Infratentorial Deep Arteriovenous Malformations: Mesencephalic 301

Mesencephalic Arteriovenous Malformations

Mesencephalic AVMs are usually
small and typically present with subarach-
noid hemorrhage and, rarely, parenchymal
hemorrhage. Angiography demonstrates
slow flow in most cases. MRI accurately
defines the relationship of the malforma-
tion nidus to regional anatomy. Preopera-
tive embolization is seldom indicated for
these small malformations. Stereotactic ra-
diosurgery is preferred for medial lesions
in patients with minimal or no neurologic
deficit. In this section the mesencephalic
AVM is a lateral lesion located at the junc-
tion of the upper fourth ventricle and supe-
rior cerebellar peduncle.

Mesencephalic AVMs are usually
small and typically present with subarach-
noid hemorrhage and, rarely, parenchymal
hemorrhage. Angiography demonstrates
slow flow in most cases. MRI accurately
defines the relationship of the malforma-
tion nidus to regional anatomy. Preopera-
tive embolization is seldom indicated for
these small malformations. Stereotactic ra-
diosurgery is preferred for medial lesions
in patients with minimal or no neurologic
deficit. In this section the mesencephalic
AVM is a lateral lesion located at the junc-
tion of the upper fourth ventricle and supe-
rior cerebellar peduncle.

Special Mesencephalic AVMs are rare lesions lo-
Considerations cated on the surface of the mesencephalon.
Medial lesions lie on the tectum, where
hemorrhage produces disturbances in ocu-
lar motility and consciousness. More lat-
erally placed lesions may lie in the upper
fourth ventricle (adjacent to the aqueduct)
and in the pia-arachnoid space (above the
root entry zone of the trigeminal nerve).
Arterial supply arises from the superior
cerebellar and anterior inferior cerebellar
arteries. Venous drainage is to superior
petrosal and superior vermian veins.

302

Arteriovenous Malformations of the Brain

4.112

Approach The approach is superior vermian. A bilat-
eral suboccipital craniotomy is performed
with the patient in the lateral oblique po-
sition (see Chapter I). A femoral artery
sheath is placed prior to positioning and
maintained with heparin solution under
pressure. The skull is maintained in a radio-
lucent head-fixation device for intraopera-
tive angiography. An armored endotracheal
tube allows the neck to be flexed, facilitat-
ing exposure of the posterior fossa. A mid-
line skin incision extends from 3 cm above
the superior nuchal line to the mid-cervical
spine.

4.112 The bilateral suboccipi-
tal craniotomy exposes the inferior edge of
the transverse sinus and 3 cm of cerebellum
on either side. The inferior margin extends
to the foramen magnum to permit adequate
decompression and removal of cerebrospi-
nal fluid from the cisterna magna.

4.113 The dural flap is reflect-
ed on its inferior base.

4.113

Infratentonal Deep Arteriovenous Malformations: Mesencephalic 303

4
I

I

4.114

I

4.115

4.114 Self-retaining retractors
are placed to expose the superior vermis.
The vermis is split to expose the upper
fourth ventricle.

4.115 The retractors are posi-
tioned to expose the malformation.

304

Arteriovenous Malformations of the Brain

Tectum

Fourth
ventricle

4.116

SCA

4.117

4.116 A sagittal view shows
the relationship of the AVM to the anatom-
ic structures: superior cerebellar peduncle,
fourth ventricle, aqueduct, and tectum of
the mesencephalon.

4.117 Dissection begins medi-
ally and proceeds around the margin of the
malformation. The fine feeding arteries are
sealed with a color-absorptive laser and bi-
polar forceps. The occluded branches are
incised in seriatim. Dissection is then di-
-rected upward toward the tectum where the
superior cerebellar artery (SCA) and the
fourth nerve are identified.

4.118 The cerebellum is re-
tracted to expose the lateral aspect of the
mesencephalon and pons. Feeding
branches from the SCA are occluded and
incised.

Infratentorial Deep Arteriovenous Malformations: Mesencephalic 305

4.119 The trigeminal nerve
and the draining vein to the superior
petrosal vein are identified at the lateral
aspect of the pens. Feeding branches from
the anterior inferior cerebellar artery are
coagulated.

4.120 The primary draining
vein is occluded with laser and sectioned.

4.120

306 Arteriovenous Malformations of the Brain

4.121

Nerve IV

,SCA

4.122

4.121 Adhesions are dissected
as the malformation is reflected laterally
from the brainstem.

4.122 The bed of the malfor-
mation is inspected for bleeding sites. Intra-
operative angiography is performed by
vertebral injection.

Infratentorial Deep Arteriovenous Malformations: Mesencephalic 307

4.124

4.123

Closure 4.123 and 4.124 Closure is completed
as for a bilateral suboccipital craniotomy
(see Chapter I).

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