent dye is adminis-

tered that allows the blood flow in vessels to be

visualized in real time. ICG can be used to con-

firm the patency of vessels and occlusion of

flow from an aneurysm dome during aneurysm

surgery. ICG can help identify inflow arteries

and draining veins during AVM surgery

(although ICG is most useful for superficial

lesions, and its application for deep AVM sur-

gery is debated). Quantitative ICG can provide

flow measurements [ 99 , 100 ]. Alternatively,

handheld Doppler ultrasonography probes can

be used to evaluate blood flow measurement in

the operating room. The best application of

this technology is for the selection of donor

and recipient vessels during bypass surgery

and for confirming patency of inflow and out-

flow vessels before and after aneurysm clip-

ping [ 101 , 102 ].

Cerebral Protection and Hypothermia

Cerebrovascular surgical procedures have the

potential for significant injury to neural structures.

Hypotension, retraction, excessive blood loss, iat-

rogenic injury, hemodilution, hypo- and hypergly-

cemia, and hypoxia can all cause ischemic injury.

Maneuvers that decrease cerebral metabolism

greatly prolong the amount of time that the brain

14 Microsurgical Management of Posterior Fossa Vascular Lesions