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Original Article
The Role of Endoscopic Assistance in Ambient Cistern Surgery: Analysis of Four Surgical
Approaches
Eberval Gadelha Figueiredo1, André Beer-Furlan1, Peter Nakaji2, Neil Crawford2, Leonardo C. Welling3,
Eduardo C. Ribas1, Manoel J. Teixeira1, Albert L. Rhoton Jr4, Robert F. Spetzler2, Mark C. Preul, MD2
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OBJECTIVE: We used microscopy with endoscopic
assistance to conduct an objective analysis of 4 surgical
approaches commonly used in the surgery of the ambient
cistern: infratentorial supracerebellar (SC), occipital interhemispheric (OI), subtemporal (ST), and transchoroideal
(TC). In addition, we performed a parahippocampalis gyrus
resection in the ST context.
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METHODS: Each approach (SC, OI, ST, TC) was performed on 3 cadaveric heads (6 sides). After the microscopic anatomic dissection, the 30-degree endoscope was
used to explore the exposure. The parahippocampalis gyrus was resected through an ST approach and the exposure
was evaluated. The quantitative analysis was based on
linear exposure of the vascular structures (linear exposure), such as the posterior choroidal artery (PChA), the
P2 and P3 segments of the posterior cerebral artery (PCA)
with their branches, the basal vein of Rosenthal, and the
area of exposure of the ambient cistern region (area of
exposure) limited by points on its superior, mesial, and
anterior walls. In all cases, a P value of less than 0.05 was
considered significant.
INTRODUCTION
RESULTS: There was a significant difference (P < 0.05) in
linear exposure of the PCA and medial PChA between
microsurgery and endoscopic assistance using the ST
approach. This approach also improved the medial, superior, and total exposure of the ambient cistern region.
he increasing use of rigid endoscopes in endonasal and
transcranial approaches has changed the way cerebral
and skull base lesions are managed. Visualization of the
anatomic structures has improved reducing exposure and brain
retraction, thereby minimizing surgical morbidity. In the current
minimally invasive trend in neurosurgery, endoscopic surgery is a
growing field and widely accepted as a means of treating intracranial lesions.1e9
The ambient cistern region is a unique cerebral compartment
and represents a challenge to the neurosurgeon because of its
deep location surrounding vital structures, narrow boundaries,
and complex tridimensional anatomy. Several surgical approaches
with modifications and in combinations have been described
to access the ambient cistern region and the posterior portion
of the posterior cerebral artery (PCA). However, selecting the
appropriate surgical route remains controversial. In addition, the
use of endoscopic assistance has not been studied thus far.10e18
We used microscopy associated with endoscopic assistance to
conduct an objective analysis of 4 surgical approaches commonly
used in the surgery of the ambient cistern: infratentorial
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CONCLUSIONS: This study demonstrates that endoscope
assistance improved exposure of the ambient cistern region when using the ST approach. Endoscopic assistance
provided similar surgical exposure compared with ST
associated with parahippocampalis resection.
T
Key words
Ambient cistern
- Endoscopy
- Endoscopic assistance
- Anatomy
- Microsurgery
- Microneurosurgery
From the 1Department of Neurology, Discipline of Neurosurgery, University of São Paulo
Medical School (FMUSP), São Paulo, Brazil; 2Barrow Neurological Institute, St Josephs
Hospital and Medical Center, Phoenix, Arizona, USA; 3State University of Ponta Grossa,
School of Medicine, Ponta Grossa, Parana, Brazil; and 4Department of Neurosurgery,
University of Florida, Gainsville, Florida, USA
Abbreviations and Acronyms
BVR: Basal vein of Rosenthal
OI: Occipital interhemispheric
PCA: Posterior cerebral artery
PChA: Medial posterior choroidal artery
SC: Infratentorial supracerebellar
ST: Subtemporal
STh: Parahippocampal gyrus resection
TC: Transchoroideal
Citation: World Neurosurg. (2015) 84, 6:1907-1915.
http://dx.doi.org/10.1016/j.wneu.2015.08.031
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WORLD NEUROSURGERY 84 [6]: 1907-1915, DECEMBER 2015
To whom correspondence should be addressed: Eberval Gadelha Figueiredo, M.D., Ph.D.
[E-mail: [email protected]]
Journal homepage: www.WORLDNEUROSURGERY.org
Available online: www.sciencedirect.com
1878-8750/$ - see front matter ª 2015 Elsevier Inc. All rights reserved.
