III

Chapter

Aneurysms of the
^lf Posterior Circulation

159

160

Aneurysms of the Posterior Circulation

In this chapter, we describe surgical
approaches to aneurysms located on the
posterior circulation. These guidelines are
also described in Chapter II (Aneurysms of
the Anterior Circulation). Regardless of the
aneurysm's location, the operative princi-
ples listed should be followed in all cases.

Guidelines to Surgery of Intracranial
Aneurysms

1. Complete Angiography

Complete angiography requires a four-
vessel study with multiple views of the an-
eurysm being considered for surgery. Col-
lateral circulation must be determined on
static images or the potential for collateral
circulation must be assessed by dynamic
maneuvers, such as the Alcock test and
temporary carotid occlusion with balloon.
Radioisotope flow studies may be valuable
in evaluating circumstances of cerebral is-
chemia.

2. Multidisciplinary Approach

Alternative methods of treatment that
must be considered include observation,
trapping of the aneurysm by proximal and
distal occlusion of the parent artery, proxi-
mal endovascular occlusion, intra-aneurys-
mal endovascular occlusion, and direct
surgery. Direct surgical procedures may be
augmented with endovascular techniques
to maintain vascular control.

3. Formulation of the Plan (Phase I)

If multiple aneurysms are present, the
aneurysm responsible for rupture can be
determined by localization of hemorrhage
on computed tomography (CT) scan, as-
sessment of hemosiderin deposit on mag-
netic resonance imaging (MRI), aneurysm
configuration on angiography, or clinical
neurologic findings. The surgical approach
is chosen based on the experience of the
team and is designed to achieve maximal
exposure of the surgical anatomy. Timing
of surgery follows the principle that a rup-
tured aneurysm should be clipped as soon
as the patient is adequately prepared for the
operation. Early surgery is preferred and is
performed as quickly as the patient and sur-
gical team are prepared.

4. Execution of the Plan (Phase II)

The most significant determinant of
surgical outcome is the experience of the
surgeon and the operating team. A com-
plete surgical team must be available. The
details of the entire surgical plan must be
effectively communicated to all members
of the team, which includes radiologists,
nurses, anesthesiologists, and surgical staff.

5. Anticipation

Early surgery of the ruptured aneu-
rysm poses an increased risk of intraopera-
tive rupture. The microscope is draped,
focused on the skin, and swung out of posi-
tion, ready to be used without delay.

6. Surgical Exposure

Brain relaxation is achieved by the ap-
propriate use of hyperventilation, osmotic
agents, diuresis, and aspiration of cere-
brospinal fluid. Cerebrospinal fluid aspira-
tion can be accomplished with a preoper-
atively placed lumbar drain (aspiration only
after craniotomy), from arachnoid cisterns,
or by direct intraoperative cannulation of
the ventricular system. Appropriate brain
resection is preferable to over-retraction of
brain tissue. We cannot overemphasize the
importance of patience until brain relax-
ation is achieved. The surgeon must be pre-
pared to delay the operation if relaxation is
judged as insufficient.

161

Guidelines to Surgery of Intracranial Aneurysms

7. Proximal Vascular Control

Proximal vascular control must be es-
tablished on the parent vessel if possible,
and if not, by isolating the proximal vascu-
lar supply in the neck. Certain lesions (e.g.,
giant aneurysms of the basilar artery) are
best controlled by temporary intravascular
occlusion with a balloon catheter. This ap-
proach is preferable to the use of systemic
circulatory arrest.

8. Precise Exposure of the Aneurysm

The sequence of exposure is the proxi-
mal parent vessel, distal parent vessel, and
aneurysm neck. Sharp dissection is prefer-
able to blunt dissection. Once the aneurysm
neck is isolated, critical vessels adjacent to
the aneurysm are defined and dissected
from the aneurysm when possible. This dis-
section is often delayed until the aneurysm
is collapsed by trapping or clip application.

9. Hypotension

Focal hypotension by trapping with
low-pressure temporary clips is preferable
to systemic hypotension. If trapping iso-
lates the distal circulation from the collat-
eral circulation, cerebral protection must
be provided by intravenous barbiturate or
equivalent agents. Trapping should be used
to avoid aneurysm rupture intraopera-
tively. If vascular control cannot be
achieved within the operative exposure, a
temporary balloon may be navigated into
the appropriate location.

