II

Chapter

Aneurysms of the Anterior
Circulation

79

Aneurysms of the Anterior Circulation

In this chapter, we describe surgical
approaches to aneurysms located on the
anterior circulation. Regardless of the an-
eurysm's location, the operative principles
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
ischemia.

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 radiolo-
gists, nurses, anesthesiologists, and surgi-
cal 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.

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.

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81

Guidelines to Surgery of Intracranial Aneurysms

I

I

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 intraoper-
atively. 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.

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,
intraoperative Doppler analysis, or intra-
operative angiography. Postoperative as-
sessment of clip placement by cerebral
angiography is no longer recommended.
Preoperative vascular bypass, intraopera-
tive 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.

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 the clip closure is best deter-
mined when the patient's blood pressure is
returned to normal.

82

Aneurysms of the Anterior Circulation

General
Considerations

Aneurysms of the Internal Carotid Artery

Aneurysms of the Intracavernous Carotid
Artery

Intracavernous carotid artery aneu-
rysms can be traumatic or spontaneous.
Traumatic aneurysms frequently occur at
two sites: along the anterior loop of the
cavernous carotid artery in association with
fracture of the anterior clinoid or, less fre-
quently, along the medial loop due to frac-
tures through the petrous bone. Most spon-
taneous intracavernous aneurysms arise
along the anterior loop of the artery and
project laterally. Patients with these aneu-
rysms present with pain, which is retro-
orbital or supraorbital in location, and vary-
ing degrees of cavernous sinus syndrome
associated with the compression of cranial
nerves III through VI. Stretching of the
sympathetic nerves around the carotid ar-
tery may produce Horner's syndrome.

Options for the treatment of intra-
cavernous aneurysms include observation,
endovascular intra-aneurysmal placement
of a balloon or thrombogenic substances,
trapping procedures by direct or endo-
vascular techniques, and direct intracaver-
nous aneurysm clipping. Regardless of the
method selected, all patients must undergo
a preoperative test to assess their tolerance
of internal carotid artery (ICA) occlusion.
The ICA is occluded by balloon (placed by
transfemoral catheterization). The patient
is awake while undergoing continuous neu-
rologic and neurophysiologic monitoring
and assessment of cerebral blood flow by a
radioisotope perfusion technique. Failure
to tolerate occlusion documents the need
for a preoperative vascular bypass graft.
We prefer a vein graft from the cervical
carotid artery to the middle cerebral artery
rather than a shorter graft, which connects
the petrous to the supraclinoid ICA seg-
ment. The latter not only is more difficult
but also prolongs the operative procedure,
fails to allow graft maturation, and prevents
preoperative documentation of patency.

When approaching intracavernous and
paraophthalmic aneurysms, we prefer to
isolate the common and internal carotid ar-
teries in the cervical region. This maneuver
ensures timely proximal control of the car-
otid and provides a conduit for intraopera-
tive angiography. The proximal carotid
artery can also be exposed in the middle
fossa (Glasscock's triangle), medial to the
eustachian tube.

If preliminary carotid occlusion is well
tolerated and there are no contralateral an-
eurysms or arteriovenous malformations,
then balloon occlusion of the aneurysm or
trapping of the aneurysm with balloons is
an acceptable option. Proximal balloon oc-
clusion is preferred to proximal ligation or
staged occlusion using mechanical clamps
on the ICA because it eliminates a long
segment of proximal carotid artery filled
with thrombus. Moreover, staged occlusion
seems to be cf little value in view of con-
temporary physiologic testing.

We reserve direct operation for aneu-
rysms of the cavernous carotid artery or
paraclinoid ICA that expand the distal dural
ring and enter the intradural space and in-
tracavernous aneurysms not amenable to
indirect techniques. Fortunately, most in-
tracavernous carotid aneurysms arise from
the proximal anterior loop and can be
clipped directly after opening the distal du-
ral ring, opening the roof of the cavernous
sinus, and partially mobilizing the lateral
wall of the cavernous sinus. The proximal
dural ring may remain intact. The inner
membranous layer of the lateral wall is dis-
sected from the base of the aneurysm. The
venous channels of the sinus are gently
packed with Surgicel.