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supracerebellar (SC), occipital interhemispheric (OI), subtemporal
(ST), and transchoroideal (TC). In addition, we performed a parahippocampalis gyrus resection (STh) in the ST context. This
study sought to analyze and compare surgical exposure provided
by these techniques using a computerized tracking system and
discuss the role of endoscopic assistance when approaching the
ambient cistern region.
METHODS
Three silicone-injected cadaveric heads (6 sides) without obvious
intracranial disease were used in this study. Before dissection, the
heads were rigidly fixated in a Mayfield headholder (Codman, Inc.
Raynham, Massachusetts). The procedures were performed using
standard microsurgical instruments under the surgical microscope
(S88; Carl Zeiss, Germany). A high-speed drill (Midas Rex, L.P.,
Fort Worth, Texas) was used to drill the bone. Endoscope assistance was provided by a 30-degree-angle glass-rod lens endoscope
(Minop System; Aesculap, Tuttlingen, Germany). The video units
were composed of 3 chip video cameras and videotape records and
monitors (Sony Corp., San Jose, California). Before dissection, the
specimens were rigidly fixed in a Mayfield headholder in a position that recreated surgical positioning.
Surgical Approaches
The detailed surgical techniques for each approach have been
well illustrated and were performed as previously described in the
literature.19e22
Briefly, suboccipital and occipital craniotomies with preservation of the bone overlying the transverse sinus were performed.
The SC approach followed by the OI approach was carried out
providing a posterior route to the ambient cistern region.
On the same side of the cadaver head, a low-set temporal
craniotomy provided access to the ST and TC approaches. The
subtemporal approach involves placing the sagittal suture parallel
to the floor with the vertex angled inferiorly to allow for maximal
visualization along the tentorial surface to the ambient cistern.
The arachnoid adhesions connecting the mesial temporal lobe to
the tentorial edge were sharply dissected and removed to expose
the underlying structures.
For the TC approach, we used the inferior temporal sulcus
to access the temporal horn of the lateral ventricle and open
the choroidal fissure between the hippocampal fimbria and the
choroid plexus.
After microscopic anatomic dissection, the 30-degree endoscope was used to explore the exposure in each approach. The
parahippocampal gyrus was resected through an ST approach in
order to improve the exposition of the ambient cistern region.
Defining the Landmarks: Ambient Cistern Anatomy
The ambient cistern is a complex arachnoid compartment that
is shaped like a “C” around the parahippocampal gyrus if viewed
from a coronal perspective (Figure 1A). It extends from the
posterior margin of the crural cistern to the lateral edge of the
midbrain colliculi.23 Some authors consider the crural cistern as
part of the anterior ambient cistern because no definite
border or separation can be observed between the 2 arachnoid
compartments.16 We consider the ambient cistern region area
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limited anteriorly by the posterior lateral surface of the cerebral
peduncle; medially by the lateral surface of the midbrain;
laterally by the tentorial edge, parahippocampal gyrus, fimbria
of the fornix, and choroidal fissure; and superiorly by the
pulvinar of the thalamus, lateral geniculate body, and optic tract
(Figure 1B).24 This region contains the anterior choroidal artery,
the P2 and P3 segments of the PCA with their branches, the
basal vein of Rosenthal (BVR), and, infrequently, a segment of
the superior cerebellar artery. The anterior choroidal artery runs
along the roof of the cistern and enters the choroidal fissure
between the pial layers of the peduncle and the uncus, which
fuse to form the tela choroidea.25e27
The P2 segment of the PCA can be subdivided into P2a and
P2p, which are bordered by the posterior edge of the peduncle.
