Arteriovenous Malformations of the Brain

Cavernous Vascular Malformations

4.125

Special Cavernous vascular malformations are
Considerations composed of sinusoidal vascular channels
with a microscopic arterial supply that is
capillary and arteriolar in size. Venous
drainage is usually prominent and diverse
in distribution. Cavernous malformations
occur throughout the brainstem, but the
pons is the most common location. These
malformations are usually occult to an-
giography and should be distinguished from
venous angiomas, which have a charac-
teristic angiographic appearance and rarely
cause hemorrhage.

Cavernous malformations are usually
diagnosed by stereotactic appearance on
magnetic resonance imaging, even though
their appearance varies greatly. The most
characteristic finding is that of hemor-

rhages of multiple ages with negative an-
giography. Calcification, contrast enhance-
ment, and progressive mass effect are typi-
cal features of cavernous malformations of
the brainstem.

The natural history of cavernous mal-
formations appears to be more benign than
that of true arteriovenous malformations
(AVMs). Surgical therapy is indicated
when progressive enlargement and neuro-
logic deterioration occur and the lesion is
accessible. In the brainstem, accessibility is
determined by the lesion's proximity to the
surface of the brainstem or floor of the
fourth ventricle. Surgical extirpation is
the only effective treatment for cavernous
malformations because intravascular embo-
lization is impossible and radiosurgery is of
unproven value.

309

Infratentorial Deep Arteriovenous Malformations: Cavernous

There are four approaches to brain-
stem vascular malformations illustrated in
this section: superior vermian, inferior ver-
mian, lateral cerebellar, and combined sub-
temporal-suboccipital transtentorial. The
cavernous malformation illustrated in this
section is located in the lateral pons, pre-
sents in the floor of the fourth ventricle,
and is reached through an inferior vermian
approach.

Approach A bilateral suboccipital craniotomy is per-
formed with the patient in the lateral
oblique position (see Chapter I). The thorax
is elevated 15 degrees. The head is main-
tained in a head-fixation device with the
neck flexed. Intraoperative angiography is
not planned for patients with these lesions.

Obex

4.126

4.126 The dura is opened by
cruciate incision. The cistern is opened and
the tonsils are retracted laterally with two
lO-mm self-retaining retractors. After the

obex is exposed, the inferior vermis
between the distal branches of the posterior
inferior cerebellar artery (PICA) is incised
with focused laser energy.

310

Arteriovenous Malformations of the Brain

4.127

4.127 The sagittal section
shows the relationship of the lesion to the
fourth ventricle, pons, and cerebellum.

Motor
nucleus V

Nucleus VI

Nucleus VII

4.128

4.128 The anatomy of the
floor of the fourth ventricle is demonstrated
in relation to the distorted appearance of
the operative view. The location of the VI.
VII, and X cranial nerve nuclei can be con-
firmed by electrical stimulation.

Infratentorial Deep Arteriovenous Malformations: Cavernous 311

4

4.130

4.129 Retractors are posi-
tioned to permit visualization from the me-
dian raphe to the lateral recess of the right
fourth ventricle. The hematoma is aspirated
at the medial edge of the superior cerebellar
peduncle, 1 cm above the site where stimu-
lation elicited a facial response.

4.130 An incision is made in
the floor of the fourth ventricle with laser
energy at the site of aspiration of the
hematoma.

4.131

312 Arteriovenous Malformations of the Brain

4.131 Opposing 2-mm retrac-
tors provide exposure of the surface of the
thrombosed malformation.

Infratentorial Deep Arteriovenous Malformations: Cavernous 313

4.132

4.133

4.132 Two additional retrac-
tors improve the exposure of the lesion.
Multiple arteriolar-like feeding arteries are
sealed and cut with laser energy. The mass
of the lesion is reduced by application of
defocused laser energy to the surface of the
malformation and aspiration of the loc-
ulated hematoma.

4.133 The lesion is reflected
medially as the lateral border of the malfor-
mation is freed from the brainstem by a
similar technique.

314 Arteriovenous Malformations of the Brain

4.134

4.134 The shrunken malfor-
mation is removed from the cavity of the
pons. The entire bed of the cavity is in-
spected for residual malformation. Color
ultrasound may help identify loculated ar-
eas of malformation. It is essential to re-
move all residual malformation by
thoroughly exploring the walls and base of
the cavity. Minute bleeding sites are coagu-
lated. Physiologic preservation of the brain-
stem nuclei is confirmed by stimulation.

Closure Closure is performed as for the bilateral
suboccipital craniotomy (see Chapter I).

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