Aneurysms of the Internal Carotid Artery: Intracavernous 83

Special The bone resection necessary for cavern-
Considerations ous sinus exploration can be performed
using an extradural or an intradural ap-
proach. Aneurysms that occupy only a por-
tion of the cavernous sinus and those
aneurysms that arise near the proximal du-
ral ring can be adequately exposed by a
totally intradural approach. This technique
will be demonstrated in the section on par-
ophthalmic aneurysms. Aneurysms that are
located entirely in the cavernous sinus, par-
ticularly those aneurysms that arise proxi-
mal to the anterior loop, are best exposed
after extensive bone removal by the extra-
dural approach shown in this section.

The bony structures that surround the
cavernous sinus region (i.e., anterior

clinoid process, lateral wall of orbital
fissure) must be removed. Removal of the
bony structure is critical in order to ade-
quately and safely mobilize the dura pro-
pria of the lateral wall as well as to gain
access to the triangles that are necessary
for intracavernous exploration (i.e., Haku-
ba's, paramedian, Parkinson's, Mullen's).
This bone resection can be done either in a
preliminary extradural phase or after the
dura is opened. We prefer intradural re-
moval of the anterior clinoid for aneurysms
that partially involve the distal ring of the
cavernous sinus. For these examples of an-
eurysms located entirely in the cavernous
sinus, we recommend resection of the bony
structures during an extradural phase. Al-
though the proximal ICA can be isolated
and controlled in Glasscock's triangle, we
prefer to expose the ICA in the neck.

84

Aneurysms of the Anterior Circulation

Approach The patient is placed in the supine position
(see Chapter I) with the head maintained at
30 degrees rotation to bring the surgeon's
line of view down the anterior clinoid pro-
cess. The craniotomy is pterional (see
Chapter I), modified to extend anteriorly to
the midpoint of the orbit flush with the or-

bital roof and inferiorly flush with the floor
of the middle fossa (orbitozygomatic osteo-
tomy is optional; see Chapter I). A lumbar
catheter for intraoperative aspiration of ce-
rebrospinal fluid decreases the need for
temporal lobe retraction.

Balloon inflated
in the medial loop
of the ICA

Aneurysm of
anterior loop

85

Aneurysms of the Internal Carotid Artery: Intracavernous

2.3

Anterior

clinoid

process

2.4

2.2 A curved incision, made
in the right side of the patient's neck, ex-
poses the bifurcation of the common ca-
rotid artery. Internal and external carotid
branches are isolated with vessel loops to
establish proximal vascular control. The
ICA is also available for catheterization,
balloon occlusion, and angiographic pro-
cedures.

2.3 Cerebrospinal fluid is
aspirated from the lumbar catheter. The
dura is retracted to expose the orbital roof,
which is entered with a high-speed drill.
The posterior one half of the orbital roof is
removed with a punch.

2.4 A rongeur is used to en-
large the posterior orbitotomy and to ex-
pose the superior orbital fissure by
removing additional bone from the lesser
wing of the sphenoid.

1

86

Aneurysms of the Anterior Circulation

Optic
canal

2.5 A high-speed pneumatic
drill with a diamond burr is used to dissect
the bone of the orbital apex and the proxi-
mal anterior clinoid process.

2.6 A diamond burr re-
moves the bony roof of the optic canal and
opens the optic foramen. Care is taken to
avoid entering the ethmoid sinus, which is
medial to the optic canal.

Ethmoid
sinus

87

Aneurysms of the Internal Carotid Artery: Intracavernous

2.7 The anterior clinoid
process is dissected until the process is suf-
ficiently thinned to expose the lateral sur-
face of the optic nerve. The remainder of
the clinoid is hollowed out to separate it
from the optic strut. The optic strut con-
nects the anterior clinoid process with the
floor of the optic canal.