The P2a begins at the junction of the posterior communicating
artery and courses through the anterior ambient or crural cistern
along the upper surface of the anterior perimesencephalic membrane. The P2p begins at the posterior border of the anterior
ambient or crural cistern and ends at the lateral edge of the
midbrain colliculi. The P2p often courses superiorly and laterally
within the ambient cistern to lie on the superior surface of the
parahippocampal gyrus. The P3 segment proceeds posteriorly
from the posterior edge of the ambient cistern into the quadrigeminal cistern.28
The medial posterior choroidal artery (PChA) typically originates as a single trunk from the P1 or P2 and courses through the
ambient cistern. It travels inferiorly and medially to the PCA
through the crural and ambient cisterns, turns medially to enter
the quadrigeminal cistern, and finally turns superiorly and anteriorly to enter the velum interpositum cistern. The lateral PChA
arteries arise most commonly from the P2p as 1 or several
branches, course laterally along the upper edge of the parahippocampal gyrus within the ambient cistern, and pass through
the choroidal fissure to enter the posterior part of the temporal
horn and atrium.29e32
The BVR passes around the upper midbrain and drains the walls
of the ambient cistern. It finally exits the ambient cistern and
enters the arachnoid envelope over the pineal region to join the
great vein or internal cerebral vein.33 Linear exposure of these
3 relevant vascular structures (PCA, medial PChA, and BVR) was
evaluated.
Evaluation of Exposure
The quantitative analysis was based on linear exposure of the
vascular structures (linear exposure) and the area of exposure
of the ambient cistern region (area of exposure) provided by
microsurgery and by endoscopic assistance. We used the Optotrak 3020 system (Northern Digital, Waterloo, Canada) with a
6-marker digitizing probe and accompanying software for data
collection.34e36 The Optotrak 3020 system is a frameless method
of stereotactic location that allows anatomic points to be spatially
located with a high level of precision.
The head was rigidly fixed in a 3-point headholder to ensure
that it remained in the same Cartesian coordinate system as the
Optotrak. A computer connected to the Optotrak 3020 system
stored the data files in the form of the x, y, and z coordinates (in
millimeters) of each vertex. The retractor was secured firmly to
prevent measurement errors, and the points were located spatially.
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Figure 1. (A) Coronal section demonstrating the “C” shape of the ambient cistern; (B) anterior and medial component
limits; (C) the areas of the triangles were calculated as half the magnitude of the vector cross product of any 2 vectors
forming 2 sides of a triangle. Amb. C., ambient cistern; BA, basilar artery; III Vent., third ventricle; Int. Caps, internal
capsule; IV vent., forth ventricle; Lat. Mes. S., lateral mesencephalic sulcus; Lat. vent., Lateral ventricle; PCA, posterior
cerebral artery; Post. Ch. A, posterior choroidal artery; SCA, superior cerebellar artery; Subic., subincullum; Sup. Pet. V,
superior petrosal vein; Wing of Amb. C., wing of ambient cistern.
A data point was acquired by touching the tip of the digitizing
probe to the anatomic points of interest while its position was
recorded.
This methodology emulates real endoscopic assistance, because
the visualization of critical structures is coupled with manipulation
with surgical instruments, represented by the probe of Optotrak.
Thus, the calculated area corresponds to the additional area that
could be visualized and manipulated during the endoscopic
assistance.
We evaluated the linear exposure of the PCA, medial PChA,
and BVR in each approach. The area of surgical exposure was
measured and calculated as in previous publications.34e36 We divided
the total area into 3 components: anterior, medial, and superior areas.
In each component, 4 points were selected forming a quadrangle or 2
triangles with a common side. The area of exposure of the anterior,
medial, and superior regions was measured by summing the areas
formed by the 2 triangles. The areas of the triangles were calculated as
half the magnitude of the vector cross product of any 2 vectors
forming 2 sides of a triangle (Figure 1B,C).
The anterior component was defined as the region immediately
lateral to the cerebral peduncle and anterior to the lateral
mesencephalic sulcus. The points selected on the anterior area
were 1) the most inferior and anterior point of surgical exposure
(point 1); 2) the most superior and anterior point of surgical
exposure (point 2); 3) the most inferior point at the lateral
WORLD NEUROSURGERY 84 [6]: 1907-1915, DECEMBER 2015
mesencephalic sulcus (point 3); 4) the most superior point at the
lateral mesencephalic sulcus (point 4) (Figure 1B).
The medial component was defined as the region immediately
lateral to the mesencephalon and posterior to the lateral mesencephalic sulcus. The points selected on the medial area were 1) the
most inferior point at the lateral mesencephalic sulcus (point 3);
2) the most superior point at the lateral mesencephalic sulcus
(point 4); 3) the most inferior and posterior point of surgical
exposure (point 5); 4) the most superior and posterior point of
surgical exposure (point 6) (Figure 1B).
