Supratentorial Deep Brain Arteriovenous Malformations: Posterior Thalamostriate 269

Posterior Thalamostriate Arteriovenous
Malformations

Special Posterior thalamostriate arteriovenous mal-
Considerations formations (AVMs) are located in the floor
of the lateral ventricle and trigone. These
AVMs extend into the posterior limb of the
internal capsule and may diffusely involve
the choroid plexus of the temporal horn.
Arterial supply is from the anterior and pos-
terior choroidal arteries, enhanced by a
vascular plexus of perforators from A,, P,,
and P₂. Venous drainage is by subependy-
mal, thalamostriate, and septal veins that
drain into the internal cerebral, basal, and
sylvian veins.

Stereotactic radiosurgery is considered
for these deep brain malformations, par-
ticularly in patients without hematoma or
neurologic deficit. However, radiosurgical
treatment may be associated with mor-
bidity approaching the same level as that of

direct surgery. Complete obliteration of the
malformation is difficult because of the dif-
fuse nature of the multiple arterial feeders.
Because posterior thalamostriate
AVMs commonly present with parenchy-
mal hemorrhage, surgical extirpation is the
preferred treatment. A parenchyma! hema-
toma often causes a neurologic deficit but
also partially dissects the malformation ni-
dus from surrounding brain, secondarily fa-
cilitating surgical removal. The network of
perforators from the basal arteries is trou-
blesome owing to its delicate architecture,
relative inaccessibility deep to the nidus of
the AVM, and passage through the sub-
stance of the thalamus and striatum to
reach the malformation. Preoperative em-
bolization of the perforating arteries from
A,, M], P_1? and P₂ reduces flow in the nidus

270 Arteriovenous Malformations of the Brain

of the malformation, thus enhancing our
ability to deal with the delicate feeding ves-
sels. We rely on a combination of strate-
gies, including preoperative embolization,
induced hypotension, progressive shrink-
age of the nidus with color-absorptive laser
energy, and bipolar coagulation. Preopera-
tive magnetic resonance imaging is helpful
to define the relationship of hematoma to
nidus as well as the relationship of the nidus
to deep brain structures. Angiography is
assessed for evidence of intranidal and pe-
dicular aneurysms that may have been a
source of deep hemorrhage. Small or medi-
um posterior thalamostriate AVMs can be
successfully removed with an acceptable
leVelof morbidity. Large malformations al-
ready associated with parenchyma! hema-
toma and major neurologic deficit can be
eliminated without aggravating the existing
deficit.

There are basically three surgical ap-
proaches to these lesions: transsylvian.
transcallosal, and transcortical through the
middle temporal gyrus or parieto-occipital
fissure. The transsylvian approach is re-
served for smaller, laterally placed lesions.
The transcallosal approach is used for pos-
teromedial lesions of the trigonal area. The
transcortical approach is preferred for larg-
er AVMs, such as those shown in this sec-
tion. The mesial optic radiation can be
spared and ample exposure gained by de-
veloping a parenchyma! corridor through
either the temporal or the parietal lobe and
operating through the trigonal area of the
ventricle.

Supratentorial Deep Brain Arteriovenous Malformations: Posterior Thalamostriate 271

4.64

Approach 4.62 A parieto-occipital cra-
niotomy is performed with the patient in the
supine position. The ipsilateral shoulder is
elevated with a gelatin pad and the thorax is
elevated 15 degrees. The head is main-
tained in a radiolucent head-fixation device
rotated contralateral to the lesion until the
sagittal suture is parallel to the floor. The
groin is prepared for intraoperative angi-
ography.

4.63 A parieto-occipital skin
flap (10 x 10 cm) is based inferiorly.

4.64 An 8 x 8 cm bone flap
is dissected with a pneumatic craniotome.
The medial margin of the flap ends 2 cm
from the sagittal sinus and the posterior
margin 2 cm from the transverse sinus.

272 Arteriovenous Malformations of the Brain

4.65 The dural flap is re-
flected inferiorly and attached to the skin
flap.

Supratentorial Deep Brain Arteriovenous Malformations: Posterior Thalamostriate 273

4.66

4.66 To define a trajectory
for exploration, a ventricular needle is in-
serted in a sulcus of the parieto-occipital
fissure and passed to the trigone of the
ventricle.

