106

Aneurysms of the Anterior Circulation

Aneurysms of the Carotid Terminus

General Aneurysms originating at the junction of
Considerations the anterior and middle cerebral arteries are
termed carotid terminus aneurysms. Be-
cause of their location, they support the
hypothesis that aneurysms develop in the
direction of blood flow at a junction of two
vessels. Carotid terminus aneurysms can
project superiorly into the anterior perfo-
rate substance or posteriorly, thus distort-
ing the optic tract. The majority of surgical
complications are attributed to the occlu-

sion of perforating arteries that arise from
the proximal segments of the anterior (A,)
and middle (M,) cerebral arteries. Compli-
cations may also occur when there is adher-
ence of the recurrent artery of Heubner or
anterior choroidal artery to the aneurysm.
The surgeon needs skill and patience to dis-
sect all adjacent vessels from the aneurys-
mal neck and to inspect the clip for
inclusion of these vessels prior to final
placement.

2.36

107

Aneurysms of the Internal Carotid Artery: Carotid Terminus

Special Large or giant carotid terminus aneurysms
Considerations may be difficult to expose, especially after
a rupture. Aspiration of cerebrospinal fluid
from a lumbar spinal catheter placed prior
to positioning will aid in brain relaxation
since the deep arachnoid cisterns may be
difficult to access. If the aneurysm has a
complex configuration, intraoperative an-
giography should be performed to assess
final clip placement and to ensure the pat-
ency of the internal carotid artery, M, and
A| cerebral arteries, and adjacent vessels.
The carotid terminus aneurysm presented
in this exercise is of medium size with nor-
mal configuration.

pterional with half-and-half exposure of the
frontal and temporal lobes. A catheter is
placed in the lumbar subarachnoid space
prior to positioning for intraoperative aspi-
ration of cerebrospinal fluid.

2.37 After the dura is opened,
two 15-mm retractors are placed on the
frontal and temporal lobes. The veins to the
sphenoparietal sinus are coagulated and
incised.

Approach The patient is placed in a supine position
(see Chapter I) with the head maintained at
45 degrees rotation. The craniotomy is

Vein entering
sphenoparietal

sinus

2.37

108

Aneurysms of the Anterior Circulation

2.38

2.38 The carotid cistern is
opened. Gentle retraction elevates the fron-
tal lobe as cerebrospinal fluid is aspirated to
enhance brain relaxation and retraction.

2.39 The proximal sylvian
fissure is opened widely. The arachnoid of
the carotid cistern is opened to expose the
aneurysm base and the origin of the A, and
M, segments.

Posterior communicating
artery

Anterior

choroidal

artery

Anterior

cerebral artery
(at segment)

Middle

cerebral artery
(mt segment)

2.39

109

Aneurysms of the Internal Carotid Artery: Carotid Terminus

2.40

2.41

2.40 The dome of the aneu-
rysm lies underneath the frontal lobe,
where it is protected from injury. The neck
of the aneurysm is dissected by sharply ex-
cising the arachnoid adhesions from the M,
segments.

2.41 Sharp dissection frees
the aneurysm from the proximal A, seg-
ments. Dissection of either complex or
large aneurysms of the terminus region is
facilitated by temporary occlusion of proxi-
mal and distal vessels.

110 Aneurysms of the Anterior Circulation

2.42 An aneurysm clip is
guided into place along the previously dis-
sected path.

2.43 A needle is inserted in
the aneurysm dome for aspiration as the
tips of the clip close. Aspiration of the an-
eurysm confirms occlusion of the aneurys-
mal neck.

2.43

Aneurysms of the Internal Carotid Artery: Carotid Terminus 111

• Medial striate arteries

2.44 The aneurysmal sac is
dissected free of its attachments to the an-
terior perforate substance, optic tract, and
other structures. All vessels must be free of
the clip blades. Further inspection must
confirm that the origins of the A, and M,
segments are not compromised by place-
ment of the clip and that the aneurysmal
base is completely obliterated.

The surgeon must take great care to
avoid tearing or fracturing the aneurysmal
base because there are no options to effec-
tively deal with this crisis. An encircling
clip does not fit the anatomic arrangement
and will occlude the perforators. The only
option may include trapping the aneurysm,
which deprives circulation to the hemi-
sphere and requires an emergency bypass.
Intraoperative angiography is advisable for
complex aneurysms of this region because
of the difficulty in accurately visualizing the
anatomy.

Closure Closure is completed as in a modified
pterional approach (see Chapter I).