The superior component was defined as the region on the
“roof” of the ambient cistern, which corresponds to the pulvinar
of the thalamus, lateral geniculate body, and optic tract. The
points selected on the superior region were 1) the most anterior
and lateral point on the roof of the cistern (point 7); 2) the most
anterior medial point on roof of the cistern (point 8); 3) the most
posterior and lateral point of the roof of the cistern (point 9);
4) the most posterior medial point of the roof of the cistern
(point 10) (Figure 2).
Statistical Analysis
Differences in surgical areas of exposure and linear exposure were
analyzed by 1-way repeated analysis of variance followed by the
Holm-Sidak method and the Dunn method. In all cases, a P value
of less than 0.05 was considered significant.
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TC ¼ 13.8 4.04 mm, SC ¼ 16.37 5.93 mm, OI ¼ 13.7 2.75
mm, and STh ¼ 13.6 5.78 mm. Using endoscopic assistance,
the linear exposure of the PCA in each approach was as follows:
ST ¼ 17.10 4.15 mm, TC ¼ 14.23 3.05 mm, SC ¼ 18.76 3.75
mm, and OI ¼ 17.66 4.73 mm. We found statistically significant
differences (P < 0.05) in linear exposure between microscopy and
endoscopy when using the ST approach (Figure 3).
Medial PChA. The microsurgical linear exposure of the medial
PChA in each approach was as follows: ST ¼ 11.47 4.81 mm,
TC ¼ 8.6 4.54 mm, SC ¼ 12.93 5.44 mm, OI ¼ 13.42 6.64
mm, and STh ¼ 13.95 4.51 mm. After endoscopic assistance, we
found ST ¼ 16.30 6.11 mm, TC ¼ 10.64 4.48 mm, SC ¼ 15.87
2.93 mm, and OI ¼ 13.97 4.31 mm. There was a statistically
significant difference (P < 0.05) in linear exposure between microscopy and endoscopy only when using the ST approach
(Figure 3).
Figure 2. Inferior perspective of the ambient cistern region. The figure
illustrates the landmark points used to evaluate the area of exposition at
the superior component. Amb. C, ambiente cistern; Cer. Ped., cerebral
peduncle; Crural C., crural cistern; Int. Ped. C., interpeduncular cistern;
Lat. Mes. S., lateral mesencephalic sulcus; Mam. Bd., mammilary body;
Opt. Tr., optic tract; Parahyp. G., parahypypocampal gyrus; Pt, point;
Quad. C., quadrigeminal cistern.
RESULTS
Linear Exposure
PCA. The linear exposure of the PCA in each approach
using the microscope was as follows: ST ¼ 5.95 3.52 mm,
Figure 3. Linear exposure of the posterior cerebral artery, medial posterior
choroidal artery, and basal vein of Rosenthal. *P < 0.05. BVR, basal vein of
Rosenthal; OI, occipital interhemispheric; PChA, posterior choroidal artery;
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BVR. It was not possible to expose the BVR on 3 sides with the
microscopy ST approach and on 2 sides with the TC approach.
With endoscope assistance, it was not possible to expose the BVR
on 1 side with the TC approach.
The linear exposure of the BVR using the microscope with each
approach was as follows: ST ¼ 11.5 8.3 mm, TC ¼ 7.34 4.2
mm, SC ¼ 9.03 2.33 mm, OI ¼ 8.6 4.7 mm, and STh ¼ 7.48
2.3 mm. The linear exposure of the BVR in each approach using
the endoscope was as follows: ST ¼ 8.66 3.51 mm, TC ¼ 13.11 10.11 mm, SC ¼ 13.60 5.99 mm, and OI ¼ 13.16 6.83 mm.
However, there were no statistical differences in linear exposure
between microscopy and endoscopy (Figure 3).
Area of Exposure
Superior Area. The results for the area of microsurgical exposure
were as follows: ST ¼ 7.9 11.1 mm2, TC ¼ 65.8 24.5 mm2,
PCA, posterior cerebral artery; SC, supracerebellar; ST, subtemporal; TC,
transchoroideal; STh, parahippocampal gyrus resection.
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Figure 4. Exposure of the superior area, medial area, anterior area, and total area. *P < 0.05. OI, occipital
interhemispheric; SC, supracerebellar; ST, subtemporal; TC, transchoroideal; STh, parahippocampal gyrus resection.