4.67

4.67 A cortical incision is
plotted in a parieto-occipital fissure.

274

Arteriovenous Malformations of the Brain

Pulvinar

Choroid plexus

4.68

4.68 The fissure is dissected
to a 3-cm opening secured with four 10-mm
self-retaining retractors. Dissection contin-
ues through the deep white matler until the
ventricle is entered. The pulvinar is visible
in the depth of the ventricle. Cerebrospinal
fluid is aspirated from the cavity.

4.69 This coronal section il-
lustrates the development of the surgical
corridor and the relationship of the malfor-
mation and hematoma to the ventricles,
choroidal fissure, basal ganglia, and thala-
mus.

Posterior
choroidal
fissure

Third
ventricle

4.69

275

Supratentorial Deep Brain Arteriovenous Malformations: Posterior Thalamostriate

4.70 The pulvinar, choroid
plexus, and medial wall of the lateral ventri-
cle are visualized by placing the retractors
deeper in the ventricle.

4.71 The systemic blood pres-
sure is lowered to 55 mmHg mean arterial
pressure. The choroid plexus is obliterated
with bipolar coagulation.

4.70

4.71

276

Arteriovenous Malformations of the Brain

4.72

4.72 Dissection proceeds in-
feriorly along the malformation base until
the posterior choroidal fissure is opened.
Here, perforators from the posterior cere-
bral and posterior choroidal arteries are co-
agulated and incised.

4.73 The coronal section shows
the trajectory of dissection through the
temporal horn to isolate the posterior cho-
roidal feeders and to begin resection of the
malformation base.

4.73

Supratentorial Deep Brain Arteriovenous Malformations: Posterior Thalamostriate 277

Corona

radiata

Septum
pellucidum

4.74

4.75

4.74 Dissection proceeds from
the temporal horn across the nidus base
and through the posterior hippocampus,
posterior limb of the internal capsule, and
Ihalamus where the plane enters the lateral
ventricle. Previous embolization and fi-
brosis induced by hemorrhage create a
nearly avascular plane. A massively dilated
thalamostriate vein is retracted laterally as
the nidus is reduced in volume,

4.75 The coronal section shows
dissection of the medial surface of the mal-
formation inside the lateral ventricle.

278 Arteriovenous Malformations of the Brain

I

f

Right
fornix

I

4.76

Right

foramen of
Monro

4.77

I

Right and left
internal cerebral veins

Supratentorial Deep Brain Arteriovenous Malformations: Posterior Thalamostriate 279

4.76 The choroid plexus is co-
agulated as the nidus mass is reflected lat-
erally to expose the foramen of Monro and
the junction of the thalamostriate and inter-
nal cerebral veins.

4.77 The septum is opened into
the tela choroidea of the posterior third
ventricle near the junction of the internal
cerebral and galenic veins.

4.78

4.78 The septum pellucidum
iif vwutfly'v*>a*:nfiri ^xjmsiry.tru³,internal .ce-
rebral veins.

280

Arteriovenous Malformations of the Brain

Left

foramen of
Monro

4.79

4.79 The internal cerebral vein
is coagulated and incised. The medial margin
of the malformation, including the fornix,
internal cerebral vein, and wall of the third
ventricle, is reflected laterally.

4.80 The malformation is re-
tracted laterally as the remaining base is
transected with laser energy.

I

4.80

Supratenlonal Deep Brain Arteriovenous Malformations: Posterior Thalamostriate 281

4.81

4.81 This coronal section
shows the medial trajectory of dissection
through the lateral third ventricle and floor
of the lateral ventricle and across the nidus

base.

4.82 A view into the cavity
documents complete excision of the malfor-
mation, elimination of bleeding sites, and
removal of the hematoma. An intraopera-
tive angiogram is performed through the
right carotid and vertebral arteries.

4.82

Closure Closure is performed as for a parieto-occip-
itul craniotomy (see Chapter I, Pterional
Approach).

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