193

Aneurysms of the Midbasilar Trunk: High Midbasilar Aneurysms

Aneurysms of the Midbasilar Trunk:
High Midbasilar Aneurysms

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I
I

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I

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Special The aneurysm in this exercise is a berry
Considerations aneurysm of the upper midbasilar artery
arising above the AICA. This aneurysm is
best approached through the space between
the trigeminal and the auditory-facial
nerves.

Approach The patient is placed in the supine position
after a catheter has been placed in the lum-
bar spine for cerebrospinal fluid aspiration.
A gelatin pad is placed under the right
shoulder and the head is rotated until the

sagittal suture is parallel to the floor. The
skull is fixed in a radiolucent head-fixation
device for possible intraoperative angiog-
raphy. Auditory-brainstem and somato-
sensory evoked potentials are monitored
throughout the procedure as a relative indi-
cator of brainstem ischemia. Anesthesia is
prepared to administer cerebral protective
agents if temporary occlusion of the basilar
artery is utilized. A petrosal approach (tem-
poral craniotomy, suboccipital craniotomy.
and posterior petrosectomy) is performed
(see Chapter I).

194 Aneurysms of the Posterior Circulation

3.43

3.43 Clips are applied to the
superior petrosal sinus. The superior
petrosal sinus and tentorium are incised
ventrally toward the incisura in a course
directed posterior to the trochlear nerve.

3.44 The surgeon gently ap-
plies lO-mm self-retaining retractors to the
posterior temporal lobe and lateral cerehel-
lar hemisphere to expose the medial tempo-
ral lobe, lateral pons, basilar artery, and
cranial nerves V through VIII.

Anterior inferior cerebellar artery

Trigeminal
nerve

3.44

195

Aneurysms of the Midbasilar Trunk: High Midbasilar Aneurysms

Basilar
artery

Trigeminal
nerve

3.45

Perforating
arteries

3.46

3.45 A 1-mm retractor dis-
places the trigeminal root upward. The
delicate arachnoid adhesions overlying
the aneurysm base and basilar artery are
explored.

3.46 The abducens nerve and
AICA are retracted laterally to expose the
basilar artery proximal to the aneurysm. A
temporary clip is applied to the basilar ar-
tery to permit safe dissection of the aneu-
rysm and distal basilar artery.

196

Aneurysms of the Posterior Circulation

3.47

3.49

3.47 A 2-mm retractor dis-
places the pons medially. A second tempo-
rary clip is placed on the basilar artery
distal to the origin of the aneurysm.

3.48 A right-angle clip is
placed across the base of the aneurysm so
that the blades are oriented parallel to the
axis of the basilar artery.

3.49 The clip blades are ap-
proximated and the dome of the aneurysm
is aspirated with a 22-gauge spinal needle.
Application of temporary clips and prelimi-
nary decompression of the aneurysm are
important maneuvers to prevent premature
rupture or tearing of the base of this deli-
cate aneurysm. Application of the clip
blades parallel to the axis of the artery fur-
ther reduces the risk of tearing the aneu-
rysm base or constricting the parent artery.

Aneurysms of the Midbasilar Trunk: High Midbasilar Aneurysms 197

3.50, 3.51 The collapsed aneurysm
sac and clip are gently manipulated to iden-
tify the perforating branches that arise from
the basilar artery and to document that they
are free of the clip blades.

Perforating
arteries

3.51

Closure

Closure is performed as for a petrosal crani-
otomy (see Chapter I).

198

Aneurysms of the Posterior Circulation

Aneurysms of the Midbasilar Trunk:
Low Midbasilar Aneurysms

Special The aneurysm in this exercise takes its ori-
Considerations gin from the lower midbasilar artery in the
region of the anterior inferior cerebellar ar-
teries (AICAs). This aneurysm is best ap-
proached in the space between the seventh-
eighth cranial nerve complex and the ninth
cranial nerve.

199

Aneurysms of the Midbasilar Trunk: High Midbasilar Aneurysms

Approach An upper lateral suboccipital craniotomy is
performed with the patient in the lateral
oblique position (see Chapter I). When the
need for intraoperative angiography is an-
ticipated, the head is maintained in a radio-
lucent head-fixation device and a femora!
artery sheath is inserted prior to positioning
and maintained with pressurized heparin
solution. Auditory-brainstem and somato-
sensory evoked potentials are monitored
throughout the procedure as a relative indi-
cator of brainstem ischemia. Anesthesia is
prepared to administer cerebral protective
agents if temporary occlusion of the basilar
artery above and below the aneurysm is
utilized.

Flocculus

3.53

3.53 The dura is incised adja-
cent to the mastoid after the mastoid bone
is removed to expose the sigmoid sinus.
The venous sinuses and adjacent dura are
retracted with dural retention sutures. A
10-mm self-retaining retractor is placed on
the lateral cerebellum and flocculus to ex-
pose the upper regions of the posterior
fossa.

200

Aneurysms of the Posterior Circulation

3.54

Internal
auditory
canal

Glossopharyngeal
nerve

Vagus
nerve

•Accessory
nerve

3.54 The arachnoid bridging
the flocculus and the seventh-eighth cranial
nerve complex is incised. A 2-mm retractor
is placed on the medulla. Gentle dissection
frees the arachnoid adhesions of the basilar
artery proximal to the aneurysm; this step
permits proximal control and clip applica-
tion. The origin of the AICA is identified
proximal to the base of the aneurysm.

3.55 The direction of the mi-
croretractor is changed to displace the sev-
enth-eighth cranial nerve complex upward.
Further dissection of the aneurysm base
continues on the distal basilar artery.

201

Aneurysms of the Midbasilar Trunk: Low Midbasilar Aneurysms

3.56 A 30-degree oblique-
angle clip is essential to allow clip place-
ment across the aneurysm base parallel to
the axis of the parent artery in a narrow
access.

3.57 The dome of the aneu-
rysm is aspirated with a 22-gauge spinal
needle. The aneurysm and clip are rotated
laterally to document that the AICA and
perforating arteries to the medulla are free
of the clip blades.

Closure Closure is performed as for an upper lateral
suboccipital craniotomy (see Chapter I).

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