SC ¼ 24.2 15.9 mm2, OI ¼ 31.3 35.1 mm2, and STh ¼ 60.3 19.9 mm2. Endoscopic assistance provided the following areas:
ST ¼ 95.1 62.2 mm2, TC ¼ 117.8 92.4 mm2, SC ¼ 62.7 46.7
mm2, OI ¼ 54.8 40.2 mm2. We found a statistically significant
difference (P < 0.05) in exposure of the superior area between
microscopy and endoscopy when using the ST approach. No
differences (P > 0.05) were found with the TC, SC, and OI
approaches.
There was also a significant difference (P < 0.05) when the
microscopic ST and STh approaches were compared (Figure 4).
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Medial Area. The medial area exposure in each approach using
microscopy was as follows: ST ¼ 54.3 17.6 mm2, TC ¼ 46.8 30.5 mm2, SC ¼ 55.5 27.5 mm2, OI ¼ 44.6 13.8 mm2, and
STh ¼ 81.0 10.5 mm2. The medial area using endoscopic
assistance in each approach Was as follows: ST ¼ 176.2 97.6
mm2, TC ¼ 88.6 57.0 mm2, SC ¼ 72.6 51.7 mm2 and OI ¼
92.7 57.4 mm2.
Statistically significant differences (P < 0.05) in exposure of the
medial area were observed between microscopy and endoscopy
when using the ST approach. There was also a significant
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difference (P < 0.05) when the microscopic ST and STh approaches were compared (Figure 4).
and BVR) (Figure 6) and areas of exposure (superior, medial,
anterior, and total areas) (Figure 7).
Anterior Area. The exposure of the anterior area with each
approach using microscopy was as follows: ST ¼ 42.6 9.9 mm2,
TC ¼ 45.6 18.3 mm2, SC ¼ 48.7 15.3 mm2, OI ¼ 42.6 14.3
mm2, and STh ¼ 69.2 9.6 mm2. The exposure of the anterior
area with each approach using endoscopic assistance was as
follows: ST ¼ 119.9 58.4 mm2, TC ¼ 110.6 55.4 mm2, and
SC ¼ 63.3 32.1 mm2, OI ¼ 96.5 58.8 mm2.
We found a statistically significant difference (P < 0.05) in
exposure of the anterior area between microscopy and endoscopy
when using the TC and OI approaches. No difference (P > 0.05)
was found with the ST and SC approaches.
There was also a significant difference (P < 0.05) when the
microscopic ST and STh approaches were compared (Figure 4).
Linear Exposure
There was a significant difference (P < 0.05) in linear exposure of
the PCA and medial PChA between microsurgery and endoscopic
assistance with the ST approach. Such increase in exposure with
the ST approach is probably due to the axis of exposure, allowing
the endoscope to inspect perpendicularly almost the entire
extension of the PChA and PCA along the ambient cistern cavity.
Among the techniques studied, there was a difference in the axis
of exploration of the ambient cistern, only for ST. The ST
approach with endoscopic assistance allowed a transverse inspection of the ambient cistern, whereas with the other openings,
the endoscope provided longitudinal inspection, the same axis as
the microsurgical technique. As a consequence, the endoscope did
not improve the linear exposure for the other approaches.
Total Area. The sum of all area components resulted in a total
area of exposure for each approach. The total area for the
microscopic exposure was as follows: ST ¼ 104.8 19.8 mm2,
TC ¼ 158.2 51.2 mm2, SC ¼ 137.2 47.7 mm2, OI ¼ 118.5 40.0 mm2, and STh ¼ 210.5 31.9 mm2. The total area for the
endoscopic-assisted exposure was as follows: ST ¼ 391.3 186.6
mm2, TC ¼ 308.8 186.7 mm2, SC ¼ 198.7 114.8 mm2, and
OI ¼ 243.9 132.8 mm2.
We found a statistically significant difference (P < 0.05) in the
total area of exposure between microscopy and endoscopy when
using the ST approach. There was also a significant difference
(P < 0.05) when the microscopic ST and STh approaches were
compared. No difference (P > 0.05) was found with the TC, SC,
and IO approaches (Figure 4).