2.8 The remaining fragment
of the anterior clinoid process is grasped,
dissected from its dural attachments, and
extracted. The optic strut, lying between
the carotid artery and the optic nerve,
should be further removed with a diamond
drill.

Optic
strut

ICA

Anterior

clinoid

process

2.8

88

Aneurysms of the Anterior Circulation

2.9 The bone, lateral to the
superior orbital fissure and medial to the
foramen rotundum, is thinned with a drill
and fractured laterally to fully expose the
superior orbital fissure.

2.10 An incision is made in
the dura medial and parallel to the superior
orbital fissure.

Optic
nerve

Aneurysms of the Internal Carotid Artery: Intracavernous 89

Proximal

dural
ring

Paraclinoid
ICA

Anterior

fossa

floor

Middle fossa
floor

Falciform
ligament

Distal dural
ring

Petroclinoid

ligament

2.11

2.12

2.11 The dural incision con-
tinues medially (perpendicular to the optic
nerve) and laterally (across the inferior
temporal lobe) to provide access to the
floor of the frontal and middle fossae.

2.12 The distal dural ring,
which encircles the carotid artery, is in-
cised sharply, using a disposable arachnoid
knife.

90

Aneurysms of the Anterior Circulation

Trochlear
nerve

Dura propria of
lateral wall

2.13

2.13 Beginning at the ocu-
lomotor foramen, an incision is made in the
lateral wall (dura propria) along the axis of
the third nerve into the midportion of the
oculomotor trigone.

2.14 Further dissection will
reflect the dura propria of the lateral wall to
expose cranial nerves III and IV and the
first division of cranial nerve V. The inner
membranous layer (formed by the perineu-
ral sheaths of the cranial nerves) allows
mobilization of the dura propria without
disruption of the venous channels of the
cavernous sinus.

2.15 Beginning at the fal-
ciform ligament, the optic sheath is incised
along the course of the nerve. The dura of
the medial sheath is excised. The optic
nerve is retracted medially to expose the
ophthalmic artery. The roof of the cavern-
ous sinus and the proximal dural ring are
incised along the medial edge of the ocu-
lomotor nerve. The oculomotor nerve is re-
tracted laterally to expose the fundus and
the aneurysmal base.

Aneurysms of the Internal Carotid Artery: Intracavernous 91

First division of
trigeminal nerve

Ophthalmic
artery

92

Aneurysms of the Anterior Circulation

2.16 The dissector identifies
the relationship of the aneurysmal base to
the anterior loop of the carotid artery and
to the distal dural ring.

2.17 A right-angle fenes-
trated clip is applied along the axis of the
carotid artery and across the base of the
aneurysm.

93

Aneurysms of the Internal Carotid Artery: Intracavernous

2.18 The aneurysm is aspi-
rated to document complete obliteration of
its neck. In some circumstances, giant an-
eurysms of the carotid cavernous region
may require collapse of the aneurysmal sac
for effective dissection and clip application.
The aneurysm may be collapsed by occlu-
sion of the ICA in the neck and distal to the
intracranial dissection site. Aspiration of
the dome permits dissection and accurate
placement of the clip. Intraoperative an-
giography should be performed on all aneu-
rysms in this region because of the
difficulty of direct inspection.

Closure Effective closure requires that air cells in
the sphenoid bone and clinoid are obliter-
ated with tissue adhesives and an overlaid
fascial graft. This maneuver is necessary to
prevent leakage of cerebrospinal fluid
through the nasal sinuses. The fluid is
drained from the lumbar catheter for 72
hours to maintain cerebrospinal fluid pres-

sure at 50 mm of water and to allow the
basal skull closure to heal. The dural edges
are approximated with absorbable suture.
The bone flap is replaced and any defects
are filled with methyl methacrylate to im-
prove the cosmetic result. The remainder of
the closure is done according to standard
pterional approach (see Chapter I).

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