112

Aneurysms of the Anterior Circulation

Aneurysms of the Middle Cerebral Artery

General Aneurysms arising from the middle cere-
Considerations bral artery (MCA) are usually bifurcation
aneurysms located adjacent to the first ma-
jor branch of the MCA. Because the ante-
rior temporal branch and the temporopolar
branch commonly arise independently from
the main trunk of the MCA or from its other
branches, true trifurcation aneurysms are
rare. Aneurysms proximal to the first major
branch of the MCA or distal to all major
branches occur infrequently. Proximal
MCA aneurysms are those occurring at the
bifurcation or trifurcation of an MCA with a
short middle (M,) segment. Mirror-image
bilateral aneurysms of the MCA and multi-
ple aneurysms are common.

Proximal, bifurcation, and trifurcation
MCA aneurysms are approached by proxi-
mal-to-distal dissection of the MCA along
the sylvian fissure. Distal MCA aneurysms
can be approached through an opening of
the lateral sylvian fissure. Aneurysms of
the MCA with significant temporal lobe he-
matoma and life-threatening mass effect are
best approached directly through the mid-
dle temporal gyrus (see Aneurysms of the
Middle Cerebral Artery: With Temporal
Lobe Hematoma, later in this chapter). An-
eurysms of the MCA that are giant, par-
tially thrombosed, or calcified pose a
unique and formidable challenge to pre-
serve the patency of distal MCA branches
(see Aneurysms of the Middle Cerebral Ar-
tery: Giant, Partially Thrombosed, later in
this chapter).

Aneurysms of the Middle Cerebral Artery: Small 113

Aneurysms of the Middle Cerebral Artery:
Small

2.45

Special The sequence for exposure of a small MCA
Considerations aneurysm (i.e., proximal, bifurcation, or
trifurcation) is proximal-to-distal dissection
of the M| segment, identification of the an-
eurysm neck, dissection of the distal MCA,
and development of the aneurysmal neck.
Local hypotension by trapping with low
pressure temporary clips is preferable to
systemic hypotension.

The surgeon must plan the dissection
and obliteration of these lesions to avoid
injury to the perforators of the proximal
M|, excessive retraction of the brain, and
maintenance of patency of the main
branches of the MCA. Distally placed aneu-
rysms can be approached more easily
through a lateral opening of the sylvian
fissure.

114 Aneurysms of the Anterior Circulation

Posterior

communicating

artery

Optic nerve

Anterior

choroidal

artery

Anterior
cerebral
artery

2.47

Uncal
artery

Approach The patient is placed in a supine position
(see Chapter I) with the head maintained at
45 degrees rotation. The craniotomy is
pterional with half-and-half exposure of the
frontal and temporal lobes.

of the

2.46 The arachnoid
carotid cistern is incised.

2.47 Two 15-mm retractors
stretch the arachnoid of the sylvian fissure
so that the arachnoid can be sharply cut and
the sylvian fissure can be opened. The sur-
geon can facilitate brain relaxation by hy-
perventilating the patient, administering
osmotic diuretics, and further removing
cerebrospinal fluid from the cisterns.

Anterior cerebral artery
segment)

2.48

115

Aneurysms of the Middle Cerebral Artery: Small

2.49

2.48 The arachnoid is split
over the distal carotid artery until the ca-
rotid bifurcation is exposed. The veins that
bridge the sylvian fissure are coagulated
and incised.

2.49 Care is exercised to
avoid damaging the perforating arterial
branches while blunt dissection continues
distal along M,.

2.50 The delicate perforating
vessels as well as the anterior temporal and
uncai arteries are preserved. Alternating
dissection and repositioning of the retractor
blades brings exposure of the bifurcation
and aneurysmal base. Arachnoid and brain
tissues are retained over the aneurysm
dome, which appears as a direct continua-
tion of the MCA.

Uncal artery

Perforating
arteries

2.50

116

Aneurysms of the Anterior Circulation

2.51

2.51, 2.52 Using sharp dis-
section, the arachnoid adhesions are sepa-
rated from the inferior and superior trunks
of the MCA to prepare a path for clip appli-
cation and to temporarily trap the M, and
M-, branches.

2.52

Aneurysms of the Middle Cerebral Artery: Small 117

2.53

2.53 A right-angle curved
clip is guided into place at the base of the
aneurysm.

2.54 The aneurysm is aspi-
rated as the clip is closed.

2.54

118 Aneurysms of the Anterior Circulation

2.55 The position of the clip
is inspected to ensure that the perforating
striate vessels and the distal branches of M₂
are not constricted. Blood is removed from
the sylvian fissure and temporal lobe if a
significant hematoma is present.

Closure Closure is completed as in a modified
pterional craniotomy (see Chapter I).

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