10. Clipping the Aneurysm

It is preferable to place the clip along
the long axis of the parent artery. Perpen-
dicular clipping increases the risk of con-
stricting the parent vessel or tearing the
aneurysm base. Application of multiple
clips is preferable to forcing a single clip
onto a complex aneurysm.

11. Assessment of Clipping

Once the primary clip is placed and
temporary clips are removed, the aneurysm
is punctured with a fine needle and aspi-
rated to check for residual filling. Large or
complex aneurysms may require aspiration
of the sac prior to placing the clip. Residual
filling of the aneurysm requires reposition-
ing of the clip, multiple clips, or a reenforc-
ing clip to increase closing pressure. The
success of clip closure is best determined
when the patient's blood pressure is re-
turned to normal.

After aspirating the aneurysm, the sur-
geon mobilizes the fundus and dome, in-
spects the aneurysm base, and eliminates
the inclusion of any major vessels or per-
forating vessels in the clip. Allowing a small
aneurysm to rest is preferable to constric-
ting the parent artery. Patency of the parent
artery is assessed by inspection, intra-
operative Doppler analysis, or intraop-
erative angiography. Postoperative assess-
ment of clip placement by cerebral an-
giography is recommended in complex an-
eurysms that are difficult to evaluate by
intraoperative procedures. Preoperative
vascular bypass, intraoperative bypass, or
vascular reanastomosis are options for
maintaining distal circulation.

12. Postoperative Care (Phase III)

Phase I of successful aneurysm sur-
gery is possessing a good plan, Phase II is
the execution of the plan, and Phase III is
aggressive postoperative care. In cases of
ruptured aneurysms, hypervolemic hemo-
dilution is monitored by the use of blood
analysis and Swan-Ganz catheter record-
ing. Acceptable agents to prevent vaso-
spasm are administered by protocol;
vasospasm is monitored by transcranial
Doppler recording. Intracranial pressure is
monitored (if indicated) and treated appro-
priately. Patients are managed by the inten-
sive care team until the risks of vasospasm,
cerebral edema, and other complications
have passed.

162

Aneurysms of the Posterior Circulation

Aneurysms of the Basilar Terminus

General Alternative techniques to directly clip an-
Considerations eurysms of the basilar terminus are limited.
Endovascular intra-aneurysmal occlusion
by balloon or thrombogenic devices is be-
ing increasingly evaluated for inoperable
cases. Concerns with endovascular tech-
niques include intraoperative rupture, oc-
clusion of perforating arteries, and incom-
plete thrombosis. Trapping procedures may
be used but most balloon procedures de-
pend on the development of progressive
thrombus formation within the aneurysm.
Propagation of thrombus to critical per-
forating vessels may complicate this tech-
nique.

Cerebral angiography in multiple views
is critical for assessing the aneurysm con-
figuration and the exact relationship of
major arterial branches to the aneurysm
neck. It is important to determine the abil-
ity of the posterior communicating arteries
(PCoAs) to maintain the patency of the pos-
terior cerebral arteries if either of the P,
arterial segments are sacrificed. This as-
sessment is made by static angiographic im-
ages or by dynamic maneuvers, such as the
Alcock test (vertebral artery injection dur-
ing carotid artery compression), if adequate
collateralization is not demonstrated on
static images.

The specific surgical approach to a
basilar terminus aneurysm is determined
largely by the location of the aneurysm
neck in relation to the posterior clinoid pro-
cess. Options include the pterional ap-
proach, modified pterional approach,
pterional approach with orbitozygomatic
osteotomy, subtemporal approach with zy-
gomatic osteotomy, and subtemporal ap-
proach with anterior petrosectomy. These
approaches are described in detail in Chap-
ter I.

Temporary occlusion of the distal basi-
lar artery (by direct clip placement or bal-
loon) and PCoAs bilaterally is a valuable
adjunct to the treatment of certain complex
terminus aneurysms. This tactic can pro-
vide the near total circulatory arrest needed
to decompress and permit dissection of
large or complex aneurysms.

