295

Infratentorial Convexity Arteriovenous Malformations: Cerebellar Vermian

Cerebellar Vermian Arteriovenous
Malformations

4.103

Special Vermian arteriovenous malformations
Considerations (AVMs) may be located in the superior or
inferior aspect of the vermis. Superior ver-
mian lesions, shown in this study, are sup-
plied by both superior cerebellar arteries
(SCAs) and, to a lesser degree, both ante-
rior inferior cerebellar and posterior inferi-
or cerebellar arteries. These lesions lie in
the vermis and extend into the roof of the
fourth ventricle. They may adhere to
the superior cerebellar peduncle and tec-
tum of the mesencephalon. Venous drain-

age is to the superior vermian, tentorial.
and precentral cerebellar veins and finally
to the transverse and straight sinus.

The surgeon must preserve the normal
venous drainage of the cerebellar hemi-
spheres while exploring and removing cere-
bellar vermian malformations. Preoperative
embolization of bilateral arterial feeders
helps to reduce flow and eliminate deep
feeders that extend through the cerebellar
peduncle and roof of the fourth ventricle.

296

Arteriovenous Malformations of the Brain

Stereotactic radiosurgery is considered
for lesions that have a prominent compo-
nent in the subarachnoid space and overlie
the tectum of the mesencephalon.

In this illustration, the superior ver-
mian malformation occupies the upper ver-
mis and is associated with recurrent
hemorrhage into the subarachnoid space
and fourth ventricle.

Approach A bilateral suboccipital craniotomy (see
Chapter I) is performed with the patient in
the right lateral oblique position. The skull
is fixed in a radiolucent head-fixation de-
vice with the neck in flexion. The thorax is
elevated 15 degrees and a sheath is posi-
tioned in the right femoral artery for intra-
operative angiography.

4.104

4.104 A midline skin incision
extends from 3 cm above the superior nu-
chal line to the mid-cervical spine. The cra-
niotomy exposes the inferior edge of the
transverse sinus and 3 cm of cerebellum on
either side. The inferior margin extends
into the foramen magnum in order to permit
adequate decompression and removal of
cerebrospinal fluid from the cisterna
magna.

The dura is opened in a cruciate man-
ner. The upper flap is lifted to expose the
inferior margin of the transverse sinus. Two
15-mm retractors are placed in the mid-ver-
mian region.

297

Infratentorial Convexity Arteriovenous Malformations: Cerebellar Vermian

4.105

ft

4.105 The retractors are
shifted upward to (he inferior margin of the
malformation where the dissection begins.
Arterial blood pressure is lowered to
60 mmHg to reduce the pressure and turgor
of the AVM. The surgeon increases retrac-
tion as the arachnoid plane is developed
between the vermian malformation and its
adjacent hemisphere. Multiple feeding arte-
ries converging over the surface of the cere-
bellum are coagulated and incised.

4.106 Application of laser en-
ergy or sequential use of bipolar coagula-
tion reduces the vascular mass of the mal-
formation.

4.106

298

Arteriovenous Malformations of the Brain

4.107

4.107 The retractors are moved
toward the fourth ventricle roof as the mal-
formation is retracted upward and sepa-
rated from deep perforating arteries.

4.108 The malformation is re-
tracted upward and medially to expose the
principal branches of the SCA that enter
the right lateral aspect of the lesion across
the dorsum of the tectum. Delicate dissec-
tion is needed to preserve the normal vas-
cular supply to the tectum and to preserve
the trochlear nerve and lateral brainstem.
Feeding arteries and draining veins enter-
ing the malformation are coagulated and
incised.

4.108

299

Infratentoria! Convexity Artenovenous Malformations: Cerebellar Vermian

4.109 The lesion is reflected
toward the opposite side and a similar dis-
section is performed.

4.110 Until the arterial supply
is eliminated, the superior vermian vein,
which is the principal drainage, is pre-
served. The malformation bed, fourth ven-
tricle, and aqueduct are thoroughly
inspected to ensure complete removal of
the malformation and hematoma and elim-
ination of potential bleeding sites. Intra-
operative angiography is performed by
vertebral injection.

4.109

4.110

Closure Closure is performed as for a bilateral sub-
occipital craniotomy (see Chapter I).

300

Arteriovenous Malformations of the Brain

Infratentorial Deep Arteriovenous
Malformations

General Arteriovenous malformations (AVMs) of
Considerations the brainstem are rare, representing only
1 percent of all AVMs. They usually pre-
sent with hemorrhage and neurologic defi-
cit. Surgery is complicated by location of
the nidus within critical brainstem or neu-
rovascular structures, surgical corridors

through the cerebellum, and arterial supply
deep to the malformation and penetrating
the brainstem. Yet, surgery is often indi-
cated owing to repeated hemorrhages and
progressive neurologic deficit. Relative un-
certainty about rates of obliteration and
brainstem necrosis after radiosurgery of
these AVMs raises a concern. Emboliza-
tion is rarely possible. Brainstem vascular
malformations include true AVMs, venous
angiomas, and cavernous malformations.
Cavernous malformations are often occult
to angiography and must be diagnosed by
typical features on magnetic resonance im-
aging (MRI).

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