DISCUSSION
Endoscopic and endoscopic-assisted surgery is a growing field in
neurosurgery and a widely accepted means of managing cerebral
and skull base lesions with minimal morbidity. The improvements
in endoscopes have provided better image quality, illumination,
and a wide view of the surgical field. The main limitation of the
technique, which is related to the reach of visualized structures,
has been overcome with the development of better endoscopic
surgical instrumentation.1e9
Its role in ambient cistern surgery has not been objectively
studied thus far. The complex tridimensional anatomy and density
of vital structures make the ambient cistern region a unique and
challenging cerebral compartment that cannot be fully exposed by
a single approach. As a result, selecting the most appropriate
surgical route has pivotal importance. However, selecting the
appropriate approach remains controversial in the literature and
depends on the neurosurgeon’s preference and experience rather
than objective anatomic data.10e18
We studied previously how the different microsurgical approaches described in this study exposed the ambient cistern region dissimilarly (unpublished data). In the current study, we
evaluated the improvement in surgical exposure using different
approaches with the assistance of a 30-degree endoscope
(Figure 5). The analysis was based on the linear exposure of the
3 main vascular structures of the cistern (PCA, medial PChA,
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Area of Exposure
There were no statistical differences between microsurgery
and endoscopic assistance for the total area of exposure when
considering the TC, SC, and OI approaches (P > 0.05). Nonetheless, endoscopic assistance did improve the medial, superior,
and total area of exposure of the ambient cistern region when used
with the ST approach. These findings were found similarly for
linear exposure. The angles of approach for the ST approach favor
endoscopic assistance, allowing a wider range of movements
along the axis of the ambient cistern and improving the surgical
area of exposure. When the STh approach was performed, endoscopic assistance did not provide any benefits; the structures
previously well exposed only by endoscopy can now be directly
visualized using the microscope (P > 0.05). Thus, endoscopic
assistance may prevent parenchymal resection when used with an
ST approach scenario. Further benefits of endoscopic assistance
include greater exposure of the lateral surface of the cerebral
peduncle (anterior area of exposure) when using the TC and OI
approaches (P < 0.05).
The advantages of the endoscopic assistance technique include
the ability to see “around the corners”, visualization of blind areas,
and provision of a secondary perspective on the anatomy. These
achievements are provided given by the use of the endoscope
“inside the anatomy”, in contrast to the microscope.
Surgery in the ambient cistern region is a challenging procedure
and endoscopic assistance is a resource that can be used to support the surgeon. Our results may help to identify real surgical
situations where endoscopic assistance may be useful and, on the
other hand, situations where it will not be useful. This study
provides valuable practical information that may improve the results of ambient cistern surgery and guide endoscopic assistance
in these settings.
Study Limitations
There are certain limitations to any anatomic study of this type.
The study was performed on chemically fixed tissues. The
response of formalin-fixed cadaveric tissue does not duplicate the
response of tissue in vivo. Such factors can affect the results in
terms of retraction or shrinkage of structures. It is difficult to
predict how these differences may have influenced the final
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Figure 5. Endoscopic perspective from the different approaches. (A,B) PCA and BVR seen through a right
transchoroideal approach. (C,D) Medial PChA and BRV seen through a right subtemporal approach. (E) PCA, medial
PChA, and BVR seen through a right supracerebellar approach. (F) PCA, medial PChA, and BVR seen through a right
occipital interhemispheric approach. BVR, basal vein of Rosenthal; m.PChA, medial posterior choroidal artery; PCA,
posterior cerebral artery.
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Figure 6. Diagrammatic sketch to show important structures (posterior
cerebral artery, basal vein of Rosenthal, lateral mesencephalic sulcus).
results, although we made every effort to maintain consistency in
retraction, among other variables. The advantage of gravity in
altering head position, cerebrospinal fluid drainage, venous
congestion, and brain tissue elasticity cannot be reproduced.
The number of the heads is only a relative limitation. The
reasons for that are multifactorial. Ethical and economic issues
worldwide have increasingly restricted the number of available
specimens and several papers have been published with a similar
number of heads with no detriment to the statistical analysis.34e36
The selection of any surgical technique should be based primarily on the anatomic exposure that it affords. However, surgical
experience and familiarity with using a particular approach also
has a decisive role. Nonetheless, objective and anatomic data such
as provided by this study helps us to understand the advantages
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Conflict of interest statement: The author declares that the
article content was composed in the absence of any
commercial or financial relationships that could be construed
as a potential conflict of interest.
Received 4 April 2015; accepted 25 August 2015
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Citation: World Neurosurg. (2015) 84, 6:1907-1915.
http://dx.doi.org/10.1016/j.wneu.2015.08.031
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