Aneurysms of the Basilar Terminus: Supraclinoidal 163

Aneurysms of the Basilar Terminus:
Supraclinoidal

3.1

Special Basilar terminus aneurysms with origin
Considerations above the level of the posterior clinoid pro-
cess, as demonstrated on the lateral projec-
tion of the cerebral angiogram, have been
called high-riding. Such lesions are difficult
to expose by a subtemporal approach
owing to the surgeon's need for a line of
sight upward and medial to the frontal lobe.
Basilar aneurysms in this location are best
¹ exposed by the pterional approach with
wide opening of the sylvian fissure. This

approach may be enhanced by orbitozy-
gomatic osteotomy to expand the operative
field and improve the surgeon's ability to
develop a trajectory from inferior to superi-
or. The surgeon may consider two options
for the deep exposure by selecting a corri-
dor either medial or lateral to the carotid
artery.

164 Aneurysms of the Posterior Circulation

When the aneurysm neck arises superi-
or to the supraclinoid segment of the inter-
nal carotid artery (ICA), the surgical trajec-
tory should be between the carotid artery
and the optic nerve. Wide opening of the
basal and sylvian cisterns is necessary to
permit upward elevation of the frontal lobe
and posterior retraction of the temporal
lobe. Perforating arteries from the first seg-
ment of the posterior cerebral artery (PCA,
pi) must be separated for clip application.
In supraclinoidal aneurysms of the basilar
artery, the perforators project downward
and are more easily visualized from a lat-
eral trajectory, which can only be achieved
by widely splitting the sylvian fissure and
mobilizing the temporal lobe.

fixation device in preparation for intra-
operative angiography and extended to
provide a maximal upward trajectory. A
lumbar spinal catheter is inserted prior to
positioning for intraoperative aspiration of
cerebrospinal fluid.

3.2 After the dura is
opened, two 15-mm retractors are placed
on the frontal and temporal lobes. Cere-
brospinal fluid is gradually aspirated to in-
crease brain relaxation and facilitate
exposure. Bridging veins between the tem-
poral tip and the sphenoparietal sinus are
coagulated and incised. The frontal lobe is
elevated and the temporal lobe is retracted
downward and posteriorly.

Approach

A pterional craniotomy modified with addi-
tional removal of temporal bone flush with
the floor of the middle cranial fossa is per-
formed with the patient in the supine posi-
tion (see Chapter I). The head is maintained
at 30 degrees rotation in a radiolucent head

Aneurysms of the Basilar Terminus: Supraclinoidal 165

Carotid -
cistern

3.3 The optic and carotid
cisterns are maximally opened. The sylvian
fissure is split to the carotid bifurcation.

3.4 The carotid artery is
displaced laterally with a 2-mm micro-
retractor. The membrane of Lillequist is
incised and cerebrospinal fluid is further
aspirated from the interpeduncular cistern.

Basilar
artery

166 Aneurysms of the Posterior Circulation

Right

PCA (P, segment)

3.5 The optic nerve is re-
tracted medially. Proximal control of the
basilar artery is secured above the posterior
clinoid process. The right P, segment is
dissected from the aneurysm base.

3.6 The left P, segment is
dissected from the aneurysm base to com-
plete the path for clip application.

Left PCA

segment)

167

Aneurysms of the Basilar Terminus: Supraclinoidal

Superior

cerebellar

artery

3.7

3.7 A small straight clip is
guided into place from an inferior trajec-
tory. The tips of the clip are angled slightly
upward in the narrow exposure. Selection
of a slender clip with long blades (>20 mm)
may improve visibility for more precise ap-
plication of the clip.

168

Aneurysms of the Posterior Circulation

I

3.9

3.8 The clip blades are ap-
proximated and the aneurysm dome is aspi-
rated with a 22-gauge needle.

3.9 The aneurysm dome
and clip are tilted forward to expose the
myriad of perforators arising from the basi-
lar terminus and P, segments. Adequate vi-
sualization must ensure that alt perforators
are excluded from the clip blades. Manipu-
lation of the clip or the distal application of
a second clip may be needed to free the
perforators or the origin of the P, segments.

Closure Closure is performed as for a pterional cra-
niotomy (see Chapter I).

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